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2802 lines
112 KiB
Plaintext
2802 lines
112 KiB
Plaintext
@c Copyright (C) 1988, 1989, 1992, 1994 Free Software Foundation, Inc.
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@c This is part of the GCC manual.
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@c For copying conditions, see the file gcc.texi.
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@node RTL
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@chapter RTL Representation
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@cindex RTL representation
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@cindex representation of RTL
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@cindex Register Transfer Language (RTL)
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Most of the work of the compiler is done on an intermediate representation
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called register transfer language. In this language, the instructions to be
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output are described, pretty much one by one, in an algebraic form that
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describes what the instruction does.
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RTL is inspired by Lisp lists. It has both an internal form, made up of
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structures that point at other structures, and a textual form that is used
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in the machine description and in printed debugging dumps. The textual
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form uses nested parentheses to indicate the pointers in the internal form.
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@menu
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* RTL Objects:: Expressions vs vectors vs strings vs integers.
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* Accessors:: Macros to access expression operands or vector elts.
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* Flags:: Other flags in an RTL expression.
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* Machine Modes:: Describing the size and format of a datum.
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* Constants:: Expressions with constant values.
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* Regs and Memory:: Expressions representing register contents or memory.
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* Arithmetic:: Expressions representing arithmetic on other expressions.
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* Comparisons:: Expressions representing comparison of expressions.
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* Bit Fields:: Expressions representing bitfields in memory or reg.
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* Conversions:: Extending, truncating, floating or fixing.
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* RTL Declarations:: Declaring volatility, constancy, etc.
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* Side Effects:: Expressions for storing in registers, etc.
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* Incdec:: Embedded side-effects for autoincrement addressing.
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* Assembler:: Representing @code{asm} with operands.
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* Insns:: Expression types for entire insns.
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* Calls:: RTL representation of function call insns.
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* Sharing:: Some expressions are unique; others *must* be copied.
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* Reading RTL:: Reading textual RTL from a file.
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@end menu
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@node RTL Objects, Accessors, RTL, RTL
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@section RTL Object Types
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@cindex RTL object types
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@cindex RTL integers
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@cindex RTL strings
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@cindex RTL vectors
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@cindex RTL expression
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@cindex RTX (See RTL)
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RTL uses five kinds of objects: expressions, integers, wide integers,
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strings and vectors. Expressions are the most important ones. An RTL
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expression (``RTX'', for short) is a C structure, but it is usually
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referred to with a pointer; a type that is given the typedef name
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@code{rtx}.
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An integer is simply an @code{int}; their written form uses decimal digits.
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A wide integer is an integral object whose type is @code{HOST_WIDE_INT}
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(@pxref{Config}); their written form uses decimal digits.
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A string is a sequence of characters. In core it is represented as a
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@code{char *} in usual C fashion, and it is written in C syntax as well.
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However, strings in RTL may never be null. If you write an empty string in
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a machine description, it is represented in core as a null pointer rather
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than as a pointer to a null character. In certain contexts, these null
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pointers instead of strings are valid. Within RTL code, strings are most
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commonly found inside @code{symbol_ref} expressions, but they appear in
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other contexts in the RTL expressions that make up machine descriptions.
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A vector contains an arbitrary number of pointers to expressions. The
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number of elements in the vector is explicitly present in the vector.
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The written form of a vector consists of square brackets
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(@samp{[@dots{}]}) surrounding the elements, in sequence and with
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whitespace separating them. Vectors of length zero are not created;
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null pointers are used instead.
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@cindex expression codes
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@cindex codes, RTL expression
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@findex GET_CODE
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@findex PUT_CODE
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Expressions are classified by @dfn{expression codes} (also called RTX
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codes). The expression code is a name defined in @file{rtl.def}, which is
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also (in upper case) a C enumeration constant. The possible expression
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codes and their meanings are machine-independent. The code of an RTX can
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be extracted with the macro @code{GET_CODE (@var{x})} and altered with
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@code{PUT_CODE (@var{x}, @var{newcode})}.
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The expression code determines how many operands the expression contains,
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and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell
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by looking at an operand what kind of object it is. Instead, you must know
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from its context---from the expression code of the containing expression.
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For example, in an expression of code @code{subreg}, the first operand is
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to be regarded as an expression and the second operand as an integer. In
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an expression of code @code{plus}, there are two operands, both of which
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are to be regarded as expressions. In a @code{symbol_ref} expression,
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there is one operand, which is to be regarded as a string.
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Expressions are written as parentheses containing the name of the
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expression type, its flags and machine mode if any, and then the operands
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of the expression (separated by spaces).
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Expression code names in the @samp{md} file are written in lower case,
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but when they appear in C code they are written in upper case. In this
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manual, they are shown as follows: @code{const_int}.
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@cindex (nil)
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@cindex nil
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In a few contexts a null pointer is valid where an expression is normally
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wanted. The written form of this is @code{(nil)}.
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@node Accessors, Flags, RTL Objects, RTL
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@section Access to Operands
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@cindex accessors
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@cindex access to operands
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@cindex operand access
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@cindex RTL format
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For each expression type @file{rtl.def} specifies the number of
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contained objects and their kinds, with four possibilities: @samp{e} for
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expression (actually a pointer to an expression), @samp{i} for integer,
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@samp{w} for wide integer, @samp{s} for string, and @samp{E} for vector
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of expressions. The sequence of letters for an expression code is
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called its @dfn{format}. Thus, the format of @code{subreg} is
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@samp{ei}.@refill
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@cindex RTL format characters
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A few other format characters are used occasionally:
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@table @code
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@item u
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@samp{u} is equivalent to @samp{e} except that it is printed differently
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in debugging dumps. It is used for pointers to insns.
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@item n
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@samp{n} is equivalent to @samp{i} except that it is printed differently
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in debugging dumps. It is used for the line number or code number of a
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@code{note} insn.
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@item S
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@samp{S} indicates a string which is optional. In the RTL objects in
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core, @samp{S} is equivalent to @samp{s}, but when the object is read,
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from an @samp{md} file, the string value of this operand may be omitted.
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An omitted string is taken to be the null string.
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@item V
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@samp{V} indicates a vector which is optional. In the RTL objects in
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core, @samp{V} is equivalent to @samp{E}, but when the object is read
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from an @samp{md} file, the vector value of this operand may be omitted.
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An omitted vector is effectively the same as a vector of no elements.
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@item 0
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@samp{0} means a slot whose contents do not fit any normal category.
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@samp{0} slots are not printed at all in dumps, and are often used in
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special ways by small parts of the compiler.
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@end table
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There are macros to get the number of operands, the format, and the
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class of an expression code:
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@table @code
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@findex GET_RTX_LENGTH
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@item GET_RTX_LENGTH (@var{code})
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Number of operands of an RTX of code @var{code}.
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@findex GET_RTX_FORMAT
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@item GET_RTX_FORMAT (@var{code})
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The format of an RTX of code @var{code}, as a C string.
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@findex GET_RTX_CLASS
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@cindex classes of RTX codes
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@item GET_RTX_CLASS (@var{code})
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A single character representing the type of RTX operation that code
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@var{code} performs.
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The following classes are defined:
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@table @code
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@item o
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An RTX code that represents an actual object, such as @code{reg} or
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@code{mem}. @code{subreg} is not in this class.
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@item <
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An RTX code for a comparison. The codes in this class are
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@code{NE}, @code{EQ}, @code{LE}, @code{LT}, @code{GE}, @code{GT},
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@code{LEU}, @code{LTU}, @code{GEU}, @code{GTU}.@refill
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@item 1
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An RTX code for a unary arithmetic operation, such as @code{neg}.
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@item c
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An RTX code for a commutative binary operation, other than @code{NE}
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and @code{EQ} (which have class @samp{<}).
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@item 2
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An RTX code for a noncommutative binary operation, such as @code{MINUS}.
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@item b
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An RTX code for a bitfield operation, either @code{ZERO_EXTRACT} or
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@code{SIGN_EXTRACT}.
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@item 3
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An RTX code for other three input operations, such as @code{IF_THEN_ELSE}.
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@item i
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An RTX code for a machine insn (@code{INSN}, @code{JUMP_INSN}, and
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@code{CALL_INSN}).@refill
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@item m
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An RTX code for something that matches in insns, such as @code{MATCH_DUP}.
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@item x
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All other RTX codes.
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@end table
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@end table
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@findex XEXP
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@findex XINT
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@findex XWINT
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@findex XSTR
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Operands of expressions are accessed using the macros @code{XEXP},
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@code{XINT}, @code{XWINT} and @code{XSTR}. Each of these macros takes
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two arguments: an expression-pointer (RTX) and an operand number
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(counting from zero). Thus,@refill
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@example
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XEXP (@var{x}, 2)
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@end example
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@noindent
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accesses operand 2 of expression @var{x}, as an expression.
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@example
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XINT (@var{x}, 2)
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@end example
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@noindent
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accesses the same operand as an integer. @code{XSTR}, used in the same
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fashion, would access it as a string.
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Any operand can be accessed as an integer, as an expression or as a string.
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You must choose the correct method of access for the kind of value actually
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stored in the operand. You would do this based on the expression code of
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the containing expression. That is also how you would know how many
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operands there are.
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For example, if @var{x} is a @code{subreg} expression, you know that it has
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two operands which can be correctly accessed as @code{XEXP (@var{x}, 0)}
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and @code{XINT (@var{x}, 1)}. If you did @code{XINT (@var{x}, 0)}, you
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would get the address of the expression operand but cast as an integer;
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that might occasionally be useful, but it would be cleaner to write
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@code{(int) XEXP (@var{x}, 0)}. @code{XEXP (@var{x}, 1)} would also
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compile without error, and would return the second, integer operand cast as
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an expression pointer, which would probably result in a crash when
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accessed. Nothing stops you from writing @code{XEXP (@var{x}, 28)} either,
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but this will access memory past the end of the expression with
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unpredictable results.@refill
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Access to operands which are vectors is more complicated. You can use the
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macro @code{XVEC} to get the vector-pointer itself, or the macros
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@code{XVECEXP} and @code{XVECLEN} to access the elements and length of a
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vector.
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@table @code
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@findex XVEC
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@item XVEC (@var{exp}, @var{idx})
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Access the vector-pointer which is operand number @var{idx} in @var{exp}.
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@findex XVECLEN
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@item XVECLEN (@var{exp}, @var{idx})
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Access the length (number of elements) in the vector which is
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in operand number @var{idx} in @var{exp}. This value is an @code{int}.
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@findex XVECEXP
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@item XVECEXP (@var{exp}, @var{idx}, @var{eltnum})
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Access element number @var{eltnum} in the vector which is
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in operand number @var{idx} in @var{exp}. This value is an RTX.
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It is up to you to make sure that @var{eltnum} is not negative
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and is less than @code{XVECLEN (@var{exp}, @var{idx})}.
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@end table
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All the macros defined in this section expand into lvalues and therefore
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can be used to assign the operands, lengths and vector elements as well as
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to access them.
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@node Flags, Machine Modes, Accessors, RTL
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@section Flags in an RTL Expression
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@cindex flags in RTL expression
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RTL expressions contain several flags (one-bit bitfields) that are used
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in certain types of expression. Most often they are accessed with the
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following macros:
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@table @code
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@findex MEM_VOLATILE_P
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@cindex @code{mem} and @samp{/v}
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@cindex @code{volatil}, in @code{mem}
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@cindex @samp{/v} in RTL dump
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@item MEM_VOLATILE_P (@var{x})
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In @code{mem} expressions, nonzero for volatile memory references.
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Stored in the @code{volatil} field and printed as @samp{/v}.
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@findex MEM_IN_STRUCT_P
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@cindex @code{mem} and @samp{/s}
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@cindex @code{in_struct}, in @code{mem}
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@cindex @samp{/s} in RTL dump
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@item MEM_IN_STRUCT_P (@var{x})
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In @code{mem} expressions, nonzero for reference to an entire
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structure, union or array, or to a component of one. Zero for
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references to a scalar variable or through a pointer to a scalar.
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Stored in the @code{in_struct} field and printed as @samp{/s}.
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@findex REG_LOOP_TEST_P
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@cindex @code{reg} and @samp{/s}
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@cindex @code{in_struct}, in @code{reg}
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@item REG_LOOP_TEST_P
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In @code{reg} expressions, nonzero if this register's entire life is
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contained in the exit test code for some loop. Stored in the
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@code{in_struct} field and printed as @samp{/s}.
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@findex REG_USERVAR_P
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@cindex @code{reg} and @samp{/v}
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@cindex @code{volatil}, in @code{reg}
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@item REG_USERVAR_P (@var{x})
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In a @code{reg}, nonzero if it corresponds to a variable present in
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the user's source code. Zero for temporaries generated internally by
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the compiler. Stored in the @code{volatil} field and printed as
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@samp{/v}.
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@cindex @samp{/i} in RTL dump
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@findex REG_FUNCTION_VALUE_P
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@cindex @code{reg} and @samp{/i}
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@cindex @code{integrated}, in @code{reg}
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@item REG_FUNCTION_VALUE_P (@var{x})
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Nonzero in a @code{reg} if it is the place in which this function's
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value is going to be returned. (This happens only in a hard
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register.) Stored in the @code{integrated} field and printed as
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@samp{/i}.
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The same hard register may be used also for collecting the values of
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functions called by this one, but @code{REG_FUNCTION_VALUE_P} is zero
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in this kind of use.
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@findex SUBREG_PROMOTED_VAR_P
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@cindex @code{subreg} and @samp{/s}
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@cindex @code{in_struct}, in @code{subreg}
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@item SUBREG_PROMOTED_VAR_P
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Nonzero in a @code{subreg} if it was made when accessing an object that
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was promoted to a wider mode in accord with the @code{PROMOTED_MODE} machine
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description macro (@pxref{Storage Layout}). In this case, the mode of
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the @code{subreg} is the declared mode of the object and the mode of
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@code{SUBREG_REG} is the mode of the register that holds the object.
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Promoted variables are always either sign- or zero-extended to the wider
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mode on every assignment. Stored in the @code{in_struct} field and
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printed as @samp{/s}.
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@findex SUBREG_PROMOTED_UNSIGNED_P
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@cindex @code{subreg} and @samp{/u}
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@cindex @code{unchanging}, in @code{subreg}
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@item SUBREG_PROMOTED_UNSIGNED_P
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Nonzero in a @code{subreg} that has @code{SUBREG_PROMOTED_VAR_P} nonzero
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if the object being referenced is kept zero-extended and zero if it
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is kept sign-extended. Stored in the @code{unchanging} field and
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printed as @samp{/u}.
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@findex RTX_UNCHANGING_P
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@cindex @code{reg} and @samp{/u}
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@cindex @code{mem} and @samp{/u}
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@cindex @code{unchanging}, in @code{reg} and @code{mem}
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@cindex @samp{/u} in RTL dump
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@item RTX_UNCHANGING_P (@var{x})
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Nonzero in a @code{reg} or @code{mem} if the value is not changed.
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(This flag is not set for memory references via pointers to constants.
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Such pointers only guarantee that the object will not be changed
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explicitly by the current function. The object might be changed by
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other functions or by aliasing.) Stored in the
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@code{unchanging} field and printed as @samp{/u}.
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@findex RTX_INTEGRATED_P
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@cindex @code{integrated}, in @code{insn}
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@item RTX_INTEGRATED_P (@var{insn})
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Nonzero in an insn if it resulted from an in-line function call.
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Stored in the @code{integrated} field and printed as @samp{/i}. This
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may be deleted; nothing currently depends on it.
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@findex SYMBOL_REF_USED
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@cindex @code{used}, in @code{symbol_ref}
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@item SYMBOL_REF_USED (@var{x})
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In a @code{symbol_ref}, indicates that @var{x} has been used. This is
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normally only used to ensure that @var{x} is only declared external
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once. Stored in the @code{used} field.
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@findex SYMBOL_REF_FLAG
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@cindex @code{symbol_ref} and @samp{/v}
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@cindex @code{volatil}, in @code{symbol_ref}
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@item SYMBOL_REF_FLAG (@var{x})
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In a @code{symbol_ref}, this is used as a flag for machine-specific purposes.
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Stored in the @code{volatil} field and printed as @samp{/v}.
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@findex LABEL_OUTSIDE_LOOP_P
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@cindex @code{label_ref} and @samp{/s}
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@cindex @code{in_struct}, in @code{label_ref}
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@item LABEL_OUTSIDE_LOOP_P
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In @code{label_ref} expressions, nonzero if this is a reference to a
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label that is outside the innermost loop containing the reference to the
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label. Stored in the @code{in_struct} field and printed as @samp{/s}.
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@findex INSN_DELETED_P
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@cindex @code{volatil}, in @code{insn}
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@item INSN_DELETED_P (@var{insn})
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In an insn, nonzero if the insn has been deleted. Stored in the
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@code{volatil} field and printed as @samp{/v}.
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@findex INSN_ANNULLED_BRANCH_P
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@cindex @code{insn} and @samp{/u}
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@cindex @code{unchanging}, in @code{insn}
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@item INSN_ANNULLED_BRANCH_P (@var{insn})
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In an @code{insn} in the delay slot of a branch insn, indicates that an
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annulling branch should be used. See the discussion under
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@code{sequence} below. Stored in the @code{unchanging} field and printed
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as @samp{/u}.
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@findex INSN_FROM_TARGET_P
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@cindex @code{insn} and @samp{/s}
|
|
@cindex @code{in_struct}, in @code{insn}
|
|
@cindex @samp{/s} in RTL dump
|
|
@item INSN_FROM_TARGET_P (@var{insn})
|
|
In an @code{insn} in a delay slot of a branch, indicates that the insn
|
|
is from the target of the branch. If the branch insn has
|
|
@code{INSN_ANNULLED_BRANCH_P} set, this insn should only be executed if
|
|
the branch is taken. For annulled branches with this bit clear, the
|
|
insn should be executed only if the branch is not taken. Stored in the
|
|
@code{in_struct} field and printed as @samp{/s}.
|
|
|
|
@findex CONSTANT_POOL_ADDRESS_P
|
|
@cindex @code{symbol_ref} and @samp{/u}
|
|
@cindex @code{unchanging}, in @code{symbol_ref}
|
|
@item CONSTANT_POOL_ADDRESS_P (@var{x})
|
|
Nonzero in a @code{symbol_ref} if it refers to part of the current
|
|
function's ``constants pool''. These are addresses close to the
|
|
beginning of the function, and GNU CC assumes they can be addressed
|
|
directly (perhaps with the help of base registers). Stored in the
|
|
@code{unchanging} field and printed as @samp{/u}.
|
|
|
|
@findex CONST_CALL_P
|
|
@cindex @code{call_insn} and @samp{/u}
|
|
@cindex @code{unchanging}, in @code{call_insn}
|
|
@item CONST_CALL_P (@var{x})
|
|
In a @code{call_insn}, indicates that the insn represents a call to a const
|
|
function. Stored in the @code{unchanging} field and printed as @samp{/u}.
|
|
|
|
@findex LABEL_PRESERVE_P
|
|
@cindex @code{code_label} and @samp{/i}
|
|
@cindex @code{in_struct}, in @code{code_label}
|
|
@item LABEL_PRESERVE_P (@var{x})
|
|
In a @code{code_label}, indicates that the label can never be deleted.
|
|
Labels referenced by a non-local goto will have this bit set. Stored
|
|
in the @code{in_struct} field and printed as @samp{/s}.
|
|
|
|
@findex SCHED_GROUP_P
|
|
@cindex @code{insn} and @samp{/i}
|
|
@cindex @code{in_struct}, in @code{insn}
|
|
@item SCHED_GROUP_P (@var{insn})
|
|
During instruction scheduling, in an insn, indicates that the previous insn
|
|
must be scheduled together with this insn. This is used to ensure that
|
|
certain groups of instructions will not be split up by the instruction
|
|
scheduling pass, for example, @code{use} insns before a @code{call_insn} may
|
|
not be separated from the @code{call_insn}. Stored in the @code{in_struct}
|
|
field and printed as @samp{/s}.
|
|
@end table
|
|
|
|
These are the fields which the above macros refer to:
|
|
|
|
@table @code
|
|
@findex used
|
|
@item used
|
|
Normally, this flag is used only momentarily, at the end of RTL
|
|
generation for a function, to count the number of times an expression
|
|
appears in insns. Expressions that appear more than once are copied,
|
|
according to the rules for shared structure (@pxref{Sharing}).
|
|
|
|
In a @code{symbol_ref}, it indicates that an external declaration for
|
|
the symbol has already been written.
|
|
|
|
In a @code{reg}, it is used by the leaf register renumbering code to ensure
|
|
that each register is only renumbered once.
|
|
|
|
@findex volatil
|
|
@item volatil
|
|
This flag is used in @code{mem}, @code{symbol_ref} and @code{reg}
|
|
expressions and in insns. In RTL dump files, it is printed as
|
|
@samp{/v}.
|
|
|
|
@cindex volatile memory references
|
|
In a @code{mem} expression, it is 1 if the memory reference is volatile.
|
|
Volatile memory references may not be deleted, reordered or combined.
|
|
|
|
In a @code{symbol_ref} expression, it is used for machine-specific
|
|
purposes.
|
|
|
|
In a @code{reg} expression, it is 1 if the value is a user-level variable.
|
|
0 indicates an internal compiler temporary.
|
|
|
|
In an insn, 1 means the insn has been deleted.
|
|
|
|
@findex in_struct
|
|
@item in_struct
|
|
In @code{mem} expressions, it is 1 if the memory datum referred to is
|
|
all or part of a structure or array; 0 if it is (or might be) a scalar
|
|
variable. A reference through a C pointer has 0 because the pointer
|
|
might point to a scalar variable. This information allows the compiler
|
|
to determine something about possible cases of aliasing.
|
|
|
|
In an insn in the delay slot of a branch, 1 means that this insn is from
|
|
the target of the branch.
|
|
|
|
During instruction scheduling, in an insn, 1 means that this insn must be
|
|
scheduled as part of a group together with the previous insn.
|
|
|
|
In @code{reg} expressions, it is 1 if the register has its entire life
|
|
contained within the test expression of some loop.
|
|
|
|
In @code{subreg} expressions, 1 means that the @code{subreg} is accessing
|
|
an object that has had its mode promoted from a wider mode.
|
|
|
|
In @code{label_ref} expressions, 1 means that the referenced label is
|
|
outside the innermost loop containing the insn in which the @code{label_ref}
|
|
was found.
|
|
|
|
In @code{code_label} expressions, it is 1 if the label may never be deleted.
|
|
This is used for labels which are the target of non-local gotos.
|
|
|
|
In an RTL dump, this flag is represented as @samp{/s}.
|
|
|
|
@findex unchanging
|
|
@item unchanging
|
|
In @code{reg} and @code{mem} expressions, 1 means
|
|
that the value of the expression never changes.
|
|
|
|
In @code{subreg} expressions, it is 1 if the @code{subreg} references an
|
|
unsigned object whose mode has been promoted to a wider mode.
|
|
|
|
In an insn, 1 means that this is an annulling branch.
|
|
|
|
In a @code{symbol_ref} expression, 1 means that this symbol addresses
|
|
something in the per-function constants pool.
|
|
|
|
In a @code{call_insn}, 1 means that this instruction is a call to a
|
|
const function.
|
|
|
|
In an RTL dump, this flag is represented as @samp{/u}.
|
|
|
|
@findex integrated
|
|
@item integrated
|
|
In some kinds of expressions, including insns, this flag means the
|
|
rtl was produced by procedure integration.
|
|
|
|
In a @code{reg} expression, this flag indicates the register
|
|
containing the value to be returned by the current function. On
|
|
machines that pass parameters in registers, the same register number
|
|
may be used for parameters as well, but this flag is not set on such
|
|
uses.
|
|
@end table
|
|
|
|
@node Machine Modes, Constants, Flags, RTL
|
|
@section Machine Modes
|
|
@cindex machine modes
|
|
|
|
@findex enum machine_mode
|
|
A machine mode describes a size of data object and the representation used
|
|
for it. In the C code, machine modes are represented by an enumeration
|
|
type, @code{enum machine_mode}, defined in @file{machmode.def}. Each RTL
|
|
expression has room for a machine mode and so do certain kinds of tree
|
|
expressions (declarations and types, to be precise).
|
|
|
|
In debugging dumps and machine descriptions, the machine mode of an RTL
|
|
expression is written after the expression code with a colon to separate
|
|
them. The letters @samp{mode} which appear at the end of each machine mode
|
|
name are omitted. For example, @code{(reg:SI 38)} is a @code{reg}
|
|
expression with machine mode @code{SImode}. If the mode is
|
|
@code{VOIDmode}, it is not written at all.
|
|
|
|
Here is a table of machine modes. The term ``byte'' below refers to an
|
|
object of @code{BITS_PER_UNIT} bits (@pxref{Storage Layout}).
|
|
|
|
@table @code
|
|
@findex QImode
|
|
@item QImode
|
|
``Quarter-Integer'' mode represents a single byte treated as an integer.
|
|
|
|
@findex HImode
|
|
@item HImode
|
|
``Half-Integer'' mode represents a two-byte integer.
|
|
|
|
@findex PSImode
|
|
@item PSImode
|
|
``Partial Single Integer'' mode represents an integer which occupies
|
|
four bytes but which doesn't really use all four. On some machines,
|
|
this is the right mode to use for pointers.
|
|
|
|
@findex SImode
|
|
@item SImode
|
|
``Single Integer'' mode represents a four-byte integer.
|
|
|
|
@findex PDImode
|
|
@item PDImode
|
|
``Partial Double Integer'' mode represents an integer which occupies
|
|
eight bytes but which doesn't really use all eight. On some machines,
|
|
this is the right mode to use for certain pointers.
|
|
|
|
@findex DImode
|
|
@item DImode
|
|
``Double Integer'' mode represents an eight-byte integer.
|
|
|
|
@findex TImode
|
|
@item TImode
|
|
``Tetra Integer'' (?) mode represents a sixteen-byte integer.
|
|
|
|
@findex SFmode
|
|
@item SFmode
|
|
``Single Floating'' mode represents a single-precision (four byte) floating
|
|
point number.
|
|
|
|
@findex DFmode
|
|
@item DFmode
|
|
``Double Floating'' mode represents a double-precision (eight byte) floating
|
|
point number.
|
|
|
|
@findex XFmode
|
|
@item XFmode
|
|
``Extended Floating'' mode represents a triple-precision (twelve byte)
|
|
floating point number. This mode is used for IEEE extended floating
|
|
point. On some systems not all bits within these bytes will actually
|
|
be used.
|
|
|
|
@findex TFmode
|
|
@item TFmode
|
|
``Tetra Floating'' mode represents a quadruple-precision (sixteen byte)
|
|
floating point number.
|
|
|
|
@findex CCmode
|
|
@item CCmode
|
|
``Condition Code'' mode represents the value of a condition code, which
|
|
is a machine-specific set of bits used to represent the result of a
|
|
comparison operation. Other machine-specific modes may also be used for
|
|
the condition code. These modes are not used on machines that use
|
|
@code{cc0} (see @pxref{Condition Code}).
|
|
|
|
@findex BLKmode
|
|
@item BLKmode
|
|
``Block'' mode represents values that are aggregates to which none of
|
|
the other modes apply. In RTL, only memory references can have this mode,
|
|
and only if they appear in string-move or vector instructions. On machines
|
|
which have no such instructions, @code{BLKmode} will not appear in RTL.
|
|
|
|
@findex VOIDmode
|
|
@item VOIDmode
|
|
Void mode means the absence of a mode or an unspecified mode.
|
|
For example, RTL expressions of code @code{const_int} have mode
|
|
@code{VOIDmode} because they can be taken to have whatever mode the context
|
|
requires. In debugging dumps of RTL, @code{VOIDmode} is expressed by
|
|
the absence of any mode.
|
|
|
|
@findex SCmode
|
|
@findex DCmode
|
|
@findex XCmode
|
|
@findex TCmode
|
|
@item SCmode, DCmode, XCmode, TCmode
|
|
These modes stand for a complex number represented as a pair of floating
|
|
point values. The floating point values are in @code{SFmode},
|
|
@code{DFmode}, @code{XFmode}, and @code{TFmode}, respectively.
|
|
|
|
@findex CQImode
|
|
@findex CHImode
|
|
@findex CSImode
|
|
@findex CDImode
|
|
@findex CTImode
|
|
@findex COImode
|
|
@item CQImode, CHImode, CSImode, CDImode, CTImode, COImode
|
|
These modes stand for a complex number represented as a pair of integer
|
|
values. The integer values are in @code{QImode}, @code{HImode},
|
|
@code{SImode}, @code{DImode}, @code{TImode}, and @code{OImode},
|
|
respectively.
|
|
@end table
|
|
|
|
The machine description defines @code{Pmode} as a C macro which expands
|
|
into the machine mode used for addresses. Normally this is the mode
|
|
whose size is @code{BITS_PER_WORD}, @code{SImode} on 32-bit machines.
|
|
|
|
The only modes which a machine description @i{must} support are
|
|
@code{QImode}, and the modes corresponding to @code{BITS_PER_WORD},
|
|
@code{FLOAT_TYPE_SIZE} and @code{DOUBLE_TYPE_SIZE}.
|
|
The compiler will attempt to use @code{DImode} for 8-byte structures and
|
|
unions, but this can be prevented by overriding the definition of
|
|
@code{MAX_FIXED_MODE_SIZE}. Alternatively, you can have the compiler
|
|
use @code{TImode} for 16-byte structures and unions. Likewise, you can
|
|
arrange for the C type @code{short int} to avoid using @code{HImode}.
|
|
|
|
@cindex mode classes
|
|
Very few explicit references to machine modes remain in the compiler and
|
|
these few references will soon be removed. Instead, the machine modes
|
|
are divided into mode classes. These are represented by the enumeration
|
|
type @code{enum mode_class} defined in @file{machmode.h}. The possible
|
|
mode classes are:
|
|
|
|
@table @code
|
|
@findex MODE_INT
|
|
@item MODE_INT
|
|
Integer modes. By default these are @code{QImode}, @code{HImode},
|
|
@code{SImode}, @code{DImode}, and @code{TImode}.
|
|
|
|
@findex MODE_PARTIAL_INT
|
|
@item MODE_PARTIAL_INT
|
|
The ``partial integer'' modes, @code{PSImode} and @code{PDImode}.
|
|
|
|
@findex MODE_FLOAT
|
|
@item MODE_FLOAT
|
|
floating point modes. By default these are @code{SFmode}, @code{DFmode},
|
|
@code{XFmode} and @code{TFmode}.
|
|
|
|
@findex MODE_COMPLEX_INT
|
|
@item MODE_COMPLEX_INT
|
|
Complex integer modes. (These are not currently implemented).
|
|
|
|
@findex MODE_COMPLEX_FLOAT
|
|
@item MODE_COMPLEX_FLOAT
|
|
Complex floating point modes. By default these are @code{SCmode},
|
|
@code{DCmode}, @code{XCmode}, and @code{TCmode}.
|
|
|
|
@findex MODE_FUNCTION
|
|
@item MODE_FUNCTION
|
|
Algol or Pascal function variables including a static chain.
|
|
(These are not currently implemented).
|
|
|
|
@findex MODE_CC
|
|
@item MODE_CC
|
|
Modes representing condition code values. These are @code{CCmode} plus
|
|
any modes listed in the @code{EXTRA_CC_MODES} macro. @xref{Jump Patterns},
|
|
also see @ref{Condition Code}.
|
|
|
|
@findex MODE_RANDOM
|
|
@item MODE_RANDOM
|
|
This is a catchall mode class for modes which don't fit into the above
|
|
classes. Currently @code{VOIDmode} and @code{BLKmode} are in
|
|
@code{MODE_RANDOM}.
|
|
@end table
|
|
|
|
Here are some C macros that relate to machine modes:
|
|
|
|
@table @code
|
|
@findex GET_MODE
|
|
@item GET_MODE (@var{x})
|
|
Returns the machine mode of the RTX @var{x}.
|
|
|
|
@findex PUT_MODE
|
|
@item PUT_MODE (@var{x}, @var{newmode})
|
|
Alters the machine mode of the RTX @var{x} to be @var{newmode}.
|
|
|
|
@findex NUM_MACHINE_MODES
|
|
@item NUM_MACHINE_MODES
|
|
Stands for the number of machine modes available on the target
|
|
machine. This is one greater than the largest numeric value of any
|
|
machine mode.
|
|
|
|
@findex GET_MODE_NAME
|
|
@item GET_MODE_NAME (@var{m})
|
|
Returns the name of mode @var{m} as a string.
|
|
|
|
@findex GET_MODE_CLASS
|
|
@item GET_MODE_CLASS (@var{m})
|
|
Returns the mode class of mode @var{m}.
|
|
|
|
@findex GET_MODE_WIDER_MODE
|
|
@item GET_MODE_WIDER_MODE (@var{m})
|
|
Returns the next wider natural mode. For example, the expression
|
|
@code{GET_MODE_WIDER_MODE (QImode)} returns @code{HImode}.
|
|
|
|
@findex GET_MODE_SIZE
|
|
@item GET_MODE_SIZE (@var{m})
|
|
Returns the size in bytes of a datum of mode @var{m}.
|
|
|
|
@findex GET_MODE_BITSIZE
|
|
@item GET_MODE_BITSIZE (@var{m})
|
|
Returns the size in bits of a datum of mode @var{m}.
|
|
|
|
@findex GET_MODE_MASK
|
|
@item GET_MODE_MASK (@var{m})
|
|
Returns a bitmask containing 1 for all bits in a word that fit within
|
|
mode @var{m}. This macro can only be used for modes whose bitsize is
|
|
less than or equal to @code{HOST_BITS_PER_INT}.
|
|
|
|
@findex GET_MODE_ALIGNMENT
|
|
@item GET_MODE_ALIGNMENT (@var{m)})
|
|
Return the required alignment, in bits, for an object of mode @var{m}.
|
|
|
|
@findex GET_MODE_UNIT_SIZE
|
|
@item GET_MODE_UNIT_SIZE (@var{m})
|
|
Returns the size in bytes of the subunits of a datum of mode @var{m}.
|
|
This is the same as @code{GET_MODE_SIZE} except in the case of complex
|
|
modes. For them, the unit size is the size of the real or imaginary
|
|
part.
|
|
|
|
@findex GET_MODE_NUNITS
|
|
@item GET_MODE_NUNITS (@var{m})
|
|
Returns the number of units contained in a mode, i.e.,
|
|
@code{GET_MODE_SIZE} divided by @code{GET_MODE_UNIT_SIZE}.
|
|
|
|
@findex GET_CLASS_NARROWEST_MODE
|
|
@item GET_CLASS_NARROWEST_MODE (@var{c})
|
|
Returns the narrowest mode in mode class @var{c}.
|
|
@end table
|
|
|
|
@findex byte_mode
|
|
@findex word_mode
|
|
The global variables @code{byte_mode} and @code{word_mode} contain modes
|
|
whose classes are @code{MODE_INT} and whose bitsizes are either
|
|
@code{BITS_PER_UNIT} or @code{BITS_PER_WORD}, respectively. On 32-bit
|
|
machines, these are @code{QImode} and @code{SImode}, respectively.
|
|
|
|
@node Constants, Regs and Memory, Machine Modes, RTL
|
|
@section Constant Expression Types
|
|
@cindex RTL constants
|
|
@cindex RTL constant expression types
|
|
|
|
The simplest RTL expressions are those that represent constant values.
|
|
|
|
@table @code
|
|
@findex const_int
|
|
@item (const_int @var{i})
|
|
This type of expression represents the integer value @var{i}. @var{i}
|
|
is customarily accessed with the macro @code{INTVAL} as in
|
|
@code{INTVAL (@var{exp})}, which is equivalent to @code{XWINT (@var{exp}, 0)}.
|
|
|
|
@findex const0_rtx
|
|
@findex const1_rtx
|
|
@findex const2_rtx
|
|
@findex constm1_rtx
|
|
There is only one expression object for the integer value zero; it is
|
|
the value of the variable @code{const0_rtx}. Likewise, the only
|
|
expression for integer value one is found in @code{const1_rtx}, the only
|
|
expression for integer value two is found in @code{const2_rtx}, and the
|
|
only expression for integer value negative one is found in
|
|
@code{constm1_rtx}. Any attempt to create an expression of code
|
|
@code{const_int} and value zero, one, two or negative one will return
|
|
@code{const0_rtx}, @code{const1_rtx}, @code{const2_rtx} or
|
|
@code{constm1_rtx} as appropriate.@refill
|
|
|
|
@findex const_true_rtx
|
|
Similarly, there is only one object for the integer whose value is
|
|
@code{STORE_FLAG_VALUE}. It is found in @code{const_true_rtx}. If
|
|
@code{STORE_FLAG_VALUE} is one, @code{const_true_rtx} and
|
|
@code{const1_rtx} will point to the same object. If
|
|
@code{STORE_FLAG_VALUE} is -1, @code{const_true_rtx} and
|
|
@code{constm1_rtx} will point to the same object.@refill
|
|
|
|
@findex const_double
|
|
@item (const_double:@var{m} @var{addr} @var{i0} @var{i1} @dots{})
|
|
Represents either a floating-point constant of mode @var{m} or an
|
|
integer constant too large to fit into @code{HOST_BITS_PER_WIDE_INT}
|
|
bits but small enough to fit within twice that number of bits (GNU CC
|
|
does not provide a mechanism to represent even larger constants). In
|
|
the latter case, @var{m} will be @code{VOIDmode}.
|
|
|
|
@findex CONST_DOUBLE_MEM
|
|
@findex CONST_DOUBLE_CHAIN
|
|
@var{addr} is used to contain the @code{mem} expression that corresponds
|
|
to the location in memory that at which the constant can be found. If
|
|
it has not been allocated a memory location, but is on the chain of all
|
|
@code{const_double} expressions in this compilation (maintained using an
|
|
undisplayed field), @var{addr} contains @code{const0_rtx}. If it is not
|
|
on the chain, @var{addr} contains @code{cc0_rtx}. @var{addr} is
|
|
customarily accessed with the macro @code{CONST_DOUBLE_MEM} and the
|
|
chain field via @code{CONST_DOUBLE_CHAIN}.@refill
|
|
|
|
@findex CONST_DOUBLE_LOW
|
|
If @var{m} is @code{VOIDmode}, the bits of the value are stored in
|
|
@var{i0} and @var{i1}. @var{i0} is customarily accessed with the macro
|
|
@code{CONST_DOUBLE_LOW} and @var{i1} with @code{CONST_DOUBLE_HIGH}.
|
|
|
|
If the constant is floating point (regardless of its precision), then
|
|
the number of integers used to store the value depends on the size of
|
|
@code{REAL_VALUE_TYPE} (@pxref{Cross-compilation}). The integers
|
|
represent a floating point number, but not precisely in the target
|
|
machine's or host machine's floating point format. To convert them to
|
|
the precise bit pattern used by the target machine, use the macro
|
|
@code{REAL_VALUE_TO_TARGET_DOUBLE} and friends (@pxref{Data Output}).
|
|
|
|
@findex CONST0_RTX
|
|
@findex CONST1_RTX
|
|
@findex CONST2_RTX
|
|
The macro @code{CONST0_RTX (@var{mode})} refers to an expression with
|
|
value 0 in mode @var{mode}. If mode @var{mode} is of mode class
|
|
@code{MODE_INT}, it returns @code{const0_rtx}. Otherwise, it returns a
|
|
@code{CONST_DOUBLE} expression in mode @var{mode}. Similarly, the macro
|
|
@code{CONST1_RTX (@var{mode})} refers to an expression with value 1 in
|
|
mode @var{mode} and similarly for @code{CONST2_RTX}.
|
|
|
|
@findex const_string
|
|
@item (const_string @var{str})
|
|
Represents a constant string with value @var{str}. Currently this is
|
|
used only for insn attributes (@pxref{Insn Attributes}) since constant
|
|
strings in C are placed in memory.
|
|
|
|
@findex symbol_ref
|
|
@item (symbol_ref:@var{mode} @var{symbol})
|
|
Represents the value of an assembler label for data. @var{symbol} is
|
|
a string that describes the name of the assembler label. If it starts
|
|
with a @samp{*}, the label is the rest of @var{symbol} not including
|
|
the @samp{*}. Otherwise, the label is @var{symbol}, usually prefixed
|
|
with @samp{_}.
|
|
|
|
The @code{symbol_ref} contains a mode, which is usually @code{Pmode}.
|
|
Usually that is the only mode for which a symbol is directly valid.
|
|
|
|
@findex label_ref
|
|
@item (label_ref @var{label})
|
|
Represents the value of an assembler label for code. It contains one
|
|
operand, an expression, which must be a @code{code_label} that appears
|
|
in the instruction sequence to identify the place where the label
|
|
should go.
|
|
|
|
The reason for using a distinct expression type for code label
|
|
references is so that jump optimization can distinguish them.
|
|
|
|
@item (const:@var{m} @var{exp})
|
|
Represents a constant that is the result of an assembly-time
|
|
arithmetic computation. The operand, @var{exp}, is an expression that
|
|
contains only constants (@code{const_int}, @code{symbol_ref} and
|
|
@code{label_ref} expressions) combined with @code{plus} and
|
|
@code{minus}. However, not all combinations are valid, since the
|
|
assembler cannot do arbitrary arithmetic on relocatable symbols.
|
|
|
|
@var{m} should be @code{Pmode}.
|
|
|
|
@findex high
|
|
@item (high:@var{m} @var{exp})
|
|
Represents the high-order bits of @var{exp}, usually a
|
|
@code{symbol_ref}. The number of bits is machine-dependent and is
|
|
normally the number of bits specified in an instruction that initializes
|
|
the high order bits of a register. It is used with @code{lo_sum} to
|
|
represent the typical two-instruction sequence used in RISC machines to
|
|
reference a global memory location.
|
|
|
|
@var{m} should be @code{Pmode}.
|
|
@end table
|
|
|
|
@node Regs and Memory, Arithmetic, Constants, RTL
|
|
@section Registers and Memory
|
|
@cindex RTL register expressions
|
|
@cindex RTL memory expressions
|
|
|
|
Here are the RTL expression types for describing access to machine
|
|
registers and to main memory.
|
|
|
|
@table @code
|
|
@findex reg
|
|
@cindex hard registers
|
|
@cindex pseudo registers
|
|
@item (reg:@var{m} @var{n})
|
|
For small values of the integer @var{n} (those that are less than
|
|
@code{FIRST_PSEUDO_REGISTER}), this stands for a reference to machine
|
|
register number @var{n}: a @dfn{hard register}. For larger values of
|
|
@var{n}, it stands for a temporary value or @dfn{pseudo register}.
|
|
The compiler's strategy is to generate code assuming an unlimited
|
|
number of such pseudo registers, and later convert them into hard
|
|
registers or into memory references.
|
|
|
|
@var{m} is the machine mode of the reference. It is necessary because
|
|
machines can generally refer to each register in more than one mode.
|
|
For example, a register may contain a full word but there may be
|
|
instructions to refer to it as a half word or as a single byte, as
|
|
well as instructions to refer to it as a floating point number of
|
|
various precisions.
|
|
|
|
Even for a register that the machine can access in only one mode,
|
|
the mode must always be specified.
|
|
|
|
The symbol @code{FIRST_PSEUDO_REGISTER} is defined by the machine
|
|
description, since the number of hard registers on the machine is an
|
|
invariant characteristic of the machine. Note, however, that not
|
|
all of the machine registers must be general registers. All the
|
|
machine registers that can be used for storage of data are given
|
|
hard register numbers, even those that can be used only in certain
|
|
instructions or can hold only certain types of data.
|
|
|
|
A hard register may be accessed in various modes throughout one
|
|
function, but each pseudo register is given a natural mode
|
|
and is accessed only in that mode. When it is necessary to describe
|
|
an access to a pseudo register using a nonnatural mode, a @code{subreg}
|
|
expression is used.
|
|
|
|
A @code{reg} expression with a machine mode that specifies more than
|
|
one word of data may actually stand for several consecutive registers.
|
|
If in addition the register number specifies a hardware register, then
|
|
it actually represents several consecutive hardware registers starting
|
|
with the specified one.
|
|
|
|
Each pseudo register number used in a function's RTL code is
|
|
represented by a unique @code{reg} expression.
|
|
|
|
@findex FIRST_VIRTUAL_REGISTER
|
|
@findex LAST_VIRTUAL_REGISTER
|
|
Some pseudo register numbers, those within the range of
|
|
@code{FIRST_VIRTUAL_REGISTER} to @code{LAST_VIRTUAL_REGISTER} only
|
|
appear during the RTL generation phase and are eliminated before the
|
|
optimization phases. These represent locations in the stack frame that
|
|
cannot be determined until RTL generation for the function has been
|
|
completed. The following virtual register numbers are defined:
|
|
|
|
@table @code
|
|
@findex VIRTUAL_INCOMING_ARGS_REGNUM
|
|
@item VIRTUAL_INCOMING_ARGS_REGNUM
|
|
This points to the first word of the incoming arguments passed on the
|
|
stack. Normally these arguments are placed there by the caller, but the
|
|
callee may have pushed some arguments that were previously passed in
|
|
registers.
|
|
|
|
@cindex @code{FIRST_PARM_OFFSET} and virtual registers
|
|
@cindex @code{ARG_POINTER_REGNUM} and virtual registers
|
|
When RTL generation is complete, this virtual register is replaced
|
|
by the sum of the register given by @code{ARG_POINTER_REGNUM} and the
|
|
value of @code{FIRST_PARM_OFFSET}.
|
|
|
|
@findex VIRTUAL_STACK_VARS_REGNUM
|
|
@cindex @code{FRAME_GROWS_DOWNWARD} and virtual registers
|
|
@item VIRTUAL_STACK_VARS_REGNUM
|
|
If @code{FRAME_GROWS_DOWNWARD} is defined, this points to immediately
|
|
above the first variable on the stack. Otherwise, it points to the
|
|
first variable on the stack.
|
|
|
|
@cindex @code{STARTING_FRAME_OFFSET} and virtual registers
|
|
@cindex @code{FRAME_POINTER_REGNUM} and virtual registers
|
|
@code{VIRTUAL_STACK_VARS_REGNUM} is replaced with the sum of the
|
|
register given by @code{FRAME_POINTER_REGNUM} and the value
|
|
@code{STARTING_FRAME_OFFSET}.
|
|
|
|
@findex VIRTUAL_STACK_DYNAMIC_REGNUM
|
|
@item VIRTUAL_STACK_DYNAMIC_REGNUM
|
|
This points to the location of dynamically allocated memory on the stack
|
|
immediately after the stack pointer has been adjusted by the amount of
|
|
memory desired.
|
|
|
|
@cindex @code{STACK_DYNAMIC_OFFSET} and virtual registers
|
|
@cindex @code{STACK_POINTER_REGNUM} and virtual registers
|
|
This virtual register is replaced by the sum of the register given by
|
|
@code{STACK_POINTER_REGNUM} and the value @code{STACK_DYNAMIC_OFFSET}.
|
|
|
|
@findex VIRTUAL_OUTGOING_ARGS_REGNUM
|
|
@item VIRTUAL_OUTGOING_ARGS_REGNUM
|
|
This points to the location in the stack at which outgoing arguments
|
|
should be written when the stack is pre-pushed (arguments pushed using
|
|
push insns should always use @code{STACK_POINTER_REGNUM}).
|
|
|
|
@cindex @code{STACK_POINTER_OFFSET} and virtual registers
|
|
This virtual register is replaced by the sum of the register given by
|
|
@code{STACK_POINTER_REGNUM} and the value @code{STACK_POINTER_OFFSET}.
|
|
@end table
|
|
|
|
@findex subreg
|
|
@item (subreg:@var{m} @var{reg} @var{wordnum})
|
|
@code{subreg} expressions are used to refer to a register in a machine
|
|
mode other than its natural one, or to refer to one register of
|
|
a multi-word @code{reg} that actually refers to several registers.
|
|
|
|
Each pseudo-register has a natural mode. If it is necessary to
|
|
operate on it in a different mode---for example, to perform a fullword
|
|
move instruction on a pseudo-register that contains a single
|
|
byte---the pseudo-register must be enclosed in a @code{subreg}. In
|
|
such a case, @var{wordnum} is zero.
|
|
|
|
Usually @var{m} is at least as narrow as the mode of @var{reg}, in which
|
|
case it is restricting consideration to only the bits of @var{reg} that
|
|
are in @var{m}.
|
|
|
|
Sometimes @var{m} is wider than the mode of @var{reg}. These
|
|
@code{subreg} expressions are often called @dfn{paradoxical}. They are
|
|
used in cases where we want to refer to an object in a wider mode but do
|
|
not care what value the additional bits have. The reload pass ensures
|
|
that paradoxical references are only made to hard registers.
|
|
|
|
The other use of @code{subreg} is to extract the individual registers of
|
|
a multi-register value. Machine modes such as @code{DImode} and
|
|
@code{TImode} can indicate values longer than a word, values which
|
|
usually require two or more consecutive registers. To access one of the
|
|
registers, use a @code{subreg} with mode @code{SImode} and a
|
|
@var{wordnum} that says which register.
|
|
|
|
Storing in a non-paradoxical @code{subreg} has undefined results for
|
|
bits belonging to the same word as the @code{subreg}. This laxity makes
|
|
it easier to generate efficient code for such instructions. To
|
|
represent an instruction that preserves all the bits outside of those in
|
|
the @code{subreg}, use @code{strict_low_part} around the @code{subreg}.
|
|
|
|
@cindex @code{WORDS_BIG_ENDIAN}, effect on @code{subreg}
|
|
The compilation parameter @code{WORDS_BIG_ENDIAN}, if set to 1, says
|
|
that word number zero is the most significant part; otherwise, it is
|
|
the least significant part.
|
|
|
|
@cindex combiner pass
|
|
@cindex reload pass
|
|
@cindex @code{subreg}, special reload handling
|
|
Between the combiner pass and the reload pass, it is possible to have a
|
|
paradoxical @code{subreg} which contains a @code{mem} instead of a
|
|
@code{reg} as its first operand. After the reload pass, it is also
|
|
possible to have a non-paradoxical @code{subreg} which contains a
|
|
@code{mem}; this usually occurs when the @code{mem} is a stack slot
|
|
which replaced a pseudo register.
|
|
|
|
Note that it is not valid to access a @code{DFmode} value in @code{SFmode}
|
|
using a @code{subreg}. On some machines the most significant part of a
|
|
@code{DFmode} value does not have the same format as a single-precision
|
|
floating value.
|
|
|
|
It is also not valid to access a single word of a multi-word value in a
|
|
hard register when less registers can hold the value than would be
|
|
expected from its size. For example, some 32-bit machines have
|
|
floating-point registers that can hold an entire @code{DFmode} value.
|
|
If register 10 were such a register @code{(subreg:SI (reg:DF 10) 1)}
|
|
would be invalid because there is no way to convert that reference to
|
|
a single machine register. The reload pass prevents @code{subreg}
|
|
expressions such as these from being formed.
|
|
|
|
@findex SUBREG_REG
|
|
@findex SUBREG_WORD
|
|
The first operand of a @code{subreg} expression is customarily accessed
|
|
with the @code{SUBREG_REG} macro and the second operand is customarily
|
|
accessed with the @code{SUBREG_WORD} macro.
|
|
|
|
@findex scratch
|
|
@cindex scratch operands
|
|
@item (scratch:@var{m})
|
|
This represents a scratch register that will be required for the
|
|
execution of a single instruction and not used subsequently. It is
|
|
converted into a @code{reg} by either the local register allocator or
|
|
the reload pass.
|
|
|
|
@code{scratch} is usually present inside a @code{clobber} operation
|
|
(@pxref{Side Effects}).
|
|
|
|
@findex cc0
|
|
@cindex condition code register
|
|
@item (cc0)
|
|
This refers to the machine's condition code register. It has no
|
|
operands and may not have a machine mode. There are two ways to use it:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
To stand for a complete set of condition code flags. This is best on
|
|
most machines, where each comparison sets the entire series of flags.
|
|
|
|
With this technique, @code{(cc0)} may be validly used in only two
|
|
contexts: as the destination of an assignment (in test and compare
|
|
instructions) and in comparison operators comparing against zero
|
|
(@code{const_int} with value zero; that is to say, @code{const0_rtx}).
|
|
|
|
@item
|
|
To stand for a single flag that is the result of a single condition.
|
|
This is useful on machines that have only a single flag bit, and in
|
|
which comparison instructions must specify the condition to test.
|
|
|
|
With this technique, @code{(cc0)} may be validly used in only two
|
|
contexts: as the destination of an assignment (in test and compare
|
|
instructions) where the source is a comparison operator, and as the
|
|
first operand of @code{if_then_else} (in a conditional branch).
|
|
@end itemize
|
|
|
|
@findex cc0_rtx
|
|
There is only one expression object of code @code{cc0}; it is the
|
|
value of the variable @code{cc0_rtx}. Any attempt to create an
|
|
expression of code @code{cc0} will return @code{cc0_rtx}.
|
|
|
|
Instructions can set the condition code implicitly. On many machines,
|
|
nearly all instructions set the condition code based on the value that
|
|
they compute or store. It is not necessary to record these actions
|
|
explicitly in the RTL because the machine description includes a
|
|
prescription for recognizing the instructions that do so (by means of
|
|
the macro @code{NOTICE_UPDATE_CC}). @xref{Condition Code}. Only
|
|
instructions whose sole purpose is to set the condition code, and
|
|
instructions that use the condition code, need mention @code{(cc0)}.
|
|
|
|
On some machines, the condition code register is given a register number
|
|
and a @code{reg} is used instead of @code{(cc0)}. This is usually the
|
|
preferable approach if only a small subset of instructions modify the
|
|
condition code. Other machines store condition codes in general
|
|
registers; in such cases a pseudo register should be used.
|
|
|
|
Some machines, such as the Sparc and RS/6000, have two sets of
|
|
arithmetic instructions, one that sets and one that does not set the
|
|
condition code. This is best handled by normally generating the
|
|
instruction that does not set the condition code, and making a pattern
|
|
that both performs the arithmetic and sets the condition code register
|
|
(which would not be @code{(cc0)} in this case). For examples, search
|
|
for @samp{addcc} and @samp{andcc} in @file{sparc.md}.
|
|
|
|
@findex pc
|
|
@item (pc)
|
|
@cindex program counter
|
|
This represents the machine's program counter. It has no operands and
|
|
may not have a machine mode. @code{(pc)} may be validly used only in
|
|
certain specific contexts in jump instructions.
|
|
|
|
@findex pc_rtx
|
|
There is only one expression object of code @code{pc}; it is the value
|
|
of the variable @code{pc_rtx}. Any attempt to create an expression of
|
|
code @code{pc} will return @code{pc_rtx}.
|
|
|
|
All instructions that do not jump alter the program counter implicitly
|
|
by incrementing it, but there is no need to mention this in the RTL.
|
|
|
|
@findex mem
|
|
@item (mem:@var{m} @var{addr})
|
|
This RTX represents a reference to main memory at an address
|
|
represented by the expression @var{addr}. @var{m} specifies how large
|
|
a unit of memory is accessed.
|
|
@end table
|
|
|
|
@node Arithmetic, Comparisons, Regs and Memory, RTL
|
|
@section RTL Expressions for Arithmetic
|
|
@cindex arithmetic, in RTL
|
|
@cindex math, in RTL
|
|
@cindex RTL expressions for arithmetic
|
|
|
|
Unless otherwise specified, all the operands of arithmetic expressions
|
|
must be valid for mode @var{m}. An operand is valid for mode @var{m}
|
|
if it has mode @var{m}, or if it is a @code{const_int} or
|
|
@code{const_double} and @var{m} is a mode of class @code{MODE_INT}.
|
|
|
|
For commutative binary operations, constants should be placed in the
|
|
second operand.
|
|
|
|
@table @code
|
|
@findex plus
|
|
@cindex RTL addition
|
|
@cindex RTL sum
|
|
@item (plus:@var{m} @var{x} @var{y})
|
|
Represents the sum of the values represented by @var{x} and @var{y}
|
|
carried out in machine mode @var{m}.
|
|
|
|
@findex lo_sum
|
|
@item (lo_sum:@var{m} @var{x} @var{y})
|
|
Like @code{plus}, except that it represents that sum of @var{x} and the
|
|
low-order bits of @var{y}. The number of low order bits is
|
|
machine-dependent but is normally the number of bits in a @code{Pmode}
|
|
item minus the number of bits set by the @code{high} code
|
|
(@pxref{Constants}).
|
|
|
|
@var{m} should be @code{Pmode}.
|
|
|
|
@findex minus
|
|
@cindex RTL subtraction
|
|
@cindex RTL difference
|
|
@item (minus:@var{m} @var{x} @var{y})
|
|
Like @code{plus} but represents subtraction.
|
|
|
|
@findex compare
|
|
@cindex RTL comparison
|
|
@item (compare:@var{m} @var{x} @var{y})
|
|
Represents the result of subtracting @var{y} from @var{x} for purposes
|
|
of comparison. The result is computed without overflow, as if with
|
|
infinite precision.
|
|
|
|
Of course, machines can't really subtract with infinite precision.
|
|
However, they can pretend to do so when only the sign of the
|
|
result will be used, which is the case when the result is stored
|
|
in the condition code. And that is the only way this kind of expression
|
|
may validly be used: as a value to be stored in the condition codes.
|
|
|
|
The mode @var{m} is not related to the modes of @var{x} and @var{y},
|
|
but instead is the mode of the condition code value. If @code{(cc0)}
|
|
is used, it is @code{VOIDmode}. Otherwise it is some mode in class
|
|
@code{MODE_CC}, often @code{CCmode}. @xref{Condition Code}.
|
|
|
|
Normally, @var{x} and @var{y} must have the same mode. Otherwise,
|
|
@code{compare} is valid only if the mode of @var{x} is in class
|
|
@code{MODE_INT} and @var{y} is a @code{const_int} or
|
|
@code{const_double} with mode @code{VOIDmode}. The mode of @var{x}
|
|
determines what mode the comparison is to be done in; thus it must not
|
|
be @code{VOIDmode}.
|
|
|
|
If one of the operands is a constant, it should be placed in the
|
|
second operand and the comparison code adjusted as appropriate.
|
|
|
|
A @code{compare} specifying two @code{VOIDmode} constants is not valid
|
|
since there is no way to know in what mode the comparison is to be
|
|
performed; the comparison must either be folded during the compilation
|
|
or the first operand must be loaded into a register while its mode is
|
|
still known.
|
|
|
|
@findex neg
|
|
@item (neg:@var{m} @var{x})
|
|
Represents the negation (subtraction from zero) of the value represented
|
|
by @var{x}, carried out in mode @var{m}.
|
|
|
|
@findex mult
|
|
@cindex multiplication
|
|
@cindex product
|
|
@item (mult:@var{m} @var{x} @var{y})
|
|
Represents the signed product of the values represented by @var{x} and
|
|
@var{y} carried out in machine mode @var{m}.
|
|
|
|
Some machines support a multiplication that generates a product wider
|
|
than the operands. Write the pattern for this as
|
|
|
|
@example
|
|
(mult:@var{m} (sign_extend:@var{m} @var{x}) (sign_extend:@var{m} @var{y}))
|
|
@end example
|
|
|
|
where @var{m} is wider than the modes of @var{x} and @var{y}, which need
|
|
not be the same.
|
|
|
|
Write patterns for unsigned widening multiplication similarly using
|
|
@code{zero_extend}.
|
|
|
|
@findex div
|
|
@cindex division
|
|
@cindex signed division
|
|
@cindex quotient
|
|
@item (div:@var{m} @var{x} @var{y})
|
|
Represents the quotient in signed division of @var{x} by @var{y},
|
|
carried out in machine mode @var{m}. If @var{m} is a floating point
|
|
mode, it represents the exact quotient; otherwise, the integerized
|
|
quotient.
|
|
|
|
Some machines have division instructions in which the operands and
|
|
quotient widths are not all the same; you should represent
|
|
such instructions using @code{truncate} and @code{sign_extend} as in,
|
|
|
|
@example
|
|
(truncate:@var{m1} (div:@var{m2} @var{x} (sign_extend:@var{m2} @var{y})))
|
|
@end example
|
|
|
|
@findex udiv
|
|
@cindex unsigned division
|
|
@cindex division
|
|
@item (udiv:@var{m} @var{x} @var{y})
|
|
Like @code{div} but represents unsigned division.
|
|
|
|
@findex mod
|
|
@findex umod
|
|
@cindex remainder
|
|
@cindex division
|
|
@item (mod:@var{m} @var{x} @var{y})
|
|
@itemx (umod:@var{m} @var{x} @var{y})
|
|
Like @code{div} and @code{udiv} but represent the remainder instead of
|
|
the quotient.
|
|
|
|
@findex smin
|
|
@findex smax
|
|
@cindex signed minimum
|
|
@cindex signed maximum
|
|
@item (smin:@var{m} @var{x} @var{y})
|
|
@itemx (smax:@var{m} @var{x} @var{y})
|
|
Represents the smaller (for @code{smin}) or larger (for @code{smax}) of
|
|
@var{x} and @var{y}, interpreted as signed integers in mode @var{m}.
|
|
|
|
@findex umin
|
|
@findex umax
|
|
@cindex unsigned minimum and maximum
|
|
@item (umin:@var{m} @var{x} @var{y})
|
|
@itemx (umax:@var{m} @var{x} @var{y})
|
|
Like @code{smin} and @code{smax}, but the values are interpreted as unsigned
|
|
integers.
|
|
|
|
@findex not
|
|
@cindex complement, bitwise
|
|
@cindex bitwise complement
|
|
@item (not:@var{m} @var{x})
|
|
Represents the bitwise complement of the value represented by @var{x},
|
|
carried out in mode @var{m}, which must be a fixed-point machine mode.
|
|
|
|
@findex and
|
|
@cindex logical-and, bitwise
|
|
@cindex bitwise logical-and
|
|
@item (and:@var{m} @var{x} @var{y})
|
|
Represents the bitwise logical-and of the values represented by
|
|
@var{x} and @var{y}, carried out in machine mode @var{m}, which must be
|
|
a fixed-point machine mode.
|
|
|
|
@findex ior
|
|
@cindex inclusive-or, bitwise
|
|
@cindex bitwise inclusive-or
|
|
@item (ior:@var{m} @var{x} @var{y})
|
|
Represents the bitwise inclusive-or of the values represented by @var{x}
|
|
and @var{y}, carried out in machine mode @var{m}, which must be a
|
|
fixed-point mode.
|
|
|
|
@findex xor
|
|
@cindex exclusive-or, bitwise
|
|
@cindex bitwise exclusive-or
|
|
@item (xor:@var{m} @var{x} @var{y})
|
|
Represents the bitwise exclusive-or of the values represented by @var{x}
|
|
and @var{y}, carried out in machine mode @var{m}, which must be a
|
|
fixed-point mode.
|
|
|
|
@findex ashift
|
|
@cindex left shift
|
|
@cindex shift
|
|
@cindex arithmetic shift
|
|
@item (ashift:@var{m} @var{x} @var{c})
|
|
Represents the result of arithmetically shifting @var{x} left by @var{c}
|
|
places. @var{x} have mode @var{m}, a fixed-point machine mode. @var{c}
|
|
be a fixed-point mode or be a constant with mode @code{VOIDmode}; which
|
|
mode is determined by the mode called for in the machine description
|
|
entry for the left-shift instruction. For example, on the Vax, the mode
|
|
of @var{c} is @code{QImode} regardless of @var{m}.
|
|
|
|
@findex lshiftrt
|
|
@cindex right shift
|
|
@findex ashiftrt
|
|
@item (lshiftrt:@var{m} @var{x} @var{c})
|
|
@itemx (ashiftrt:@var{m} @var{x} @var{c})
|
|
Like @code{ashift} but for right shift. Unlike the case for left shift,
|
|
these two operations are distinct.
|
|
|
|
@findex rotate
|
|
@cindex rotate
|
|
@cindex left rotate
|
|
@findex rotatert
|
|
@cindex right rotate
|
|
@item (rotate:@var{m} @var{x} @var{c})
|
|
@itemx (rotatert:@var{m} @var{x} @var{c})
|
|
Similar but represent left and right rotate. If @var{c} is a constant,
|
|
use @code{rotate}.
|
|
|
|
@findex abs
|
|
@cindex absolute value
|
|
@item (abs:@var{m} @var{x})
|
|
Represents the absolute value of @var{x}, computed in mode @var{m}.
|
|
|
|
@findex sqrt
|
|
@cindex square root
|
|
@item (sqrt:@var{m} @var{x})
|
|
Represents the square root of @var{x}, computed in mode @var{m}.
|
|
Most often @var{m} will be a floating point mode.
|
|
|
|
@findex ffs
|
|
@item (ffs:@var{m} @var{x})
|
|
Represents one plus the index of the least significant 1-bit in
|
|
@var{x}, represented as an integer of mode @var{m}. (The value is
|
|
zero if @var{x} is zero.) The mode of @var{x} need not be @var{m};
|
|
depending on the target machine, various mode combinations may be
|
|
valid.
|
|
@end table
|
|
|
|
@node Comparisons, Bit Fields, Arithmetic, RTL
|
|
@section Comparison Operations
|
|
@cindex RTL comparison operations
|
|
|
|
Comparison operators test a relation on two operands and are considered
|
|
to represent a machine-dependent nonzero value described by, but not
|
|
necessarily equal to, @code{STORE_FLAG_VALUE} (@pxref{Misc})
|
|
if the relation holds, or zero if it does not. The mode of the
|
|
comparison operation is independent of the mode of the data being
|
|
compared. If the comparison operation is being tested (e.g., the first
|
|
operand of an @code{if_then_else}), the mode must be @code{VOIDmode}.
|
|
If the comparison operation is producing data to be stored in some
|
|
variable, the mode must be in class @code{MODE_INT}. All comparison
|
|
operations producing data must use the same mode, which is
|
|
machine-specific.
|
|
|
|
@cindex condition codes
|
|
There are two ways that comparison operations may be used. The
|
|
comparison operators may be used to compare the condition codes
|
|
@code{(cc0)} against zero, as in @code{(eq (cc0) (const_int 0))}. Such
|
|
a construct actually refers to the result of the preceding instruction
|
|
in which the condition codes were set. The instructing setting the
|
|
condition code must be adjacent to the instruction using the condition
|
|
code; only @code{note} insns may separate them.
|
|
|
|
Alternatively, a comparison operation may directly compare two data
|
|
objects. The mode of the comparison is determined by the operands; they
|
|
must both be valid for a common machine mode. A comparison with both
|
|
operands constant would be invalid as the machine mode could not be
|
|
deduced from it, but such a comparison should never exist in RTL due to
|
|
constant folding.
|
|
|
|
In the example above, if @code{(cc0)} were last set to
|
|
@code{(compare @var{x} @var{y})}, the comparison operation is
|
|
identical to @code{(eq @var{x} @var{y})}. Usually only one style
|
|
of comparisons is supported on a particular machine, but the combine
|
|
pass will try to merge the operations to produce the @code{eq} shown
|
|
in case it exists in the context of the particular insn involved.
|
|
|
|
Inequality comparisons come in two flavors, signed and unsigned. Thus,
|
|
there are distinct expression codes @code{gt} and @code{gtu} for signed and
|
|
unsigned greater-than. These can produce different results for the same
|
|
pair of integer values: for example, 1 is signed greater-than -1 but not
|
|
unsigned greater-than, because -1 when regarded as unsigned is actually
|
|
@code{0xffffffff} which is greater than 1.
|
|
|
|
The signed comparisons are also used for floating point values. Floating
|
|
point comparisons are distinguished by the machine modes of the operands.
|
|
|
|
@table @code
|
|
@findex eq
|
|
@cindex equal
|
|
@item (eq:@var{m} @var{x} @var{y})
|
|
1 if the values represented by @var{x} and @var{y} are equal,
|
|
otherwise 0.
|
|
|
|
@findex ne
|
|
@cindex not equal
|
|
@item (ne:@var{m} @var{x} @var{y})
|
|
1 if the values represented by @var{x} and @var{y} are not equal,
|
|
otherwise 0.
|
|
|
|
@findex gt
|
|
@cindex greater than
|
|
@item (gt:@var{m} @var{x} @var{y})
|
|
1 if the @var{x} is greater than @var{y}. If they are fixed-point,
|
|
the comparison is done in a signed sense.
|
|
|
|
@findex gtu
|
|
@cindex greater than
|
|
@cindex unsigned greater than
|
|
@item (gtu:@var{m} @var{x} @var{y})
|
|
Like @code{gt} but does unsigned comparison, on fixed-point numbers only.
|
|
|
|
@findex lt
|
|
@cindex less than
|
|
@findex ltu
|
|
@cindex unsigned less than
|
|
@item (lt:@var{m} @var{x} @var{y})
|
|
@itemx (ltu:@var{m} @var{x} @var{y})
|
|
Like @code{gt} and @code{gtu} but test for ``less than''.
|
|
|
|
@findex ge
|
|
@cindex greater than
|
|
@findex geu
|
|
@cindex unsigned greater than
|
|
@item (ge:@var{m} @var{x} @var{y})
|
|
@itemx (geu:@var{m} @var{x} @var{y})
|
|
Like @code{gt} and @code{gtu} but test for ``greater than or equal''.
|
|
|
|
@findex le
|
|
@cindex less than or equal
|
|
@findex leu
|
|
@cindex unsigned less than
|
|
@item (le:@var{m} @var{x} @var{y})
|
|
@itemx (leu:@var{m} @var{x} @var{y})
|
|
Like @code{gt} and @code{gtu} but test for ``less than or equal''.
|
|
|
|
@findex if_then_else
|
|
@item (if_then_else @var{cond} @var{then} @var{else})
|
|
This is not a comparison operation but is listed here because it is
|
|
always used in conjunction with a comparison operation. To be
|
|
precise, @var{cond} is a comparison expression. This expression
|
|
represents a choice, according to @var{cond}, between the value
|
|
represented by @var{then} and the one represented by @var{else}.
|
|
|
|
On most machines, @code{if_then_else} expressions are valid only
|
|
to express conditional jumps.
|
|
|
|
@findex cond
|
|
@item (cond [@var{test1} @var{value1} @var{test2} @var{value2} @dots{}] @var{default})
|
|
Similar to @code{if_then_else}, but more general. Each of @var{test1},
|
|
@var{test2}, @dots{} is performed in turn. The result of this expression is
|
|
the @var{value} corresponding to the first non-zero test, or @var{default} if
|
|
none of the tests are non-zero expressions.
|
|
|
|
This is currently not valid for instruction patterns and is supported only
|
|
for insn attributes. @xref{Insn Attributes}.
|
|
@end table
|
|
|
|
@node Bit Fields, Conversions, Comparisons, RTL
|
|
@section Bit Fields
|
|
@cindex bit fields
|
|
|
|
Special expression codes exist to represent bitfield instructions.
|
|
These types of expressions are lvalues in RTL; they may appear
|
|
on the left side of an assignment, indicating insertion of a value
|
|
into the specified bit field.
|
|
|
|
@table @code
|
|
@findex sign_extract
|
|
@cindex @code{BITS_BIG_ENDIAN}, effect on @code{sign_extract}
|
|
@item (sign_extract:@var{m} @var{loc} @var{size} @var{pos})
|
|
This represents a reference to a sign-extended bit field contained or
|
|
starting in @var{loc} (a memory or register reference). The bit field
|
|
is @var{size} bits wide and starts at bit @var{pos}. The compilation
|
|
option @code{BITS_BIG_ENDIAN} says which end of the memory unit
|
|
@var{pos} counts from.
|
|
|
|
If @var{loc} is in memory, its mode must be a single-byte integer mode.
|
|
If @var{loc} is in a register, the mode to use is specified by the
|
|
operand of the @code{insv} or @code{extv} pattern
|
|
(@pxref{Standard Names}) and is usually a full-word integer mode.
|
|
|
|
The mode of @var{pos} is machine-specific and is also specified
|
|
in the @code{insv} or @code{extv} pattern.
|
|
|
|
The mode @var{m} is the same as the mode that would be used for
|
|
@var{loc} if it were a register.
|
|
|
|
@findex zero_extract
|
|
@item (zero_extract:@var{m} @var{loc} @var{size} @var{pos})
|
|
Like @code{sign_extract} but refers to an unsigned or zero-extended
|
|
bit field. The same sequence of bits are extracted, but they
|
|
are filled to an entire word with zeros instead of by sign-extension.
|
|
@end table
|
|
|
|
@node Conversions, RTL Declarations, Bit Fields, RTL
|
|
@section Conversions
|
|
@cindex conversions
|
|
@cindex machine mode conversions
|
|
|
|
All conversions between machine modes must be represented by
|
|
explicit conversion operations. For example, an expression
|
|
which is the sum of a byte and a full word cannot be written as
|
|
@code{(plus:SI (reg:QI 34) (reg:SI 80))} because the @code{plus}
|
|
operation requires two operands of the same machine mode.
|
|
Therefore, the byte-sized operand is enclosed in a conversion
|
|
operation, as in
|
|
|
|
@example
|
|
(plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))
|
|
@end example
|
|
|
|
The conversion operation is not a mere placeholder, because there
|
|
may be more than one way of converting from a given starting mode
|
|
to the desired final mode. The conversion operation code says how
|
|
to do it.
|
|
|
|
For all conversion operations, @var{x} must not be @code{VOIDmode}
|
|
because the mode in which to do the conversion would not be known.
|
|
The conversion must either be done at compile-time or @var{x}
|
|
must be placed into a register.
|
|
|
|
@table @code
|
|
@findex sign_extend
|
|
@item (sign_extend:@var{m} @var{x})
|
|
Represents the result of sign-extending the value @var{x}
|
|
to machine mode @var{m}. @var{m} must be a fixed-point mode
|
|
and @var{x} a fixed-point value of a mode narrower than @var{m}.
|
|
|
|
@findex zero_extend
|
|
@item (zero_extend:@var{m} @var{x})
|
|
Represents the result of zero-extending the value @var{x}
|
|
to machine mode @var{m}. @var{m} must be a fixed-point mode
|
|
and @var{x} a fixed-point value of a mode narrower than @var{m}.
|
|
|
|
@findex float_extend
|
|
@item (float_extend:@var{m} @var{x})
|
|
Represents the result of extending the value @var{x}
|
|
to machine mode @var{m}. @var{m} must be a floating point mode
|
|
and @var{x} a floating point value of a mode narrower than @var{m}.
|
|
|
|
@findex truncate
|
|
@item (truncate:@var{m} @var{x})
|
|
Represents the result of truncating the value @var{x}
|
|
to machine mode @var{m}. @var{m} must be a fixed-point mode
|
|
and @var{x} a fixed-point value of a mode wider than @var{m}.
|
|
|
|
@findex float_truncate
|
|
@item (float_truncate:@var{m} @var{x})
|
|
Represents the result of truncating the value @var{x}
|
|
to machine mode @var{m}. @var{m} must be a floating point mode
|
|
and @var{x} a floating point value of a mode wider than @var{m}.
|
|
|
|
@findex float
|
|
@item (float:@var{m} @var{x})
|
|
Represents the result of converting fixed point value @var{x},
|
|
regarded as signed, to floating point mode @var{m}.
|
|
|
|
@findex unsigned_float
|
|
@item (unsigned_float:@var{m} @var{x})
|
|
Represents the result of converting fixed point value @var{x},
|
|
regarded as unsigned, to floating point mode @var{m}.
|
|
|
|
@findex fix
|
|
@item (fix:@var{m} @var{x})
|
|
When @var{m} is a fixed point mode, represents the result of
|
|
converting floating point value @var{x} to mode @var{m}, regarded as
|
|
signed. How rounding is done is not specified, so this operation may
|
|
be used validly in compiling C code only for integer-valued operands.
|
|
|
|
@findex unsigned_fix
|
|
@item (unsigned_fix:@var{m} @var{x})
|
|
Represents the result of converting floating point value @var{x} to
|
|
fixed point mode @var{m}, regarded as unsigned. How rounding is done
|
|
is not specified.
|
|
|
|
@findex fix
|
|
@item (fix:@var{m} @var{x})
|
|
When @var{m} is a floating point mode, represents the result of
|
|
converting floating point value @var{x} (valid for mode @var{m}) to an
|
|
integer, still represented in floating point mode @var{m}, by rounding
|
|
towards zero.
|
|
@end table
|
|
|
|
@node RTL Declarations, Side Effects, Conversions, RTL
|
|
@section Declarations
|
|
@cindex RTL declarations
|
|
@cindex declarations, RTL
|
|
|
|
Declaration expression codes do not represent arithmetic operations
|
|
but rather state assertions about their operands.
|
|
|
|
@table @code
|
|
@findex strict_low_part
|
|
@cindex @code{subreg}, in @code{strict_low_part}
|
|
@item (strict_low_part (subreg:@var{m} (reg:@var{n} @var{r}) 0))
|
|
This expression code is used in only one context: as the destination operand of a
|
|
@code{set} expression. In addition, the operand of this expression
|
|
must be a non-paradoxical @code{subreg} expression.
|
|
|
|
The presence of @code{strict_low_part} says that the part of the
|
|
register which is meaningful in mode @var{n}, but is not part of
|
|
mode @var{m}, is not to be altered. Normally, an assignment to such
|
|
a subreg is allowed to have undefined effects on the rest of the
|
|
register when @var{m} is less than a word.
|
|
@end table
|
|
|
|
@node Side Effects, Incdec, RTL Declarations, RTL
|
|
@section Side Effect Expressions
|
|
@cindex RTL side effect expressions
|
|
|
|
The expression codes described so far represent values, not actions.
|
|
But machine instructions never produce values; they are meaningful
|
|
only for their side effects on the state of the machine. Special
|
|
expression codes are used to represent side effects.
|
|
|
|
The body of an instruction is always one of these side effect codes;
|
|
the codes described above, which represent values, appear only as
|
|
the operands of these.
|
|
|
|
@table @code
|
|
@findex set
|
|
@item (set @var{lval} @var{x})
|
|
Represents the action of storing the value of @var{x} into the place
|
|
represented by @var{lval}. @var{lval} must be an expression
|
|
representing a place that can be stored in: @code{reg} (or
|
|
@code{subreg} or @code{strict_low_part}), @code{mem}, @code{pc} or
|
|
@code{cc0}.@refill
|
|
|
|
If @var{lval} is a @code{reg}, @code{subreg} or @code{mem}, it has a
|
|
machine mode; then @var{x} must be valid for that mode.@refill
|
|
|
|
If @var{lval} is a @code{reg} whose machine mode is less than the full
|
|
width of the register, then it means that the part of the register
|
|
specified by the machine mode is given the specified value and the
|
|
rest of the register receives an undefined value. Likewise, if
|
|
@var{lval} is a @code{subreg} whose machine mode is narrower than
|
|
the mode of the register, the rest of the register can be changed in
|
|
an undefined way.
|
|
|
|
If @var{lval} is a @code{strict_low_part} of a @code{subreg}, then the
|
|
part of the register specified by the machine mode of the
|
|
@code{subreg} is given the value @var{x} and the rest of the register
|
|
is not changed.@refill
|
|
|
|
If @var{lval} is @code{(cc0)}, it has no machine mode, and @var{x} may
|
|
be either a @code{compare} expression or a value that may have any mode.
|
|
The latter case represents a ``test'' instruction. The expression
|
|
@code{(set (cc0) (reg:@var{m} @var{n}))} is equivalent to
|
|
@code{(set (cc0) (compare (reg:@var{m} @var{n}) (const_int 0)))}.
|
|
Use the former expression to save space during the compilation.
|
|
|
|
@cindex jump instructions and @code{set}
|
|
@cindex @code{if_then_else} usage
|
|
If @var{lval} is @code{(pc)}, we have a jump instruction, and the
|
|
possibilities for @var{x} are very limited. It may be a
|
|
@code{label_ref} expression (unconditional jump). It may be an
|
|
@code{if_then_else} (conditional jump), in which case either the
|
|
second or the third operand must be @code{(pc)} (for the case which
|
|
does not jump) and the other of the two must be a @code{label_ref}
|
|
(for the case which does jump). @var{x} may also be a @code{mem} or
|
|
@code{(plus:SI (pc) @var{y})}, where @var{y} may be a @code{reg} or a
|
|
@code{mem}; these unusual patterns are used to represent jumps through
|
|
branch tables.@refill
|
|
|
|
If @var{lval} is neither @code{(cc0)} nor @code{(pc)}, the mode of
|
|
@var{lval} must not be @code{VOIDmode} and the mode of @var{x} must be
|
|
valid for the mode of @var{lval}.
|
|
|
|
@findex SET_DEST
|
|
@findex SET_SRC
|
|
@var{lval} is customarily accessed with the @code{SET_DEST} macro and
|
|
@var{x} with the @code{SET_SRC} macro.
|
|
|
|
@findex return
|
|
@item (return)
|
|
As the sole expression in a pattern, represents a return from the
|
|
current function, on machines where this can be done with one
|
|
instruction, such as Vaxes. On machines where a multi-instruction
|
|
``epilogue'' must be executed in order to return from the function,
|
|
returning is done by jumping to a label which precedes the epilogue, and
|
|
the @code{return} expression code is never used.
|
|
|
|
Inside an @code{if_then_else} expression, represents the value to be
|
|
placed in @code{pc} to return to the caller.
|
|
|
|
Note that an insn pattern of @code{(return)} is logically equivalent to
|
|
@code{(set (pc) (return))}, but the latter form is never used.
|
|
|
|
@findex call
|
|
@item (call @var{function} @var{nargs})
|
|
Represents a function call. @var{function} is a @code{mem} expression
|
|
whose address is the address of the function to be called.
|
|
@var{nargs} is an expression which can be used for two purposes: on
|
|
some machines it represents the number of bytes of stack argument; on
|
|
others, it represents the number of argument registers.
|
|
|
|
Each machine has a standard machine mode which @var{function} must
|
|
have. The machine description defines macro @code{FUNCTION_MODE} to
|
|
expand into the requisite mode name. The purpose of this mode is to
|
|
specify what kind of addressing is allowed, on machines where the
|
|
allowed kinds of addressing depend on the machine mode being
|
|
addressed.
|
|
|
|
@findex clobber
|
|
@item (clobber @var{x})
|
|
Represents the storing or possible storing of an unpredictable,
|
|
undescribed value into @var{x}, which must be a @code{reg},
|
|
@code{scratch} or @code{mem} expression.
|
|
|
|
One place this is used is in string instructions that store standard
|
|
values into particular hard registers. It may not be worth the
|
|
trouble to describe the values that are stored, but it is essential to
|
|
inform the compiler that the registers will be altered, lest it
|
|
attempt to keep data in them across the string instruction.
|
|
|
|
If @var{x} is @code{(mem:BLK (const_int 0))}, it means that all memory
|
|
locations must be presumed clobbered.
|
|
|
|
Note that the machine description classifies certain hard registers as
|
|
``call-clobbered''. All function call instructions are assumed by
|
|
default to clobber these registers, so there is no need to use
|
|
@code{clobber} expressions to indicate this fact. Also, each function
|
|
call is assumed to have the potential to alter any memory location,
|
|
unless the function is declared @code{const}.
|
|
|
|
If the last group of expressions in a @code{parallel} are each a
|
|
@code{clobber} expression whose arguments are @code{reg} or
|
|
@code{match_scratch} (@pxref{RTL Template}) expressions, the combiner
|
|
phase can add the appropriate @code{clobber} expressions to an insn it
|
|
has constructed when doing so will cause a pattern to be matched.
|
|
|
|
This feature can be used, for example, on a machine that whose multiply
|
|
and add instructions don't use an MQ register but which has an
|
|
add-accumulate instruction that does clobber the MQ register. Similarly,
|
|
a combined instruction might require a temporary register while the
|
|
constituent instructions might not.
|
|
|
|
When a @code{clobber} expression for a register appears inside a
|
|
@code{parallel} with other side effects, the register allocator
|
|
guarantees that the register is unoccupied both before and after that
|
|
insn. However, the reload phase may allocate a register used for one of
|
|
the inputs unless the @samp{&} constraint is specified for the selected
|
|
alternative (@pxref{Modifiers}). You can clobber either a specific hard
|
|
register, a pseudo register, or a @code{scratch} expression; in the
|
|
latter two cases, GNU CC will allocate a hard register that is available
|
|
there for use as a temporary.
|
|
|
|
For instructions that require a temporary register, you should use
|
|
@code{scratch} instead of a pseudo-register because this will allow the
|
|
combiner phase to add the @code{clobber} when required. You do this by
|
|
coding (@code{clobber} (@code{match_scratch} @dots{})). If you do
|
|
clobber a pseudo register, use one which appears nowhere else---generate
|
|
a new one each time. Otherwise, you may confuse CSE.
|
|
|
|
There is one other known use for clobbering a pseudo register in a
|
|
@code{parallel}: when one of the input operands of the insn is also
|
|
clobbered by the insn. In this case, using the same pseudo register in
|
|
the clobber and elsewhere in the insn produces the expected results.
|
|
|
|
@findex use
|
|
@item (use @var{x})
|
|
Represents the use of the value of @var{x}. It indicates that the
|
|
value in @var{x} at this point in the program is needed, even though
|
|
it may not be apparent why this is so. Therefore, the compiler will
|
|
not attempt to delete previous instructions whose only effect is to
|
|
store a value in @var{x}. @var{x} must be a @code{reg} expression.
|
|
|
|
During the delayed branch scheduling phase, @var{x} may be an insn.
|
|
This indicates that @var{x} previously was located at this place in the
|
|
code and its data dependencies need to be taken into account. These
|
|
@code{use} insns will be deleted before the delayed branch scheduling
|
|
phase exits.
|
|
|
|
@findex parallel
|
|
@item (parallel [@var{x0} @var{x1} @dots{}])
|
|
Represents several side effects performed in parallel. The square
|
|
brackets stand for a vector; the operand of @code{parallel} is a
|
|
vector of expressions. @var{x0}, @var{x1} and so on are individual
|
|
side effect expressions---expressions of code @code{set}, @code{call},
|
|
@code{return}, @code{clobber} or @code{use}.@refill
|
|
|
|
``In parallel'' means that first all the values used in the individual
|
|
side-effects are computed, and second all the actual side-effects are
|
|
performed. For example,
|
|
|
|
@example
|
|
(parallel [(set (reg:SI 1) (mem:SI (reg:SI 1)))
|
|
(set (mem:SI (reg:SI 1)) (reg:SI 1))])
|
|
@end example
|
|
|
|
@noindent
|
|
says unambiguously that the values of hard register 1 and the memory
|
|
location addressed by it are interchanged. In both places where
|
|
@code{(reg:SI 1)} appears as a memory address it refers to the value
|
|
in register 1 @emph{before} the execution of the insn.
|
|
|
|
It follows that it is @emph{incorrect} to use @code{parallel} and
|
|
expect the result of one @code{set} to be available for the next one.
|
|
For example, people sometimes attempt to represent a jump-if-zero
|
|
instruction this way:
|
|
|
|
@example
|
|
(parallel [(set (cc0) (reg:SI 34))
|
|
(set (pc) (if_then_else
|
|
(eq (cc0) (const_int 0))
|
|
(label_ref @dots{})
|
|
(pc)))])
|
|
@end example
|
|
|
|
@noindent
|
|
But this is incorrect, because it says that the jump condition depends
|
|
on the condition code value @emph{before} this instruction, not on the
|
|
new value that is set by this instruction.
|
|
|
|
@cindex peephole optimization, RTL representation
|
|
Peephole optimization, which takes place together with final assembly
|
|
code output, can produce insns whose patterns consist of a @code{parallel}
|
|
whose elements are the operands needed to output the resulting
|
|
assembler code---often @code{reg}, @code{mem} or constant expressions.
|
|
This would not be well-formed RTL at any other stage in compilation,
|
|
but it is ok then because no further optimization remains to be done.
|
|
However, the definition of the macro @code{NOTICE_UPDATE_CC}, if
|
|
any, must deal with such insns if you define any peephole optimizations.
|
|
|
|
@findex sequence
|
|
@item (sequence [@var{insns} @dots{}])
|
|
Represents a sequence of insns. Each of the @var{insns} that appears
|
|
in the vector is suitable for appearing in the chain of insns, so it
|
|
must be an @code{insn}, @code{jump_insn}, @code{call_insn},
|
|
@code{code_label}, @code{barrier} or @code{note}.
|
|
|
|
A @code{sequence} RTX is never placed in an actual insn during RTL
|
|
generation. It represents the sequence of insns that result from a
|
|
@code{define_expand} @emph{before} those insns are passed to
|
|
@code{emit_insn} to insert them in the chain of insns. When actually
|
|
inserted, the individual sub-insns are separated out and the
|
|
@code{sequence} is forgotten.
|
|
|
|
After delay-slot scheduling is completed, an insn and all the insns that
|
|
reside in its delay slots are grouped together into a @code{sequence}.
|
|
The insn requiring the delay slot is the first insn in the vector;
|
|
subsequent insns are to be placed in the delay slot.
|
|
|
|
@code{INSN_ANNULLED_BRANCH_P} is set on an insn in a delay slot to
|
|
indicate that a branch insn should be used that will conditionally annul
|
|
the effect of the insns in the delay slots. In such a case,
|
|
@code{INSN_FROM_TARGET_P} indicates that the insn is from the target of
|
|
the branch and should be executed only if the branch is taken; otherwise
|
|
the insn should be executed only if the branch is not taken.
|
|
@xref{Delay Slots}.
|
|
@end table
|
|
|
|
These expression codes appear in place of a side effect, as the body of
|
|
an insn, though strictly speaking they do not always describe side
|
|
effects as such:
|
|
|
|
@table @code
|
|
@findex asm_input
|
|
@item (asm_input @var{s})
|
|
Represents literal assembler code as described by the string @var{s}.
|
|
|
|
@findex unspec
|
|
@findex unspec_volatile
|
|
@item (unspec [@var{operands} @dots{}] @var{index})
|
|
@itemx (unspec_volatile [@var{operands} @dots{}] @var{index})
|
|
Represents a machine-specific operation on @var{operands}. @var{index}
|
|
selects between multiple machine-specific operations.
|
|
@code{unspec_volatile} is used for volatile operations and operations
|
|
that may trap; @code{unspec} is used for other operations.
|
|
|
|
These codes may appear inside a @code{pattern} of an
|
|
insn, inside a @code{parallel}, or inside an expression.
|
|
|
|
@findex addr_vec
|
|
@item (addr_vec:@var{m} [@var{lr0} @var{lr1} @dots{}])
|
|
Represents a table of jump addresses. The vector elements @var{lr0},
|
|
etc., are @code{label_ref} expressions. The mode @var{m} specifies
|
|
how much space is given to each address; normally @var{m} would be
|
|
@code{Pmode}.
|
|
|
|
@findex addr_diff_vec
|
|
@item (addr_diff_vec:@var{m} @var{base} [@var{lr0} @var{lr1} @dots{}])
|
|
Represents a table of jump addresses expressed as offsets from
|
|
@var{base}. The vector elements @var{lr0}, etc., are @code{label_ref}
|
|
expressions and so is @var{base}. The mode @var{m} specifies how much
|
|
space is given to each address-difference.@refill
|
|
@end table
|
|
|
|
@node Incdec, Assembler, Side Effects, RTL
|
|
@section Embedded Side-Effects on Addresses
|
|
@cindex RTL preincrement
|
|
@cindex RTL postincrement
|
|
@cindex RTL predecrement
|
|
@cindex RTL postdecrement
|
|
|
|
Four special side-effect expression codes appear as memory addresses.
|
|
|
|
@table @code
|
|
@findex pre_dec
|
|
@item (pre_dec:@var{m} @var{x})
|
|
Represents the side effect of decrementing @var{x} by a standard
|
|
amount and represents also the value that @var{x} has after being
|
|
decremented. @var{x} must be a @code{reg} or @code{mem}, but most
|
|
machines allow only a @code{reg}. @var{m} must be the machine mode
|
|
for pointers on the machine in use. The amount @var{x} is decremented
|
|
by is the length in bytes of the machine mode of the containing memory
|
|
reference of which this expression serves as the address. Here is an
|
|
example of its use:@refill
|
|
|
|
@example
|
|
(mem:DF (pre_dec:SI (reg:SI 39)))
|
|
@end example
|
|
|
|
@noindent
|
|
This says to decrement pseudo register 39 by the length of a @code{DFmode}
|
|
value and use the result to address a @code{DFmode} value.
|
|
|
|
@findex pre_inc
|
|
@item (pre_inc:@var{m} @var{x})
|
|
Similar, but specifies incrementing @var{x} instead of decrementing it.
|
|
|
|
@findex post_dec
|
|
@item (post_dec:@var{m} @var{x})
|
|
Represents the same side effect as @code{pre_dec} but a different
|
|
value. The value represented here is the value @var{x} has @i{before}
|
|
being decremented.
|
|
|
|
@findex post_inc
|
|
@item (post_inc:@var{m} @var{x})
|
|
Similar, but specifies incrementing @var{x} instead of decrementing it.
|
|
@end table
|
|
|
|
These embedded side effect expressions must be used with care. Instruction
|
|
patterns may not use them. Until the @samp{flow} pass of the compiler,
|
|
they may occur only to represent pushes onto the stack. The @samp{flow}
|
|
pass finds cases where registers are incremented or decremented in one
|
|
instruction and used as an address shortly before or after; these cases are
|
|
then transformed to use pre- or post-increment or -decrement.
|
|
|
|
If a register used as the operand of these expressions is used in
|
|
another address in an insn, the original value of the register is used.
|
|
Uses of the register outside of an address are not permitted within the
|
|
same insn as a use in an embedded side effect expression because such
|
|
insns behave differently on different machines and hence must be treated
|
|
as ambiguous and disallowed.
|
|
|
|
An instruction that can be represented with an embedded side effect
|
|
could also be represented using @code{parallel} containing an additional
|
|
@code{set} to describe how the address register is altered. This is not
|
|
done because machines that allow these operations at all typically
|
|
allow them wherever a memory address is called for. Describing them as
|
|
additional parallel stores would require doubling the number of entries
|
|
in the machine description.
|
|
|
|
@node Assembler, Insns, Incdec, RTL
|
|
@section Assembler Instructions as Expressions
|
|
@cindex assembler instructions in RTL
|
|
|
|
@cindex @code{asm_operands}, usage
|
|
The RTX code @code{asm_operands} represents a value produced by a
|
|
user-specified assembler instruction. It is used to represent
|
|
an @code{asm} statement with arguments. An @code{asm} statement with
|
|
a single output operand, like this:
|
|
|
|
@smallexample
|
|
asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z));
|
|
@end smallexample
|
|
|
|
@noindent
|
|
is represented using a single @code{asm_operands} RTX which represents
|
|
the value that is stored in @code{outputvar}:
|
|
|
|
@smallexample
|
|
(set @var{rtx-for-outputvar}
|
|
(asm_operands "foo %1,%2,%0" "a" 0
|
|
[@var{rtx-for-addition-result} @var{rtx-for-*z}]
|
|
[(asm_input:@var{m1} "g")
|
|
(asm_input:@var{m2} "di")]))
|
|
@end smallexample
|
|
|
|
@noindent
|
|
Here the operands of the @code{asm_operands} RTX are the assembler
|
|
template string, the output-operand's constraint, the index-number of the
|
|
output operand among the output operands specified, a vector of input
|
|
operand RTX's, and a vector of input-operand modes and constraints. The
|
|
mode @var{m1} is the mode of the sum @code{x+y}; @var{m2} is that of
|
|
@code{*z}.
|
|
|
|
When an @code{asm} statement has multiple output values, its insn has
|
|
several such @code{set} RTX's inside of a @code{parallel}. Each @code{set}
|
|
contains a @code{asm_operands}; all of these share the same assembler
|
|
template and vectors, but each contains the constraint for the respective
|
|
output operand. They are also distinguished by the output-operand index
|
|
number, which is 0, 1, @dots{} for successive output operands.
|
|
|
|
@node Insns, Calls, Assembler, RTL
|
|
@section Insns
|
|
@cindex insns
|
|
|
|
The RTL representation of the code for a function is a doubly-linked
|
|
chain of objects called @dfn{insns}. Insns are expressions with
|
|
special codes that are used for no other purpose. Some insns are
|
|
actual instructions; others represent dispatch tables for @code{switch}
|
|
statements; others represent labels to jump to or various sorts of
|
|
declarative information.
|
|
|
|
In addition to its own specific data, each insn must have a unique
|
|
id-number that distinguishes it from all other insns in the current
|
|
function (after delayed branch scheduling, copies of an insn with the
|
|
same id-number may be present in multiple places in a function, but
|
|
these copies will always be identical and will only appear inside a
|
|
@code{sequence}), and chain pointers to the preceding and following
|
|
insns. These three fields occupy the same position in every insn,
|
|
independent of the expression code of the insn. They could be accessed
|
|
with @code{XEXP} and @code{XINT}, but instead three special macros are
|
|
always used:
|
|
|
|
@table @code
|
|
@findex INSN_UID
|
|
@item INSN_UID (@var{i})
|
|
Accesses the unique id of insn @var{i}.
|
|
|
|
@findex PREV_INSN
|
|
@item PREV_INSN (@var{i})
|
|
Accesses the chain pointer to the insn preceding @var{i}.
|
|
If @var{i} is the first insn, this is a null pointer.
|
|
|
|
@findex NEXT_INSN
|
|
@item NEXT_INSN (@var{i})
|
|
Accesses the chain pointer to the insn following @var{i}.
|
|
If @var{i} is the last insn, this is a null pointer.
|
|
@end table
|
|
|
|
@findex get_insns
|
|
@findex get_last_insn
|
|
The first insn in the chain is obtained by calling @code{get_insns}; the
|
|
last insn is the result of calling @code{get_last_insn}. Within the
|
|
chain delimited by these insns, the @code{NEXT_INSN} and
|
|
@code{PREV_INSN} pointers must always correspond: if @var{insn} is not
|
|
the first insn,
|
|
|
|
@example
|
|
NEXT_INSN (PREV_INSN (@var{insn})) == @var{insn}
|
|
@end example
|
|
|
|
@noindent
|
|
is always true and if @var{insn} is not the last insn,
|
|
|
|
@example
|
|
PREV_INSN (NEXT_INSN (@var{insn})) == @var{insn}
|
|
@end example
|
|
|
|
@noindent
|
|
is always true.
|
|
|
|
After delay slot scheduling, some of the insns in the chain might be
|
|
@code{sequence} expressions, which contain a vector of insns. The value
|
|
of @code{NEXT_INSN} in all but the last of these insns is the next insn
|
|
in the vector; the value of @code{NEXT_INSN} of the last insn in the vector
|
|
is the same as the value of @code{NEXT_INSN} for the @code{sequence} in
|
|
which it is contained. Similar rules apply for @code{PREV_INSN}.
|
|
|
|
This means that the above invariants are not necessarily true for insns
|
|
inside @code{sequence} expressions. Specifically, if @var{insn} is the
|
|
first insn in a @code{sequence}, @code{NEXT_INSN (PREV_INSN (@var{insn}))}
|
|
is the insn containing the @code{sequence} expression, as is the value
|
|
of @code{PREV_INSN (NEXT_INSN (@var{insn}))} is @var{insn} is the last
|
|
insn in the @code{sequence} expression. You can use these expressions
|
|
to find the containing @code{sequence} expression.@refill
|
|
|
|
Every insn has one of the following six expression codes:
|
|
|
|
@table @code
|
|
@findex insn
|
|
@item insn
|
|
The expression code @code{insn} is used for instructions that do not jump
|
|
and do not do function calls. @code{sequence} expressions are always
|
|
contained in insns with code @code{insn} even if one of those insns
|
|
should jump or do function calls.
|
|
|
|
Insns with code @code{insn} have four additional fields beyond the three
|
|
mandatory ones listed above. These four are described in a table below.
|
|
|
|
@findex jump_insn
|
|
@item jump_insn
|
|
The expression code @code{jump_insn} is used for instructions that may
|
|
jump (or, more generally, may contain @code{label_ref} expressions). If
|
|
there is an instruction to return from the current function, it is
|
|
recorded as a @code{jump_insn}.
|
|
|
|
@findex JUMP_LABEL
|
|
@code{jump_insn} insns have the same extra fields as @code{insn} insns,
|
|
accessed in the same way and in addition contain a field
|
|
@code{JUMP_LABEL} which is defined once jump optimization has completed.
|
|
|
|
For simple conditional and unconditional jumps, this field contains the
|
|
@code{code_label} to which this insn will (possibly conditionally)
|
|
branch. In a more complex jump, @code{JUMP_LABEL} records one of the
|
|
labels that the insn refers to; the only way to find the others
|
|
is to scan the entire body of the insn.
|
|
|
|
Return insns count as jumps, but since they do not refer to any labels,
|
|
they have zero in the @code{JUMP_LABEL} field.
|
|
|
|
@findex call_insn
|
|
@item call_insn
|
|
The expression code @code{call_insn} is used for instructions that may do
|
|
function calls. It is important to distinguish these instructions because
|
|
they imply that certain registers and memory locations may be altered
|
|
unpredictably.
|
|
|
|
@findex CALL_INSN_FUNCTION_USAGE
|
|
@code{call_insn} insns have the same extra fields as @code{insn} insns,
|
|
accessed in the same way and in addition contain a field
|
|
@code{CALL_INSN_FUNCTION_USAGE}, which contains a list (chain of
|
|
@code{expr_list} expressions) containing @code{use} and @code{clobber}
|
|
expressions that denote hard registers used or clobbered by the called
|
|
function. A register specified in a @code{clobber} in this list is
|
|
modified @emph{after} the execution of the @code{call_insn}, while a
|
|
register in a @code{clobber} in the body of the @code{call_insn} is
|
|
clobbered before the insn completes execution. @code{clobber}
|
|
expressions in this list augment registers specified in
|
|
@code{CALL_USED_REGISTERS} (@pxref{Register Basics}).
|
|
|
|
@findex code_label
|
|
@findex CODE_LABEL_NUMBER
|
|
@item code_label
|
|
A @code{code_label} insn represents a label that a jump insn can jump
|
|
to. It contains two special fields of data in addition to the three
|
|
standard ones. @code{CODE_LABEL_NUMBER} is used to hold the @dfn{label
|
|
number}, a number that identifies this label uniquely among all the
|
|
labels in the compilation (not just in the current function).
|
|
Ultimately, the label is represented in the assembler output as an
|
|
assembler label, usually of the form @samp{L@var{n}} where @var{n} is
|
|
the label number.
|
|
|
|
When a @code{code_label} appears in an RTL expression, it normally
|
|
appears within a @code{label_ref} which represents the address of
|
|
the label, as a number.
|
|
|
|
@findex LABEL_NUSES
|
|
The field @code{LABEL_NUSES} is only defined once the jump optimization
|
|
phase is completed and contains the number of times this label is
|
|
referenced in the current function.
|
|
|
|
@findex barrier
|
|
@item barrier
|
|
Barriers are placed in the instruction stream when control cannot flow
|
|
past them. They are placed after unconditional jump instructions to
|
|
indicate that the jumps are unconditional and after calls to
|
|
@code{volatile} functions, which do not return (e.g., @code{exit}).
|
|
They contain no information beyond the three standard fields.
|
|
|
|
@findex note
|
|
@findex NOTE_LINE_NUMBER
|
|
@findex NOTE_SOURCE_FILE
|
|
@item note
|
|
@code{note} insns are used to represent additional debugging and
|
|
declarative information. They contain two nonstandard fields, an
|
|
integer which is accessed with the macro @code{NOTE_LINE_NUMBER} and a
|
|
string accessed with @code{NOTE_SOURCE_FILE}.
|
|
|
|
If @code{NOTE_LINE_NUMBER} is positive, the note represents the
|
|
position of a source line and @code{NOTE_SOURCE_FILE} is the source file name
|
|
that the line came from. These notes control generation of line
|
|
number data in the assembler output.
|
|
|
|
Otherwise, @code{NOTE_LINE_NUMBER} is not really a line number but a
|
|
code with one of the following values (and @code{NOTE_SOURCE_FILE}
|
|
must contain a null pointer):
|
|
|
|
@table @code
|
|
@findex NOTE_INSN_DELETED
|
|
@item NOTE_INSN_DELETED
|
|
Such a note is completely ignorable. Some passes of the compiler
|
|
delete insns by altering them into notes of this kind.
|
|
|
|
@findex NOTE_INSN_BLOCK_BEG
|
|
@findex NOTE_INSN_BLOCK_END
|
|
@item NOTE_INSN_BLOCK_BEG
|
|
@itemx NOTE_INSN_BLOCK_END
|
|
These types of notes indicate the position of the beginning and end
|
|
of a level of scoping of variable names. They control the output
|
|
of debugging information.
|
|
|
|
@findex NOTE_INSN_LOOP_BEG
|
|
@findex NOTE_INSN_LOOP_END
|
|
@item NOTE_INSN_LOOP_BEG
|
|
@itemx NOTE_INSN_LOOP_END
|
|
These types of notes indicate the position of the beginning and end
|
|
of a @code{while} or @code{for} loop. They enable the loop optimizer
|
|
to find loops quickly.
|
|
|
|
@findex NOTE_INSN_LOOP_CONT
|
|
@item NOTE_INSN_LOOP_CONT
|
|
Appears at the place in a loop that @code{continue} statements jump to.
|
|
|
|
@findex NOTE_INSN_LOOP_VTOP
|
|
@item NOTE_INSN_LOOP_VTOP
|
|
This note indicates the place in a loop where the exit test begins for
|
|
those loops in which the exit test has been duplicated. This position
|
|
becomes another virtual start of the loop when considering loop
|
|
invariants.
|
|
|
|
@findex NOTE_INSN_FUNCTION_END
|
|
@item NOTE_INSN_FUNCTION_END
|
|
Appears near the end of the function body, just before the label that
|
|
@code{return} statements jump to (on machine where a single instruction
|
|
does not suffice for returning). This note may be deleted by jump
|
|
optimization.
|
|
|
|
@findex NOTE_INSN_SETJMP
|
|
@item NOTE_INSN_SETJMP
|
|
Appears following each call to @code{setjmp} or a related function.
|
|
@end table
|
|
|
|
These codes are printed symbolically when they appear in debugging dumps.
|
|
@end table
|
|
|
|
@cindex @code{HImode}, in @code{insn}
|
|
@cindex @code{QImode}, in @code{insn}
|
|
The machine mode of an insn is normally @code{VOIDmode}, but some
|
|
phases use the mode for various purposes; for example, the reload pass
|
|
sets it to @code{HImode} if the insn needs reloading but not register
|
|
elimination and @code{QImode} if both are required. The common
|
|
subexpression elimination pass sets the mode of an insn to @code{QImode}
|
|
when it is the first insn in a block that has already been processed.
|
|
|
|
Here is a table of the extra fields of @code{insn}, @code{jump_insn}
|
|
and @code{call_insn} insns:
|
|
|
|
@table @code
|
|
@findex PATTERN
|
|
@item PATTERN (@var{i})
|
|
An expression for the side effect performed by this insn. This must be
|
|
one of the following codes: @code{set}, @code{call}, @code{use},
|
|
@code{clobber}, @code{return}, @code{asm_input}, @code{asm_output},
|
|
@code{addr_vec}, @code{addr_diff_vec}, @code{trap_if}, @code{unspec},
|
|
@code{unspec_volatile}, @code{parallel}, or @code{sequence}. If it is a @code{parallel},
|
|
each element of the @code{parallel} must be one these codes, except that
|
|
@code{parallel} expressions cannot be nested and @code{addr_vec} and
|
|
@code{addr_diff_vec} are not permitted inside a @code{parallel} expression.
|
|
|
|
@findex INSN_CODE
|
|
@item INSN_CODE (@var{i})
|
|
An integer that says which pattern in the machine description matches
|
|
this insn, or -1 if the matching has not yet been attempted.
|
|
|
|
Such matching is never attempted and this field remains -1 on an insn
|
|
whose pattern consists of a single @code{use}, @code{clobber},
|
|
@code{asm_input}, @code{addr_vec} or @code{addr_diff_vec} expression.
|
|
|
|
@findex asm_noperands
|
|
Matching is also never attempted on insns that result from an @code{asm}
|
|
statement. These contain at least one @code{asm_operands} expression.
|
|
The function @code{asm_noperands} returns a non-negative value for
|
|
such insns.
|
|
|
|
In the debugging output, this field is printed as a number followed by
|
|
a symbolic representation that locates the pattern in the @file{md}
|
|
file as some small positive or negative offset from a named pattern.
|
|
|
|
@findex LOG_LINKS
|
|
@item LOG_LINKS (@var{i})
|
|
A list (chain of @code{insn_list} expressions) giving information about
|
|
dependencies between instructions within a basic block. Neither a jump
|
|
nor a label may come between the related insns.
|
|
|
|
@findex REG_NOTES
|
|
@item REG_NOTES (@var{i})
|
|
A list (chain of @code{expr_list} and @code{insn_list} expressions)
|
|
giving miscellaneous information about the insn. It is often information
|
|
pertaining to the registers used in this insn.
|
|
@end table
|
|
|
|
The @code{LOG_LINKS} field of an insn is a chain of @code{insn_list}
|
|
expressions. Each of these has two operands: the first is an insn,
|
|
and the second is another @code{insn_list} expression (the next one in
|
|
the chain). The last @code{insn_list} in the chain has a null pointer
|
|
as second operand. The significant thing about the chain is which
|
|
insns appear in it (as first operands of @code{insn_list}
|
|
expressions). Their order is not significant.
|
|
|
|
This list is originally set up by the flow analysis pass; it is a null
|
|
pointer until then. Flow only adds links for those data dependencies
|
|
which can be used for instruction combination. For each insn, the flow
|
|
analysis pass adds a link to insns which store into registers values
|
|
that are used for the first time in this insn. The instruction
|
|
scheduling pass adds extra links so that every dependence will be
|
|
represented. Links represent data dependencies, antidependencies and
|
|
output dependencies; the machine mode of the link distinguishes these
|
|
three types: antidependencies have mode @code{REG_DEP_ANTI}, output
|
|
dependencies have mode @code{REG_DEP_OUTPUT}, and data dependencies have
|
|
mode @code{VOIDmode}.
|
|
|
|
The @code{REG_NOTES} field of an insn is a chain similar to the
|
|
@code{LOG_LINKS} field but it includes @code{expr_list} expressions in
|
|
addition to @code{insn_list} expressions. There are several kinds
|
|
of register notes, which are distinguished by the machine mode, which
|
|
in a register note is really understood as being an @code{enum reg_note}.
|
|
The first operand @var{op} of the note is data whose meaning depends on
|
|
the kind of note.
|
|
|
|
@findex REG_NOTE_KIND
|
|
@findex PUT_REG_NOTE_KIND
|
|
The macro @code{REG_NOTE_KIND (@var{x})} returns the kind of
|
|
register note. Its counterpart, the macro @code{PUT_REG_NOTE_KIND
|
|
(@var{x}, @var{newkind})} sets the register note type of @var{x} to be
|
|
@var{newkind}.
|
|
|
|
Register notes are of three classes: They may say something about an
|
|
input to an insn, they may say something about an output of an insn, or
|
|
they may create a linkage between two insns. There are also a set
|
|
of values that are only used in @code{LOG_LINKS}.
|
|
|
|
These register notes annotate inputs to an insn:
|
|
|
|
@table @code
|
|
@findex REG_DEAD
|
|
@item REG_DEAD
|
|
The value in @var{op} dies in this insn; that is to say, altering the
|
|
value immediately after this insn would not affect the future behavior
|
|
of the program.
|
|
|
|
This does not necessarily mean that the register @var{op} has no useful
|
|
value after this insn since it may also be an output of the insn. In
|
|
such a case, however, a @code{REG_DEAD} note would be redundant and is
|
|
usually not present until after the reload pass, but no code relies on
|
|
this fact.
|
|
|
|
@findex REG_INC
|
|
@item REG_INC
|
|
The register @var{op} is incremented (or decremented; at this level
|
|
there is no distinction) by an embedded side effect inside this insn.
|
|
This means it appears in a @code{post_inc}, @code{pre_inc},
|
|
@code{post_dec} or @code{pre_dec} expression.
|
|
|
|
@findex REG_NONNEG
|
|
@item REG_NONNEG
|
|
The register @var{op} is known to have a nonnegative value when this
|
|
insn is reached. This is used so that decrement and branch until zero
|
|
instructions, such as the m68k dbra, can be matched.
|
|
|
|
The @code{REG_NONNEG} note is added to insns only if the machine
|
|
description has a @samp{decrement_and_branch_until_zero} pattern.
|
|
|
|
@findex REG_NO_CONFLICT
|
|
@item REG_NO_CONFLICT
|
|
This insn does not cause a conflict between @var{op} and the item
|
|
being set by this insn even though it might appear that it does.
|
|
In other words, if the destination register and @var{op} could
|
|
otherwise be assigned the same register, this insn does not
|
|
prevent that assignment.
|
|
|
|
Insns with this note are usually part of a block that begins with a
|
|
@code{clobber} insn specifying a multi-word pseudo register (which will
|
|
be the output of the block), a group of insns that each set one word of
|
|
the value and have the @code{REG_NO_CONFLICT} note attached, and a final
|
|
insn that copies the output to itself with an attached @code{REG_EQUAL}
|
|
note giving the expression being computed. This block is encapsulated
|
|
with @code{REG_LIBCALL} and @code{REG_RETVAL} notes on the first and
|
|
last insns, respectively.
|
|
|
|
@findex REG_LABEL
|
|
@item REG_LABEL
|
|
This insn uses @var{op}, a @code{code_label}, but is not a
|
|
@code{jump_insn}. The presence of this note allows jump optimization to
|
|
be aware that @var{op} is, in fact, being used.
|
|
@end table
|
|
|
|
The following notes describe attributes of outputs of an insn:
|
|
|
|
@table @code
|
|
@findex REG_EQUIV
|
|
@findex REG_EQUAL
|
|
@item REG_EQUIV
|
|
@itemx REG_EQUAL
|
|
This note is only valid on an insn that sets only one register and
|
|
indicates that that register will be equal to @var{op} at run time; the
|
|
scope of this equivalence differs between the two types of notes. The
|
|
value which the insn explicitly copies into the register may look
|
|
different from @var{op}, but they will be equal at run time. If the
|
|
output of the single @code{set} is a @code{strict_low_part} expression,
|
|
the note refers to the register that is contained in @code{SUBREG_REG}
|
|
of the @code{subreg} expression.
|
|
|
|
For @code{REG_EQUIV}, the register is equivalent to @var{op} throughout
|
|
the entire function, and could validly be replaced in all its
|
|
occurrences by @var{op}. (``Validly'' here refers to the data flow of
|
|
the program; simple replacement may make some insns invalid.) For
|
|
example, when a constant is loaded into a register that is never
|
|
assigned any other value, this kind of note is used.
|
|
|
|
When a parameter is copied into a pseudo-register at entry to a function,
|
|
a note of this kind records that the register is equivalent to the stack
|
|
slot where the parameter was passed. Although in this case the register
|
|
may be set by other insns, it is still valid to replace the register
|
|
by the stack slot throughout the function.
|
|
|
|
In the case of @code{REG_EQUAL}, the register that is set by this insn
|
|
will be equal to @var{op} at run time at the end of this insn but not
|
|
necessarily elsewhere in the function. In this case, @var{op}
|
|
is typically an arithmetic expression. For example, when a sequence of
|
|
insns such as a library call is used to perform an arithmetic operation,
|
|
this kind of note is attached to the insn that produces or copies the
|
|
final value.
|
|
|
|
These two notes are used in different ways by the compiler passes.
|
|
@code{REG_EQUAL} is used by passes prior to register allocation (such as
|
|
common subexpression elimination and loop optimization) to tell them how
|
|
to think of that value. @code{REG_EQUIV} notes are used by register
|
|
allocation to indicate that there is an available substitute expression
|
|
(either a constant or a @code{mem} expression for the location of a
|
|
parameter on the stack) that may be used in place of a register if
|
|
insufficient registers are available.
|
|
|
|
Except for stack homes for parameters, which are indicated by a
|
|
@code{REG_EQUIV} note and are not useful to the early optimization
|
|
passes and pseudo registers that are equivalent to a memory location
|
|
throughout there entire life, which is not detected until later in
|
|
the compilation, all equivalences are initially indicated by an attached
|
|
@code{REG_EQUAL} note. In the early stages of register allocation, a
|
|
@code{REG_EQUAL} note is changed into a @code{REG_EQUIV} note if
|
|
@var{op} is a constant and the insn represents the only set of its
|
|
destination register.
|
|
|
|
Thus, compiler passes prior to register allocation need only check for
|
|
@code{REG_EQUAL} notes and passes subsequent to register allocation
|
|
need only check for @code{REG_EQUIV} notes.
|
|
|
|
@findex REG_UNUSED
|
|
@item REG_UNUSED
|
|
The register @var{op} being set by this insn will not be used in a
|
|
subsequent insn. This differs from a @code{REG_DEAD} note, which
|
|
indicates that the value in an input will not be used subsequently.
|
|
These two notes are independent; both may be present for the same
|
|
register.
|
|
|
|
@findex REG_WAS_0
|
|
@item REG_WAS_0
|
|
The single output of this insn contained zero before this insn.
|
|
@var{op} is the insn that set it to zero. You can rely on this note if
|
|
it is present and @var{op} has not been deleted or turned into a @code{note};
|
|
its absence implies nothing.
|
|
@end table
|
|
|
|
These notes describe linkages between insns. They occur in pairs: one
|
|
insn has one of a pair of notes that points to a second insn, which has
|
|
the inverse note pointing back to the first insn.
|
|
|
|
@table @code
|
|
@findex REG_RETVAL
|
|
@item REG_RETVAL
|
|
This insn copies the value of a multi-insn sequence (for example, a
|
|
library call), and @var{op} is the first insn of the sequence (for a
|
|
library call, the first insn that was generated to set up the arguments
|
|
for the library call).
|
|
|
|
Loop optimization uses this note to treat such a sequence as a single
|
|
operation for code motion purposes and flow analysis uses this note to
|
|
delete such sequences whose results are dead.
|
|
|
|
A @code{REG_EQUAL} note will also usually be attached to this insn to
|
|
provide the expression being computed by the sequence.
|
|
|
|
@findex REG_LIBCALL
|
|
@item REG_LIBCALL
|
|
This is the inverse of @code{REG_RETVAL}: it is placed on the first
|
|
insn of a multi-insn sequence, and it points to the last one.
|
|
|
|
@findex REG_CC_SETTER
|
|
@findex REG_CC_USER
|
|
@item REG_CC_SETTER
|
|
@itemx REG_CC_USER
|
|
On machines that use @code{cc0}, the insns which set and use @code{cc0}
|
|
set and use @code{cc0} are adjacent. However, when branch delay slot
|
|
filling is done, this may no longer be true. In this case a
|
|
@code{REG_CC_USER} note will be placed on the insn setting @code{cc0} to
|
|
point to the insn using @code{cc0} and a @code{REG_CC_SETTER} note will
|
|
be placed on the insn using @code{cc0} to point to the insn setting
|
|
@code{cc0}.@refill
|
|
@end table
|
|
|
|
These values are only used in the @code{LOG_LINKS} field, and indicate
|
|
the type of dependency that each link represents. Links which indicate
|
|
a data dependence (a read after write dependence) do not use any code,
|
|
they simply have mode @code{VOIDmode}, and are printed without any
|
|
descriptive text.
|
|
|
|
@table @code
|
|
@findex REG_DEP_ANTI
|
|
@item REG_DEP_ANTI
|
|
This indicates an anti dependence (a write after read dependence).
|
|
|
|
@findex REG_DEP_OUTPUT
|
|
@item REG_DEP_OUTPUT
|
|
This indicates an output dependence (a write after write dependence).
|
|
@end table
|
|
|
|
For convenience, the machine mode in an @code{insn_list} or
|
|
@code{expr_list} is printed using these symbolic codes in debugging dumps.
|
|
|
|
@findex insn_list
|
|
@findex expr_list
|
|
The only difference between the expression codes @code{insn_list} and
|
|
@code{expr_list} is that the first operand of an @code{insn_list} is
|
|
assumed to be an insn and is printed in debugging dumps as the insn's
|
|
unique id; the first operand of an @code{expr_list} is printed in the
|
|
ordinary way as an expression.
|
|
|
|
@node Calls, Sharing, Insns, RTL
|
|
@section RTL Representation of Function-Call Insns
|
|
@cindex calling functions in RTL
|
|
@cindex RTL function-call insns
|
|
@cindex function-call insns
|
|
|
|
Insns that call subroutines have the RTL expression code @code{call_insn}.
|
|
These insns must satisfy special rules, and their bodies must use a special
|
|
RTL expression code, @code{call}.
|
|
|
|
@cindex @code{call} usage
|
|
A @code{call} expression has two operands, as follows:
|
|
|
|
@example
|
|
(call (mem:@var{fm} @var{addr}) @var{nbytes})
|
|
@end example
|
|
|
|
@noindent
|
|
Here @var{nbytes} is an operand that represents the number of bytes of
|
|
argument data being passed to the subroutine, @var{fm} is a machine mode
|
|
(which must equal as the definition of the @code{FUNCTION_MODE} macro in
|
|
the machine description) and @var{addr} represents the address of the
|
|
subroutine.
|
|
|
|
For a subroutine that returns no value, the @code{call} expression as
|
|
shown above is the entire body of the insn, except that the insn might
|
|
also contain @code{use} or @code{clobber} expressions.
|
|
|
|
@cindex @code{BLKmode}, and function return values
|
|
For a subroutine that returns a value whose mode is not @code{BLKmode},
|
|
the value is returned in a hard register. If this register's number is
|
|
@var{r}, then the body of the call insn looks like this:
|
|
|
|
@example
|
|
(set (reg:@var{m} @var{r})
|
|
(call (mem:@var{fm} @var{addr}) @var{nbytes}))
|
|
@end example
|
|
|
|
@noindent
|
|
This RTL expression makes it clear (to the optimizer passes) that the
|
|
appropriate register receives a useful value in this insn.
|
|
|
|
When a subroutine returns a @code{BLKmode} value, it is handled by
|
|
passing to the subroutine the address of a place to store the value.
|
|
So the call insn itself does not ``return'' any value, and it has the
|
|
same RTL form as a call that returns nothing.
|
|
|
|
On some machines, the call instruction itself clobbers some register,
|
|
for example to contain the return address. @code{call_insn} insns
|
|
on these machines should have a body which is a @code{parallel}
|
|
that contains both the @code{call} expression and @code{clobber}
|
|
expressions that indicate which registers are destroyed. Similarly,
|
|
if the call instruction requires some register other than the stack
|
|
pointer that is not explicitly mentioned it its RTL, a @code{use}
|
|
subexpression should mention that register.
|
|
|
|
Functions that are called are assumed to modify all registers listed in
|
|
the configuration macro @code{CALL_USED_REGISTERS} (@pxref{Register
|
|
Basics}) and, with the exception of @code{const} functions and library
|
|
calls, to modify all of memory.
|
|
|
|
Insns containing just @code{use} expressions directly precede the
|
|
@code{call_insn} insn to indicate which registers contain inputs to the
|
|
function. Similarly, if registers other than those in
|
|
@code{CALL_USED_REGISTERS} are clobbered by the called function, insns
|
|
containing a single @code{clobber} follow immediately after the call to
|
|
indicate which registers.
|
|
|
|
@node Sharing
|
|
@section Structure Sharing Assumptions
|
|
@cindex sharing of RTL components
|
|
@cindex RTL structure sharing assumptions
|
|
|
|
The compiler assumes that certain kinds of RTL expressions are unique;
|
|
there do not exist two distinct objects representing the same value.
|
|
In other cases, it makes an opposite assumption: that no RTL expression
|
|
object of a certain kind appears in more than one place in the
|
|
containing structure.
|
|
|
|
These assumptions refer to a single function; except for the RTL
|
|
objects that describe global variables and external functions,
|
|
and a few standard objects such as small integer constants,
|
|
no RTL objects are common to two functions.
|
|
|
|
@itemize @bullet
|
|
@cindex @code{reg}, RTL sharing
|
|
@item
|
|
Each pseudo-register has only a single @code{reg} object to represent it,
|
|
and therefore only a single machine mode.
|
|
|
|
@cindex symbolic label
|
|
@cindex @code{symbol_ref}, RTL sharing
|
|
@item
|
|
For any symbolic label, there is only one @code{symbol_ref} object
|
|
referring to it.
|
|
|
|
@cindex @code{const_int}, RTL sharing
|
|
@item
|
|
There is only one @code{const_int} expression with value 0, only
|
|
one with value 1, and only one with value @minus{}1.
|
|
Some other integer values are also stored uniquely.
|
|
|
|
@cindex @code{pc}, RTL sharing
|
|
@item
|
|
There is only one @code{pc} expression.
|
|
|
|
@cindex @code{cc0}, RTL sharing
|
|
@item
|
|
There is only one @code{cc0} expression.
|
|
|
|
@cindex @code{const_double}, RTL sharing
|
|
@item
|
|
There is only one @code{const_double} expression with value 0 for
|
|
each floating point mode. Likewise for values 1 and 2.
|
|
|
|
@cindex @code{label_ref}, RTL sharing
|
|
@cindex @code{scratch}, RTL sharing
|
|
@item
|
|
No @code{label_ref} or @code{scratch} appears in more than one place in
|
|
the RTL structure; in other words, it is safe to do a tree-walk of all
|
|
the insns in the function and assume that each time a @code{label_ref}
|
|
or @code{scratch} is seen it is distinct from all others that are seen.
|
|
|
|
@cindex @code{mem}, RTL sharing
|
|
@item
|
|
Only one @code{mem} object is normally created for each static
|
|
variable or stack slot, so these objects are frequently shared in all
|
|
the places they appear. However, separate but equal objects for these
|
|
variables are occasionally made.
|
|
|
|
@cindex @code{asm_operands}, RTL sharing
|
|
@item
|
|
When a single @code{asm} statement has multiple output operands, a
|
|
distinct @code{asm_operands} expression is made for each output operand.
|
|
However, these all share the vector which contains the sequence of input
|
|
operands. This sharing is used later on to test whether two
|
|
@code{asm_operands} expressions come from the same statement, so all
|
|
optimizations must carefully preserve the sharing if they copy the
|
|
vector at all.
|
|
|
|
@item
|
|
No RTL object appears in more than one place in the RTL structure
|
|
except as described above. Many passes of the compiler rely on this
|
|
by assuming that they can modify RTL objects in place without unwanted
|
|
side-effects on other insns.
|
|
|
|
@findex unshare_all_rtl
|
|
@item
|
|
During initial RTL generation, shared structure is freely introduced.
|
|
After all the RTL for a function has been generated, all shared
|
|
structure is copied by @code{unshare_all_rtl} in @file{emit-rtl.c},
|
|
after which the above rules are guaranteed to be followed.
|
|
|
|
@findex copy_rtx_if_shared
|
|
@item
|
|
During the combiner pass, shared structure within an insn can exist
|
|
temporarily. However, the shared structure is copied before the
|
|
combiner is finished with the insn. This is done by calling
|
|
@code{copy_rtx_if_shared}, which is a subroutine of
|
|
@code{unshare_all_rtl}.
|
|
@end itemize
|
|
|
|
@node Reading RTL
|
|
@section Reading RTL
|
|
|
|
To read an RTL object from a file, call @code{read_rtx}. It takes one
|
|
argument, a stdio stream, and returns a single RTL object.
|
|
|
|
Reading RTL from a file is very slow. This is not currently a
|
|
problem since reading RTL occurs only as part of building the
|
|
compiler.
|
|
|
|
People frequently have the idea of using RTL stored as text in a file as
|
|
an interface between a language front end and the bulk of GNU CC. This
|
|
idea is not feasible.
|
|
|
|
GNU CC was designed to use RTL internally only. Correct RTL for a given
|
|
program is very dependent on the particular target machine. And the RTL
|
|
does not contain all the information about the program.
|
|
|
|
The proper way to interface GNU CC to a new language front end is with
|
|
the ``tree'' data structure. There is no manual for this data
|
|
structure, but it is described in the files @file{tree.h} and
|
|
@file{tree.def}.
|