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Fix GC bugs --with-wide-int and Qnil == 0
Use the same alignment for the !USE_LSB_TAG case as for the more-typical USE_LSB_TAG case. The attempt to support arbitrary alignments with !USE_LSB_TAG had subtle bugs in garbage collection once we changed the representation of symbols so that Qnil == 0. Problem reported by Eli Zaretskii (Bug#20862). * src/alloc.c (XMALLOC_HEADER_ALIGNMENT) [XMALLOC_OVERRUN_CHECK]: * src/alloc.c (vector_alignment, union aligned_Lisp_Symbol) (union aligned_Lisp_Misc, maybe_lisp_pointer, pure_alloc): Use same alignment for !USE_LSB_TAG as for USE_LSB_TAG. * src/alloc.c (POINTERS_MIGHT_HIDE_IN_OBJECTS): Remove. This optimization in the !USE_LSB_TAG case is no longer valid when symbols are represented via offsets. Change the only use to assume that pointers might hide in objects. * src/lisp.h (alignas) [!USE_LSB_TAG]: Require support in this case, too. (TAG_SYMOFFSET, XSYMBOL) [!USE_LSB_TAG]: Do not shift the offset. This is OK, because the !USE_LSB_TAG case now applies only when Lisp_Object is wider than void *, so there's no longer any need to shift the offset. Not shifting the offset means that symbol representations have the same alignment as pointers, which the GC assumes.
This commit is contained in:
Paul Eggert
parent
f230b2ff31
commit
93f4f67ba9
53
src/alloc.c
53
src/alloc.c
@ -528,12 +528,8 @@ buffer_memory_full (ptrdiff_t nbytes)
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alignment that Emacs needs for C types and for USE_LSB_TAG. */
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#define XMALLOC_BASE_ALIGNMENT alignof (max_align_t)
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#if USE_LSB_TAG
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#define XMALLOC_HEADER_ALIGNMENT \
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COMMON_MULTIPLE (GCALIGNMENT, XMALLOC_BASE_ALIGNMENT)
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#else
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# define XMALLOC_HEADER_ALIGNMENT XMALLOC_BASE_ALIGNMENT
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#endif
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#define XMALLOC_OVERRUN_SIZE_SIZE \
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(((XMALLOC_OVERRUN_CHECK_SIZE + sizeof (size_t) \
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+ XMALLOC_HEADER_ALIGNMENT - 1) \
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@ -2730,7 +2726,7 @@ enum
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{
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/* Alignment of struct Lisp_Vector objects. */
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vector_alignment = COMMON_MULTIPLE (ALIGNOF_STRUCT_LISP_VECTOR,
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USE_LSB_TAG ? GCALIGNMENT : 1),
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GCALIGNMENT),
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/* Vector size requests are a multiple of this. */
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roundup_size = COMMON_MULTIPLE (vector_alignment, word_size)
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@ -3299,15 +3295,13 @@ usage: (make-byte-code ARGLIST BYTE-CODE CONSTANTS DEPTH &optional DOCSTRING INT
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***********************************************************************/
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/* Like struct Lisp_Symbol, but padded so that the size is a multiple
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of the required alignment if LSB tags are used. */
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of the required alignment. */
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union aligned_Lisp_Symbol
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{
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struct Lisp_Symbol s;
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#if USE_LSB_TAG
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unsigned char c[(sizeof (struct Lisp_Symbol) + GCALIGNMENT - 1)
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& -GCALIGNMENT];
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#endif
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};
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/* Each symbol_block is just under 1020 bytes long, since malloc
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@ -3411,15 +3405,13 @@ Its value is void, and its function definition and property list are nil. */)
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***********************************************************************/
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/* Like union Lisp_Misc, but padded so that its size is a multiple of
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the required alignment when LSB tags are used. */
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the required alignment. */
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union aligned_Lisp_Misc
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{
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union Lisp_Misc m;
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#if USE_LSB_TAG
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unsigned char c[(sizeof (union Lisp_Misc) + GCALIGNMENT - 1)
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& -GCALIGNMENT];
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#endif
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};
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/* Allocation of markers and other objects that share that structure.
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@ -4628,13 +4620,13 @@ mark_maybe_object (Lisp_Object obj)
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}
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/* Return true if P can point to Lisp data, and false otherwise.
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USE_LSB_TAG needs Lisp data to be aligned on multiples of GCALIGNMENT.
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Otherwise, assume that Lisp data is aligned on even addresses. */
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Symbols are implemented via offsets not pointers, but the offsets
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are also multiples of GCALIGNMENT. */
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static bool
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maybe_lisp_pointer (void *p)
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{
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return !((intptr_t) p % (USE_LSB_TAG ? GCALIGNMENT : 2));
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return (uintptr_t) p % GCALIGNMENT == 0;
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}
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/* If P points to Lisp data, mark that as live if it isn't already
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@ -4722,27 +4714,6 @@ mark_maybe_pointer (void *p)
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miss objects if __alignof__ were used. */
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#define GC_POINTER_ALIGNMENT alignof (void *)
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/* Define POINTERS_MIGHT_HIDE_IN_OBJECTS to 1 if marking via C pointers does
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not suffice, which is the typical case. A host where a Lisp_Object is
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wider than a pointer might allocate a Lisp_Object in non-adjacent halves.
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If USE_LSB_TAG, the bottom half is not a valid pointer, but it should
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suffice to widen it to to a Lisp_Object and check it that way. */
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#if USE_LSB_TAG || VAL_MAX < UINTPTR_MAX
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# if !USE_LSB_TAG && VAL_MAX < UINTPTR_MAX >> GCTYPEBITS
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/* If tag bits straddle pointer-word boundaries, neither mark_maybe_pointer
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nor mark_maybe_object can follow the pointers. This should not occur on
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any practical porting target. */
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# error "MSB type bits straddle pointer-word boundaries"
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# endif
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/* Marking via C pointers does not suffice, because Lisp_Objects contain
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pointer words that hold pointers ORed with type bits. */
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# define POINTERS_MIGHT_HIDE_IN_OBJECTS 1
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#else
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/* Marking via C pointers suffices, because Lisp_Objects contain pointer
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words that hold unmodified pointers. */
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# define POINTERS_MIGHT_HIDE_IN_OBJECTS 0
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#endif
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/* Mark Lisp objects referenced from the address range START+OFFSET..END
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or END+OFFSET..START. */
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@ -4788,7 +4759,6 @@ mark_memory (void *start, void *end)
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{
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void *p = *(void **) ((char *) pp + i);
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mark_maybe_pointer (p);
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if (POINTERS_MIGHT_HIDE_IN_OBJECTS)
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mark_maybe_object (XIL ((intptr_t) p));
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}
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}
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@ -5148,22 +5118,13 @@ static void *
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pure_alloc (size_t size, int type)
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{
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void *result;
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#if USE_LSB_TAG
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size_t alignment = GCALIGNMENT;
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#else
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size_t alignment = alignof (EMACS_INT);
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/* Give Lisp_Floats an extra alignment. */
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if (type == Lisp_Float)
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alignment = alignof (struct Lisp_Float);
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#endif
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again:
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if (type >= 0)
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{
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/* Allocate space for a Lisp object from the beginning of the free
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space with taking account of alignment. */
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result = ALIGN (purebeg + pure_bytes_used_lisp, alignment);
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result = ALIGN (purebeg + pure_bytes_used_lisp, GCALIGNMENT);
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pure_bytes_used_lisp = ((char *)result - (char *)purebeg) + size;
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}
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else
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11
src/lisp.h
11
src/lisp.h
@ -278,10 +278,7 @@ error !;
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#endif
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#ifndef alignas
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# define alignas(alignment) /* empty */
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# if USE_LSB_TAG
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# error "USE_LSB_TAG requires alignas"
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# endif
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# error "alignas not defined"
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#endif
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#ifdef HAVE_STRUCT_ATTRIBUTE_ALIGNED
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@ -731,9 +728,7 @@ struct Lisp_Symbol
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/* Yield an integer that contains a symbol tag along with OFFSET.
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OFFSET should be the offset in bytes from 'lispsym' to the symbol. */
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#define TAG_SYMOFFSET(offset) \
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TAG_PTR (Lisp_Symbol, \
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((uintptr_t) (offset) >> (USE_LSB_TAG ? 0 : GCTYPEBITS)))
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#define TAG_SYMOFFSET(offset) TAG_PTR (Lisp_Symbol, offset)
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/* XLI_BUILTIN_LISPSYM (iQwhatever) is equivalent to
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XLI (builtin_lisp_symbol (Qwhatever)),
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@ -899,8 +894,6 @@ INLINE struct Lisp_Symbol *
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XSYMBOL (Lisp_Object a)
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{
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uintptr_t i = (uintptr_t) XUNTAG (a, Lisp_Symbol);
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if (! USE_LSB_TAG)
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i <<= GCTYPEBITS;
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void *p = (char *) lispsym + i;
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return p;
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}
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