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Code Issues Releases Activity

Avoid raising unexpected floating point exceptions in libm

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When using clang with x86_64 CPUs that support AVX, some floating point
transformations may raise exceptions that would not have been raised by
the original code. To avoid this, use the -fp-exception-behavior=maytrap
flag, introduced in clang 10.0.0.

In particular, this fixes a number of test failures with ctanhf(3) and
ctanf(3), when libm is compiled with -mavx. An unexpected FE_INVALID
exception is then raised, because clang emits vdivps instructions to
perform certain divides. (The vdivps instruction operates on multiple
single-precision float operands simultaneously, but the exceptions may
be influenced by unused parts of the XMM registers. In this particular
case, it was calculating 0 / 0, which results in FE_INVALID.)

If -fp-exception-behavior=maytrap is specified however, clang uses
vdivss instructions instead, which work on one operand, and should not
raise unexpected exceptions.

Reported by:	olivier
Reviewed by:	arichardson
PR:		254911
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D29686
This commit is contained in:
Dimitry Andric 2021-04-10 01:07:54 +02:00
parent 44faff9f4a
commit 3b00222f15
1 changed files with 10 additions and 1 deletions
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11
lib/msun/Makefile
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@ -34,6 +34,16 @@ CFLAGS+= -I${.CURDIR}/ld128
CFLAGS+= -I${.CURDIR}/${ARCH_SUBDIR}
.include <bsd.compiler.mk>
.if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 100000
# When using clang with x86_64 CPUs that support AVX, some floating point
# transformations may raise exceptions that would not have been raised by the
# original code. To avoid this, use the -fp-exception-behavior=maytrap flag,
# introduced in clang 10.0.0.
# See also: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254911
CFLAGS+= -ffp-exception-behavior=maytrap
.endif
.PATH: ${.CURDIR}/bsdsrc
.PATH: ${.CURDIR}/src
.PATH: ${.CURDIR}/man
@ -113,7 +123,6 @@ COMMON_SRCS+= catrigl.c \
# 'long double' [-Werror=overflow]
# if( y >= LDBL_MAX )
# See also: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=130067
.include <bsd.compiler.mk>
.if ${COMPILER_TYPE} == "gcc"
CFLAGS.e_powl.c+= -Wno-error=overflow
.endif