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Work around GCC bug 58416 on 32-bit x86

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* configure.ac (C_SWITCH_MATCHINE): On 32-bit x86 with GCC 4+,
append -mfpmath=sse (if SSE2 is known to work) or -fno-tree-sra
(otherwise) to work around GCC bug 58416.
* etc/NEWS: Mention this.
This commit is contained in:
Paul Eggert 2024-07-19 13:39:21 -07:00
parent 524e9d50a7
commit 9f4fc66082
2 changed files with 51 additions and 0 deletions
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45
configure.ac
View file

@ -2333,6 +2333,51 @@ case $canonical in
fi
;;
esac
AC_CACHE_CHECK([for flags to work around GCC bug 58416],
[emacs_cv_gcc_bug_58416_CFLAGS],
[emacs_cv_gcc_bug_58416_CFLAGS='none needed'
AS_CASE([$canonical],
[[i[3456]86-* | x86_64-*]],
[AS_IF([test "$GCC" = yes],
[old_CFLAGS=$CFLAGS
# If no flags are needed (e.g., not GCC 4+), don't use any.
# Otherwise, use -mfpmath=sse if already assuming SSE2.
# Otherwise, use -fno-tree-sra.
for emacs_cv_gcc_bug_58416_CFLAGS in \
'none needed' -mfpmath=sse -fno-tree-sra
do
AS_CASE([$emacs_cv_gcc_bug_58416_CFLAGS],
['none needed'], [],
[-fno-tree-sra], [break],
[CFLAGS="$old_CFLAGS $emacs_cv_gcc_bug_58416_CFLAGS"])
AC_COMPILE_IFELSE(
[AC_LANG_DEFINES_PROVIDED
[/* Work around GCC bug with double in unions on x86,
where the generated insns copy non-floating-point data
via fldl/fstpl instruction pairs. This can misbehave
the data's bit pattern looks like a NaN. See, e.g.:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58416#c10
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71460
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93271
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659
Problem observed with 'gcc -m32' with GCC 14.1.1
20240607 (Red Hat 14.1.1-5) on x86-64. */
#include <float.h>
#if \
(4 <= __GNUC__ && !defined __clang__ \
&& (defined __i386__ || defined __x86_64__) \
&& ! (0 <= FLT_EVAL_METHOD && FLT_EVAL_METHOD <= 1))
# error "GCC bug 58416 is possibly present"
#endif
]],
[break])
done
CFLAGS=$old_CFLAGS])])])
AS_CASE([$emacs_cv_gcc_bug_58416_CFLAGS],
[-*],
[C_SWITCH_MACHINE="$C_SWITCH_MACHINE $emacs_cv_gcc_bug_58416_CFLAGS"])
AC_SUBST([C_SWITCH_MACHINE])
C_SWITCH_SYSTEM=

6
etc/NEWS
View file

@ -24,6 +24,12 @@ applies, and please also update docstrings as needed.
* Installation Changes in Emacs 31.1
** When using GCC 4 or later to build Emacs on 32-bit x86 systems,
'configure' now defaults to using the GCC options -mfpmath=sse (if the
host system supports SSE2) or -fno-tree-sra (if not). These GCC options
work around GCC bug 58416, which can cause Emacs to behave incorrectly
in rare cases.
* Startup Changes in Emacs 31.1