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Work around GCC bug 117423

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Problem reported by Oliver Reiter and Pip Cet (Emacs bug#78473).
It looks like GCC tree-sra is trouble in general with how Emacs
uses unions, so disable tree-sra until the GCC bug is fixed.
* configure.ac (C_SWITCH_MACHINE): Also work around GCC bug 117423.
This commit is contained in:
Paul Eggert 2025-05-21 10:07:04 -07:00
parent 8605221ef6
commit 9dfc605654
1 changed files with 43 additions and 42 deletions
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85
configure.ac
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@ -2209,49 +2209,50 @@ esac
C_SWITCH_MACHINE=
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 $emacs_cv_gcc_bug_58416_CFLAGS"])
AC_COMPILE_IFELSE(
[AC_LANG_DEFINES_PROVIDED
[/* Work around GCC bug with double in unions on x86,
where the generated insns copy non-floating-point data
via fldl/fstpl instruction pairs. This can misbehave if
the data's bit pattern looks like a NaN. See, e.g.:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58416#c10
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71460
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93271
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659
Problem observed with 'gcc -m32' with GCC 14.1.1
20240607 (Red Hat 14.1.1-5) on x86-64. */
#include <float.h>
#if \
(4 <= __GNUC__ && __GNUC__ <= 14 && !defined __clang__ \
&& (defined __i386__ || defined __x86_64__) \
&& ! (0 <= FLT_EVAL_METHOD && FLT_EVAL_METHOD <= 1))
# error "GCC bug 58416 is possibly present"
#endif
]],
[break])
done
CFLAGS=$old_CFLAGS])])])
AS_CASE([$emacs_cv_gcc_bug_58416_CFLAGS],
AC_CACHE_CHECK([for flag to work around GCC bug 117423],
[emacs_cv_gcc_bug_117423_CFLAGS],
[emacs_cv_gcc_bug_117423_CFLAGS='none needed'
AS_IF([test "$GCC" = yes],
[old_CFLAGS=$CFLAGS
# If no flags are needed (e.g., not GCC 4+), don't use any.
# Otherwise, use -fno-tree-sra.
for emacs_cv_gcc_bug_117423_CFLAGS in \
'none needed' -fno-tree-sra
do
AS_CASE([$emacs_cv_gcc_bug_117423_CFLAGS],
['none needed'], [],
[-fno-tree-sra], [break],
[CFLAGS="$old_CFLAGS $emacs_cv_gcc_bug_117423_CFLAGS"])
AC_COMPILE_IFELSE(
[AC_LANG_DEFINES_PROVIDED
[/* Work around GCC bug 117423 with unions containing holes:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117423
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119085
GCC bug 117423 is present even in GCC 15.1,
the current version as of this writing; for now,
assume it is present in all GCC versions starting with GCC 4.
Working wround GCC bug 117423 also works around GCC bug 58416
with double in unions on x86, where the generated insns
copy non-floating-point data via fldl/fstpl instruction pairs.
This can misbehave if the data's bit pattern looks like a NaN.
See, e.g.:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58416#c10
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71460
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93271
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659
Although GCC bug 58416 is fixed in GCC 15.1,
GCC bug 117423 is still present there. */
#if 4 <= __GNUC__ && !defined __clang__
# error "GCC bug 117423 possibly present"
#endif
]],
[break])
done
CFLAGS=$old_CFLAGS])])
AS_CASE([$emacs_cv_gcc_bug_117423_CFLAGS],
[-*],
[C_SWITCH_MACHINE="$C_SWITCH_MACHINE $emacs_cv_gcc_bug_58416_CFLAGS"])
[C_SWITCH_MACHINE="$C_SWITCH_MACHINE $emacs_cv_gcc_bug_117423_CFLAGS"])
AC_SUBST([C_SWITCH_MACHINE])