Work around GCC bug 58416 on 32-bit x86

* 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.

2 files changed, 51 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index b6acdf2e456..67da852667d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -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=
diff --git a/etc/NEWS b/etc/NEWS
index 5429db1dded..0e13f471c74 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -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