[gcc r16-7357] libstdc++: Prevent std::stacktrace from using atomics on arm-eabi [PR120567]

[Jonathan Wakely]

https://gcc.gnu.org/g:4046caad61a0406e42cae0c38a6aa26f5975a197

commit r16-7357-g4046caad61a0406e42cae0c38a6aa26f5975a197
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Dec 9 21:02:56 2025 +0000

    libstdc++: Prevent std::stacktrace from using atomics on arm-eabi [PR120567]
    
    The conftest.cc in GLIBCXX_ENABLE_BACKTRACE checks if calls to
    __atomic_load and __atomic_store result in libatomic calls. For arm-eabi
    those expand to normal loads with a call to __sync_synchronize to ensure
    correct ordering. That means the generated assembly does not match the
    grep pattern "__atomic_" and so configure incorrectly assumes we can use
    atomics in libbacktrace.
    
    The fix is to grep for "__atomic_|__sync_" instead of just "__atomic_".
    
    As explained in the bug comments, the similar grep for "__atomic_" in
    the GLIBCXX_ENABLE_ATOMIC_BUILTINS macro doesn't need the same change.
    That conftest.cc program does emit a call to a libatomic function that
    matches the grep pattern "__atomic_" so the test gives the right answer
    for arm-eabi.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/120567
            * acinclude.m4 (GLIBCXX_ENABLE_BACKTRACE): Include "__sync_" in
            grep command to check for extern calls to libatomic.
            * configure: Regenerate.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/acinclude.m4 | 2 +-
 libstdc++-v3/configure    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 38030e432d81..8dc9e17b214c 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -5318,7 +5318,7 @@ EOF
 
     AC_MSG_CHECKING([for atomic builtins for libbacktrace])
     if AC_TRY_EVAL(ac_compile); then
-      if grep __atomic_ conftest.s >/dev/null 2>&1 ; then
+      if grep -E '__atomic_|__sync_' conftest.s >/dev/null 2>&1 ; then
 	glibcxx_cv_libbacktrace_atomics=no
       else
 	glibcxx_cv_libbacktrace_atomics=yes
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index 4f144382e1b1..6713e4504b1c 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -54172,7 +54172,7 @@ $as_echo_n "checking for atomic builtins for libbacktrace... " >&6; }
   ac_status=$?
   $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
   test $ac_status = 0; }; then
-      if grep __atomic_ conftest.s >/dev/null 2>&1 ; then
+      if grep -E '__atomic_|__sync_' conftest.s >/dev/null 2>&1 ; then
 	glibcxx_cv_libbacktrace_atomics=no
       else
 	glibcxx_cv_libbacktrace_atomics=yes