From 20f7c98c9e3df2462ad6a4d5eeda3ac4362d0082 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Tue, 14 Apr 2026 15:15:43 +0200
Subject: [PATCH] gh-148535: Don't use gcc -fprofile-update=atomic flag on i686

The -fprofile-update=atomic flag was added to fix a random GCC
internal error on PGO build (gh-145801) caused by corruption of
profile data (.gcda files). The problem is that it makes the PGO
build way slower (up to 47x slower) on i686. Since the GCC internal
error was not seen on i686 so far, don't use -fprofile-update=atomic
on i686.
---
 ...-04-14-15-20-29.gh-issue-148535.JjKiaa.rst |  6 ++
 configure                                     | 59 ++++++++++++++++++-
 configure.ac                                  | 30 ++++++++--
 3 files changed, 88 insertions(+), 7 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Build/2026-04-14-15-20-29.gh-issue-148535.JjKiaa.rst

diff --git a/Misc/NEWS.d/next/Build/2026-04-14-15-20-29.gh-issue-148535.JjKiaa.rst b/Misc/NEWS.d/next/Build/2026-04-14-15-20-29.gh-issue-148535.JjKiaa.rst
new file mode 100644
index 00000000000000..39f37acb14e098
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2026-04-14-15-20-29.gh-issue-148535.JjKiaa.rst
@@ -0,0 +1,6 @@
+No longer use the ``gcc -fprofile-update=atomic`` flag on i686. The flag has
+been added to fix a random GCC internal error on PGO build (:gh:`145801`)
+caused by corruption of profile data (.gcda files). The problem is that it
+makes the PGO build way slower (up to 47x slower) on i686. Since the GCC
+internal error was not seen on i686 so far, don't use
+``-fprofile-update=atomic`` on i686 anymore. Patch by Victor Stinner.
diff --git a/configure b/configure
index 07c16a4e37874e..6ef8c193d4b209 100755
--- a/configure
+++ b/configure
@@ -9092,7 +9092,57 @@ case "$ac_cv_cc_name" in
     fi
     ;;
   gcc)
-    { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether C compiler accepts -fprofile-update=atomic" >&5
+    # Check for 32-bit x86 ISA
+    { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for i686" >&5
+printf %s "checking for i686... " >&6; }
+if test ${ac_cv_i686+y}
+then :
+  printf %s "(cached) " >&6
+else case e in #(
+  e)
+        cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+
+                #ifdef __i386__
+                #  error "i386"
+                #endif
+
+int
+main (void)
+{
+
+  ;
+  return 0;
+}
+
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"
+then :
+  ac_cv_i686=no
+else case e in #(
+  e) ac_cv_i686=yes ;;
+esac
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.beam conftest.$ac_ext
+     ;;
+esac
+fi
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $ac_cv_i686" >&5
+printf "%s\n" "$ac_cv_i686" >&6; }
+
+    PGO_PROF_GEN_FLAG="-fprofile-generate"
+
+    # Use -fprofile-update=atomic to fix a random GCC internal error on PGO
+    # build (gh-145801) caused by corruption of profile data (.gcda files).
+    #
+    # gh-148535: On i686, using -fprofile-update=atomic makes the PGO build
+    # way slower (up to 47x slower). So far, the GCC internal error on PGO
+    # build was not seen on i686, so don't use this flag on i686.
+    if test "x$ac_cv_i686" = xno
+then :
+
+        { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether C compiler accepts -fprofile-update=atomic" >&5
 printf %s "checking whether C compiler accepts -fprofile-update=atomic... " >&6; }
 if test ${ax_cv_check_cflags___fprofile_update_atomic+y}
 then :
@@ -9127,10 +9177,13 @@ fi
 printf "%s\n" "$ax_cv_check_cflags___fprofile_update_atomic" >&6; }
 if test "x$ax_cv_check_cflags___fprofile_update_atomic" = xyes
 then :
-  PGO_PROF_GEN_FLAG="-fprofile-generate -fprofile-update=atomic"
+  PGO_PROF_GEN_FLAG="$PGO_PROF_GEN_FLAG -fprofile-update=atomic"
 else case e in #(
-  e) PGO_PROF_GEN_FLAG="-fprofile-generate" ;;
+  e) : ;;
 esac
+fi
+
+
 fi
 
     PGO_PROF_USE_FLAG="-fprofile-use -fprofile-correction"
diff --git a/configure.ac b/configure.ac
index d6e152d42b765e..ee922113e89d12 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2090,10 +2090,32 @@ case "$ac_cv_cc_name" in
     fi
     ;;
   gcc)
-    AX_CHECK_COMPILE_FLAG(
-        [-fprofile-update=atomic],
-        [PGO_PROF_GEN_FLAG="-fprofile-generate -fprofile-update=atomic"],
-        [PGO_PROF_GEN_FLAG="-fprofile-generate"])
+    # Check for 32-bit x86 ISA
+    AC_CACHE_CHECK([for i686], [ac_cv_i686], [
+        AC_COMPILE_IFELSE([
+            AC_LANG_PROGRAM([
+                #ifdef __i386__
+                #  error "i386"
+                #endif
+            ], [])
+        ],[ac_cv_i686=no],[ac_cv_i686=yes])
+    ])
+
+    PGO_PROF_GEN_FLAG="-fprofile-generate"
+
+    # Use -fprofile-update=atomic to fix a random GCC internal error on PGO
+    # build (gh-145801) caused by corruption of profile data (.gcda files).
+    #
+    # gh-148535: On i686, using -fprofile-update=atomic makes the PGO build
+    # way slower (up to 47x slower). So far, the GCC internal error on PGO
+    # build was not seen on i686, so don't use this flag on i686.
+    AS_VAR_IF([ac_cv_i686], [no], [
+        AX_CHECK_COMPILE_FLAG(
+            [-fprofile-update=atomic],
+            [PGO_PROF_GEN_FLAG="$PGO_PROF_GEN_FLAG -fprofile-update=atomic"],
+            [])
+    ])
+
     PGO_PROF_USE_FLAG="-fprofile-use -fprofile-correction"
     LLVM_PROF_MERGER="true"
     LLVM_PROF_FILE=""