From 83d84e67cd203cc75687152e57572895f56245fa Mon Sep 17 00:00:00 2001
From: Dong-hee Na <donghee.na@python.org>
Date: Fri, 23 Sep 2022 11:00:44 +0900
Subject: [PATCH] gh-96761: Fix build process of the clang compiler for
 _bootstrap_python (gh-96945)

Co-authored-by: Matthias Goergens <matthias.goergens@gmail.com>
---
 ...2-09-20-12-43-44.gh-issue-96761.IF29kR.rst |  2 +
 configure                                     | 39 ++++++++++++++++++-
 configure.ac                                  |  6 ++-
 3 files changed, 43 insertions(+), 4 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Build/2022-09-20-12-43-44.gh-issue-96761.IF29kR.rst

diff --git a/Misc/NEWS.d/next/Build/2022-09-20-12-43-44.gh-issue-96761.IF29kR.rst b/Misc/NEWS.d/next/Build/2022-09-20-12-43-44.gh-issue-96761.IF29kR.rst
new file mode 100644
index 00000000000..18f75ac4891
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2022-09-20-12-43-44.gh-issue-96761.IF29kR.rst
@@ -0,0 +1,2 @@
+Fix the build process of clang compiler for :program:`_bootstrap_python` if
+LTO optimization is applied. Patch by Matthias Görgens and Dong-hee Na.
diff --git a/configure b/configure
index 08ec2161cba..0e9f72faa7b 100755
--- a/configure
+++ b/configure
@@ -7640,7 +7640,7 @@ fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PROFILE_TASK" >&5
 $as_echo "$PROFILE_TASK" >&6; }
 
-# Make llvm-relatec checks work on systems where llvm tools are not installed with their
+# Make llvm-related checks work on systems where llvm tools are not installed with their
 # normal names in the default $PATH (ie: Ubuntu).  They exist under the
 # non-suffixed name in their versioned llvm directory.
 
@@ -7705,7 +7705,42 @@ fi
 if test "$Py_LTO" = 'true' ; then
   case $CC in
     *clang*)
-            LDFLAGS_NOLTO="-fno-lto"
+      LDFLAGS_NOLTO="-fno-lto"
+                  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether C compiler accepts -flto=thin" >&5
+$as_echo_n "checking whether C compiler accepts -flto=thin... " >&6; }
+if ${ax_cv_check_cflags___flto_thin+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+
+  ax_check_save_flags=$CFLAGS
+  CFLAGS="$CFLAGS  -flto=thin"
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  ax_cv_check_cflags___flto_thin=yes
+else
+  ax_cv_check_cflags___flto_thin=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  CFLAGS=$ax_check_save_flags
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ax_cv_check_cflags___flto_thin" >&5
+$as_echo "$ax_cv_check_cflags___flto_thin" >&6; }
+if test "x$ax_cv_check_cflags___flto_thin" = xyes; then :
+  LDFLAGS_NOLTO="-flto=thin"
+else
+  LDFLAGS_NOLTO="-flto"
+fi
+
 
       if test -n "$ac_tool_prefix"; then
   # Extract the first word of "${ac_tool_prefix}llvm-ar", so it can be a program name with args.
diff --git a/configure.ac b/configure.ac
index 7d2e83cac81..a4f3b0ab846 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1774,7 +1774,7 @@ then
 fi
 AC_MSG_RESULT($PROFILE_TASK)
 
-# Make llvm-relatec checks work on systems where llvm tools are not installed with their
+# Make llvm-related checks work on systems where llvm tools are not installed with their
 # normal names in the default $PATH (ie: Ubuntu).  They exist under the
 # non-suffixed name in their versioned llvm directory.
 
@@ -1828,8 +1828,10 @@ esac
 if test "$Py_LTO" = 'true' ; then
   case $CC in
     *clang*)
-      dnl flag to disable lto during linking
       LDFLAGS_NOLTO="-fno-lto"
+      dnl Clang linker requires -flto in order to link objects with LTO information.
+      dnl Thin LTO is faster and works for object files with full LTO information, too.
+      AX_CHECK_COMPILE_FLAG([-flto=thin],[LDFLAGS_NOLTO="-flto=thin"],[LDFLAGS_NOLTO="-flto"])
       AC_SUBST(LLVM_AR)
       AC_PATH_TOOL(LLVM_AR, llvm-ar, '', ${llvm_path})
       AC_SUBST(LLVM_AR_FOUND)