pythongh-89536: Use ThinLTO policy if possible (pythongh-96766)

Doc/using/configure.rst

@@ -232,6 +232,9 @@ also be used to improve performance.
    .. versionadded:: 3.11
       To use ThinLTO feature, use ``--with-lto=thin`` on Clang.
 
+   .. versionchanged:: 3.12
+      Use ThinLTO as the default optimization policy on Clang if the compiler accepts the flag.
+
 .. cmdoption:: --enable-bolt
 
    Enable usage of the `BOLT post-link binary optimizer

Doc/whatsnew/3.12.rst

@@ -453,6 +453,10 @@ Build Changes
   ``va_start()`` is no longer called with a single parameter.
   (Contributed by Kumar Aditya in :gh:`93207`.)
 
+* CPython now uses the ThinLTO option as the default link time optimization policy
+  if the Clang compiler accepts the flag.
+  (Contributed by Dong-hee Na in :gh:`89536`.)
+
 
 C API Changes
 =============

Misc/NEWS.d/next/Build/2022-09-12-18-34-51.gh-issue-85936.tX4VCU.rst

@@ -0,0 +1,2 @@
+CPython now uses the ThinLTO option as the default policy if the Clang
+compiler accepts the flag. Patch by Dong-hee Na.

configure.ac

@@ -1863,8 +1863,15 @@ if test "$Py_LTO" = 'true' ; then
           # Any changes made here should be reflected in the GCC+Darwin case below
           if test $Py_LTO_POLICY = default
           then
-            LTOFLAGS="-flto -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"
-            LTOCFLAGS="-flto"
+            # Check that ThinLTO is accepted.
+            AX_CHECK_COMPILE_FLAG([-flto=thin],[
+              LTOFLAGS="-flto=thin -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"
+              LTOCFLAGS="-flto=thin"
+              ],[
+              LTOFLAGS="-flto -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"
+              LTOCFLAGS="-flto"
+              ]
+            )
           else
             LTOFLAGS="-flto=${Py_LTO_POLICY} -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"
             LTOCFLAGS="-flto=${Py_LTO_POLICY}"
@@ -1873,7 +1880,8 @@ if test "$Py_LTO" = 'true' ; then
         *)
           if test $Py_LTO_POLICY = default
           then
-            LTOFLAGS="-flto"
+            # Check that ThinLTO is accepted
+            AX_CHECK_COMPILE_FLAG([-flto=thin],[LTOFLAGS="-flto=thin"],[LTOFLAGS="-flto"])
           else
             LTOFLAGS="-flto=${Py_LTO_POLICY}"
           fi