gh-130048: Reintroduce full LTO as default on Clang (GH-130049)

Author: Fidget-Spinner

Doc/using/configure.rst

@@ -575,6 +575,9 @@ also be used to improve performance.
    .. versionchanged:: 3.12
       Use ThinLTO as the default optimization policy on Clang if the compiler accepts the flag.
 
+   .. versionchanged:: next
+      Revert to using full LTO as the default optimization policy on Clang.
+
 .. option:: --enable-bolt
 
    Enable usage of the `BOLT post-link binary optimizer

Doc/whatsnew/3.14.rst

@@ -1279,6 +1279,10 @@ Build changes
 * GNU Autoconf 2.72 is now required to generate :file:`configure`.
   (Contributed by Erlend Aasland in :gh:`115765`.)
 
+* CPython now uses Full LTO as the default link time optimization policy
+  on Clang. This reverts an earlier change in CPython 3.12.
+  (Contributed by Ken Jin in :gh:`130049`.)
+
 .. _whatsnew314-pep761:
 
 PEP 761: Discontinuation of PGP signatures

Misc/NEWS.d/next/Build/2025-02-12-19-51-19.gh-issue-130048.kHNkzP.rst

@@ -0,0 +1 @@
+CPython now uses Full LTO as the default link time optimization policy on Clang. This reverts an earlier change in CPython 3.12.

configure

@@ -1986,15 +1986,8 @@ if test "$Py_LTO" = 'true' ; then
           # Any changes made here should be reflected in the GCC+Darwin case below
           if test $Py_LTO_POLICY = default
           then
-            # Check that ThinLTO is accepted.
-            AX_CHECK_COMPILE_FLAG([-flto=thin],[
-              LTOFLAGS="-flto=thin -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"
-              LTOCFLAGS="-flto=thin"
-              ],[
-              LTOFLAGS="-flto -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"
-              LTOCFLAGS="-flto"
-              ]
-            )
+            LTOFLAGS="-flto -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"
+            LTOCFLAGS="-flto"
           else
             LTOFLAGS="-flto=${Py_LTO_POLICY} -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"
             LTOCFLAGS="-flto=${Py_LTO_POLICY}"
@@ -2003,8 +1996,7 @@ if test "$Py_LTO" = 'true' ; then
         *)
           if test $Py_LTO_POLICY = default
           then
-            # Check that ThinLTO is accepted
-            AX_CHECK_COMPILE_FLAG([-flto=thin],[LTOFLAGS="-flto=thin"],[LTOFLAGS="-flto"])
+            LTOFLAGS="-flto"
           else
             LTOFLAGS="-flto=${Py_LTO_POLICY}"
           fi