Issue #25702: A --with-lto configure option has been added that will

enable link time optimizations at build time during a make profile-opt.

Makefile.pre.in

@@ -504,7 +504,7 @@ profile-opt:
 	$(MAKE) profile-removal
 
 build_all_generate_profile:
-	$(MAKE) all CFLAGS_NODIST="$(CFLAGS) $(PGO_PROF_GEN_FLAG)" LDFLAGS="$(LDFLAGS) $(PGO_PROF_GEN_FLAG)" LIBS="$(LIBS)"
+	$(MAKE) all CFLAGS_NODIST="$(CFLAGS) $(PGO_PROF_GEN_FLAG) @LTOFLAGS@" LDFLAGS="$(LDFLAGS) $(PGO_PROF_GEN_FLAG) @LTOFLAGS@" LIBS="$(LIBS)"
 
 run_profile_task:
 	: # FIXME: can't run for a cross build
@@ -514,7 +514,7 @@ build_all_merge_profile:
 	$(LLVM_PROF_MERGER)
 
 build_all_use_profile:
-	$(MAKE) all CFLAGS_NODIST="$(CFLAGS) $(PGO_PROF_USE_FLAG)"
+	$(MAKE) all CFLAGS_NODIST="$(CFLAGS) $(PGO_PROF_USE_FLAG) @LTOFLAGS@" LDFLAGS="$(LDFLAGS) @LTOFLAGS@"
 
 # Compile and run with gcov
 .PHONY=coverage coverage-lcov coverage-report

Misc/NEWS

@@ -388,6 +388,12 @@ Tests
 Build
 -----
 
+- Issue #25702: A --with-lto configure option has been added that will
+  enable link time optimizations at build time during a make profile-opt.
+  Some compilers and toolchains are known to not produce stable code when
+  using LTO, be sure to test things thoroughly before relying on it.
+  It can provide a few % speed up over profile-opt alone.
+  
 - Issue #26624: Adds validation of ucrtbase[d].dll version with warning
   for old versions.
 

configure

@@ -673,6 +673,7 @@ LLVM_PROF_FILE
 LLVM_PROF_MERGER
 PGO_PROF_USE_FLAG
 PGO_PROF_GEN_FLAG
+LTOFLAGS
 ABIFLAGS
 LN
 MKDIR_P
@@ -807,6 +808,7 @@ with_suffix
 enable_shared
 enable_profiling
 with_pydebug
+with_lto
 with_hash_algorithm
 with_address_sanitizer
 with_libs
@@ -1487,6 +1489,8 @@ Optional Packages:
                           compiler
   --with-suffix=.exe      set executable suffix
   --with-pydebug          build with Py_DEBUG defined
+  --with-lto              Enable Link Time Optimization in PGO builds.
+                          Disabled by default.
   --with-hash-algorithm=[fnv|siphash24]
                           select hash algorithm
   --with-address-sanitizer
@@ -6560,6 +6564,48 @@ $as_echo "no" >&6; }
 fi
 
 
+# Enable LTO flags
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-lto" >&5
+$as_echo_n "checking for --with-lto... " >&6; }
+
+# Check whether --with-lto was given.
+if test "${with_lto+set}" = set; then :
+  withval=$with_lto;
+if test "$withval" != no
+then
+  Py_LTO='true'
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; };
+else
+  Py_LTO='false'
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; };
+fi
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+if test "$Py_LTO" = 'true' ; then
+  case $CC in
+    *clang*)
+      # Any changes made here should be reflected in the GCC+Darwin case below
+      LTOFLAGS="-flto"
+      ;;
+    *gcc*)
+      case $ac_sys_system in
+        Darwin*)
+          LTOFLAGS="-flto"
+          ;;
+        *)
+          LTOFLAGS="-flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none"
+          ;;
+      esac
+      ;;
+  esac
+fi
+
 # Enable PGO flags.
 
 

configure.ac

@@ -1235,6 +1235,39 @@ else AC_MSG_RESULT(no); Py_DEBUG='false'
 fi],
 [AC_MSG_RESULT(no)])
 
+# Enable LTO flags
+AC_SUBST(LTOFLAGS)
+AC_MSG_CHECKING(for --with-lto)
+AC_ARG_WITH(lto, AS_HELP_STRING([--with-lto], [Enable Link Time Optimization in PGO builds. Disabled by default.]),
+[
+if test "$withval" != no
+then
+  Py_LTO='true'
+  AC_MSG_RESULT(yes);
+else
+  Py_LTO='false'
+  AC_MSG_RESULT(no);
+fi],
+[AC_MSG_RESULT(no)])
+if test "$Py_LTO" = 'true' ; then
+  case $CC in
+    *clang*)
+      # Any changes made here should be reflected in the GCC+Darwin case below
+      LTOFLAGS="-flto"
+      ;;
+    *gcc*)
+      case $ac_sys_system in
+        Darwin*)
+          LTOFLAGS="-flto"
+          ;;
+        *)
+          LTOFLAGS="-flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none"
+          ;;
+      esac
+      ;;
+  esac
+fi
+
 # Enable PGO flags.
 AC_SUBST(PGO_PROF_GEN_FLAG)
 AC_SUBST(PGO_PROF_USE_FLAG)