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.

Makefile.pre.in

@@ -451,7 +451,7 @@ profile-opt:
 	$(MAKE) profile-removal
 
 build_all_generate_profile:
-	$(MAKE) all CFLAGS="$(CFLAGS) $(PGO_PROF_GEN_FLAG)" LDFLAGS="$(LDFLAGS) $(PGO_PROF_GEN_FLAG)" LIBS="$(LIBS)"
+	$(MAKE) all CFLAGS="$(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
@@ -461,7 +461,7 @@ build_all_merge_profile:
 	$(LLVM_PROF_MERGER)
 
 build_all_use_profile:
-	$(MAKE) all CFLAGS="$(CFLAGS) $(PGO_PROF_USE_FLAG)"
+	$(MAKE) all CFLAGS="$(CFLAGS) $(PGO_PROF_USE_FLAG) @LTOFLAGS@" LDFLAGS="$(LDFLAGS) @LTOFLAGS@"
 
 # Compile and run with gcov
 .PHONY=coverage coverage-lcov coverage-report

Misc/NEWS

@@ -10,6 +10,12 @@ What's New in Python 2.7.12?
 Core and Builtins
 -----------------
 
+- 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 #26168: Fixed possible refleaks in failing Py_BuildValue() with the "N"
   format unit.
 

configure

@@ -667,6 +667,7 @@ LLVM_PROF_FILE
 LLVM_PROF_MERGER
 PGO_PROF_USE_FLAG
 PGO_PROF_GEN_FLAG
+LTOFLAGS
 UNIVERSAL_ARCH_FLAGS
 BASECFLAGS
 OPT
@@ -796,6 +797,7 @@ with_suffix
 enable_shared
 enable_profiling
 with_pydebug
+with_lto
 enable_toolbox_glue
 with_libs
 with_system_expat
@@ -1477,6 +1479,8 @@ 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-libs='lib1 ...'  link against additional libs
   --with-system-expat     build pyexpat module using an installed expat
                           library
@@ -6366,6 +6370,49 @@ rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 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

@@ -1377,6 +1377,40 @@ then
 fi
 
 
+# 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)