From d82da9f7f22cd0e81e4de3d7331668d3aa9b0811 Mon Sep 17 00:00:00 2001
From: "Gregory P. Smith" <greg@krypto.org>
Date: Fri, 15 Apr 2016 16:57:04 -0700
Subject: [PATCH] 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 |  4 ++--
 Misc/NEWS       |  6 ++++++
 configure       | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 configure.ac    | 33 +++++++++++++++++++++++++++++++++
 4 files changed, 87 insertions(+), 2 deletions(-)

diff --git a/Makefile.pre.in b/Makefile.pre.in
index cfa6a5f4ef5..17c0b5785d0 100644
--- a/Makefile.pre.in
+++ b/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
diff --git a/Misc/NEWS b/Misc/NEWS
index 124866423d8..8bf583f4d04 100644
--- a/Misc/NEWS
+++ b/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.
 
diff --git a/configure b/configure
index 0eff26d0054..68970848702 100755
--- a/configure
+++ b/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.
 
 
diff --git a/configure.ac b/configure.ac
index 71498300141..b13cf059a75 100644
--- a/configure.ac
+++ b/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)