From d21f63d327d3cef6753ffe34db13a9354813c861 Mon Sep 17 00:00:00 2001
From: Dennis Luxen <info@project-osrm.org>
Date: Tue, 1 Apr 2014 17:54:41 +0200
Subject: [PATCH] activate LTO on platforms that support it, ie. most recent
 compilers

---
 CMakeLists.txt                   | 15 +++++++++++++++
 cmake/CheckCXXCompilerFlag.cmake | 29 +++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 cmake/CheckCXXCompilerFlag.cmake

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8ddaf1219..471bfde7f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,6 +1,7 @@
 cmake_minimum_required(VERSION 2.6)
 project(OSRM)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+include(CheckCXXCompilerFlag)
 include(FindPackageHandleStandardArgs)
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")
@@ -103,6 +104,20 @@ elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
   # using Visual Studio C++
 endif()
 
+# Check if LTO is available
+set(LTO_FLAGS "")
+CHECK_CXX_COMPILER_FLAG("-flto" HAS_LTO_FLAG)
+if    (HAS_LTO_FLAG)
+  set(LTO_FLAGS "${LTO_FLAGS} -flto")
+else  (HAS_LTO_FLAG)
+  set(LTO_FLAGS "${LTO_FLAGS} -flto")
+endif (HAS_LTO_FLAG)
+
+# Add Link-Time-Optimization flags, if supported (GCC >= 4.5) and enabled
+set(CMAKE_CXX_FLAGS           "${CMAKE_CXX_FLAGS}           ${LTO_FLAGS}")
+set(CMAKE_EXE_LINKER_FLAGS    "${CMAKE_EXE_LINKER_FLAGS}    ${LTO_FLAGS}")
+set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${LTO_FLAGS}")
+
 # Configuring other platform dependencies
 if(APPLE)
   set(CMAKE_OSX_ARCHITECTURES "x86_64")
diff --git a/cmake/CheckCXXCompilerFlag.cmake b/cmake/CheckCXXCompilerFlag.cmake
new file mode 100644
index 000000000..e396f75a9
--- /dev/null
+++ b/cmake/CheckCXXCompilerFlag.cmake
@@ -0,0 +1,29 @@
+# - Check whether the CXX compiler supports a given flag.
+# CHECK_CXX_COMPILER_FLAG(<flag> <var>)
+#  <flag> - the compiler flag
+#  <var>  - variable to store the result
+# This internally calls the check_cxx_source_compiles macro.  See help
+# for CheckCXXSourceCompiles for a listing of variables that can
+# modify the build.
+
+# Copyright (c) 2006, Alexander Neundorf, <neundorf@kde.org>
+#
+# Redistribution and use is allowed according to the terms of the BSD license.
+# For details see the accompanying COPYING-CMAKE-SCRIPTS file.
+
+
+INCLUDE(CheckCXXSourceCompiles)
+
+MACRO (CHECK_CXX_COMPILER_FLAG _FLAG _RESULT)
+   SET(SAFE_CMAKE_REQUIRED_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS}")
+   SET(CMAKE_REQUIRED_DEFINITIONS "${_FLAG}")
+   CHECK_CXX_SOURCE_COMPILES("int main() { return 0;}" ${_RESULT}
+     # Some compilers do not fail with a bad flag
+     FAIL_REGEX "unrecognized .*option"                     # GNU
+     FAIL_REGEX "ignoring unknown option"                   # MSVC
+     FAIL_REGEX "[Uu]nknown option"                         # HP
+     FAIL_REGEX "[Ww]arning: [Oo]ption"                     # SunPro
+     FAIL_REGEX "command option .* is not recognized"       # XL
+     )
+   SET (CMAKE_REQUIRED_DEFINITIONS "${SAFE_CMAKE_REQUIRED_DEFINITIONS}")
+ENDMACRO (CHECK_CXX_COMPILER_FLAG)