Build: Add LTO/IPO support

Enable Link Time Optimization, also known as Interprocedural Optimization
if the compiler supports it.

Added a CMake option (ENABLE_LTO), defaulted to ON only on Windows

Change-Id: Iea02b00aac12cc9a62595eeb8ff52382f1c4ddcd
Reviewed-on: https://code.wireshark.org/review/37573
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

CMakeLists.txt

@@ -13,6 +13,9 @@ if(WIN32)
 else()
 	cmake_minimum_required(VERSION 3.5)
 endif()
+if(POLICY CMP0069)
+	cmake_policy(SET CMP0069 NEW)
+endif()
 if(POLICY CMP0074)
 	cmake_policy(SET CMP0074 NEW)
 endif()
@@ -56,6 +59,21 @@ message(STATUS "Generating build using CMake ${CMAKE_VERSION}")
 #Where to find local cmake scripts
 set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/modules)
 
+# CMake >= 3.9.0 enables LTO/IPO
+# Policy CMP0069 enables this behavior when we set the minimum CMake version < 3.9.0
+if (ENABLE_LTO)
+	include(CheckIPOSupported)
+	check_ipo_supported(RESULT lto_supported)
+	if(lto_supported)
+		set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
+	endif()
+endif()
+if (CMAKE_INTERPROCEDURAL_OPTIMIZATION)
+	message(STATUS "LTO/IPO is enabled")
+else()
+	message(STATUS "LTO/LPO is not enabled")
+endif()
+
 # If our target platform is enforced by our generator, set
 # WIRESHARK_TARGET_PLATFORM accordingly. Otherwise use
 # %WIRESHARK_TARGET_PLATFORM%.

CMakeOptions.txt

@@ -49,6 +49,12 @@ option(ENABLE_FUZZER "Enable libFuzzer instrumentation for use with fuzzshark" O
 option(ENABLE_CHECKHF_CONFLICT "Enable hf conflict check for debugging (start-up may be slower)" OFF)
 option(ENABLE_CCACHE "Speed up compiling and linking using ccache if possible" OFF)
 
+if (WIN32)
+	option(ENABLE_LTO "Improves performance using Link time Optimization" ON)
+else()
+	option(ENABLE_LTO "Improves performance using Link time Optimization" OFF)
+endif()
+
 option(ENABLE_PCAP       "Enable libpcap support (required for capturing)" ON)
 #
 # AirPcap support is available only on Windows.  It might be nice to have it