CMake: add lto options

Author: enetheru

cmake/common_compiler_flags.cmake

@@ -78,6 +78,13 @@ function( common_compiler_flags )
                 $<${IS_GNU}:-fno-gnu-unique>
             >
 
+            $<${IS_LTO}:
+                $<${IS_MSVC}: /GL >
+                $<${IS_CLANG}: -flto=${LTO_TYPE} >
+                $<${IS_APPLECLANG}: -flto=${LTO_TYPE} >
+                $<${IS_GNU}: -flto >
+            >
+
         # MSVC only
         $<${IS_MSVC}:
             # /MP isn't valid for clang-cl with msvc frontend
@@ -170,6 +177,13 @@ function( common_compiler_flags )
                 $<${IS_CLANG}:-s>
                 $<${IS_APPLECLANG}:-Wl,-S -Wl,-x -Wl,-dead_strip>
             >
+
+            $<${IS_LTO}:
+                $<${IS_MSVC}: /LTCG:INCREMENTAL >
+                $<${IS_CLANG}: -flto=${LTO_TYPE} >
+                $<${IS_APPLECLANG}: -flto=${LTO_TYPE} >
+                $<${IS_GNU}: -flto >
+            >
     )
 
 endfunction()

cmake/godotcpp.cmake

@@ -32,6 +32,9 @@ include( ${CMAKE_CURRENT_SOURCE_DIR}/cmake/web.cmake)
 include( ${CMAKE_CURRENT_SOURCE_DIR}/cmake/windows.cmake)
 include( ${CMAKE_CURRENT_SOURCE_DIR}/cmake/python_callouts.cmake)
 
+# IPO Check for when GODOT_LTO is set to fail more gracefully.
+include(CheckIPOSupported)
+
 # Detect number of processors
 include(ProcessorCount)
 ProcessorCount(PROC_MAX)
@@ -127,6 +130,9 @@ function( godotcpp_options )
 
     #TODO optimize
 
+    set( GODOT_LTO "none" CACHE STRING "Link-time optimization" )
+    set_property( CACHE GODOT_LTO PROPERTY STRINGS "none;auto;thin;full" )
+
     option( GODOT_DEV_BUILD "Developer build with dev-only debugging code (DEV_ENABLED)" OFF )
 
     #[[ debug_symbols
@@ -159,6 +165,28 @@ endfunction()
 
 # Function to configure and generate the targets
 function( godotcpp_generate )
+    #[[ Link-Time Optimization ]]
+    if( GODOT_LTO STREQUAL "none") # Default Value
+        set( IS_LTO 0 )
+    elseif( GODOT_LTO STREQUAL "thin" )
+        set( IS_LTO 1 )
+        set( LTO_TYPE "thin" )
+    else() # auto or full
+        set( IS_LTO 1 )
+        set( LTO_TYPE "full" )
+    endif()
+    if( IS_LTO )
+        check_ipo_supported(RESULT result OUTPUT output LANGUAGES CXX )
+        if( NOT result)
+            message( FATAL_ERROR "GODOT_LTO=${GODOT_LTO}, but IPO/LTO is not supported by the current compiler")
+        endif()
+        if( CMAKE_CXX_COMPILER_ID MATCHES "Clang" )
+            message( STATUS "Using LTO: ${LTO_TYPE}" )
+        else(  )
+            message( STATUS "Using LTO" )
+        endif(  )
+    endif()
+
     #[[ Multi-Threaded MSVC Compilation
     When using the MSVC compiler the build command -j <n> only specifies
     parallel jobs or targets, and not multi-threaded compilation To speed up