Enable LTO in release builds

Fixes #693

CMakeLists.txt

@@ -6,8 +6,8 @@
 # SPDX-License-Identifier: MIT
 #
 
-cmake_minimum_required(VERSION 3.0)
-cmake_policy(VERSION 3.0)
+# 3.9 required for LTO checks
+cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
 
 project(rgbds
         LANGUAGES C)
@@ -23,10 +23,8 @@ if(srcdir STREQUAL bindir)
   message(FATAL_ERROR "Terminating configuration")
 endif()
 
-include_directories("${PROJECT_SOURCE_DIR}/include")
-
-option(SANITIZERS "Build with sanitizers enabled" OFF)
-option(MORE_WARNINGS "Turn on more warnings" OFF)
+option(SANITIZERS "Build with sanitizers enabled" OFF) # Ignored on MSVC
+option(MORE_WARNINGS "Turn on more warnings" OFF) # Ignored on MSVC
 option(TRACE_PARSER "Trace parser execution" OFF)
 option(TRACE_LEXER "Trace lexer execution" OFF)
 
@@ -67,6 +65,8 @@ execute_process(COMMAND git describe --tags --dirty --always
                 ERROR_QUIET)
 string(STRIP "${GIT_REV}" GIT_REV)
 
+include_directories("${PROJECT_SOURCE_DIR}/include")
+
 add_definitions(-DBUILD_VERSION_STRING="${GIT_REV}")
 
 set(CMAKE_C_STANDARD 11)
@@ -74,6 +74,20 @@ set(CMAKE_C_STANDARD_REQUIRED True)
 
 add_subdirectory(src)
 
+# By default, build in Release mode; Debug mode must be explicitly requested
+# (You may want to augment it with the options above)
+if(CMAKE_BUILD_TYPE STREQUAL "Release")
+  message(CHECK_START "Checking if LTO is supported")
+  include(CheckIPOSupported)
+  check_ipo_supported(RESULT enable_lto)
+  if(enable_lto)
+    message(CHECK_PASS "yes")
+    set_property(TARGET rgbasm rgblink rgbfix rgbgfx PROPERTY INTERPROCEDURAL_OPTIMIZATION ON)
+  else()
+    message(CHECK_FAIL "no")
+  endif()
+endif()
+
 if(TRACE_PARSER)
   target_compile_definitions(rgbasm PRIVATE -DYYDEBUG)
 endif()