[build] Guard Windows LTO with flags

ghstack-source-id: 045bd93
Pull Request resolved: taichi-dev/taichi#7989

Author: feisuzhu

CMakeLists.txt

@@ -70,6 +70,7 @@ option(USE_LLD "Use lld (from llvm) linker" OFF)
 option(USE_MOLD "Use mold (A Modern Linker)" OFF)
 option(TI_WITH_BACKTRACE "Use backward-cpp to print out C++ stack trace upon failure" OFF)  # wheel-tag: bt
 option(TI_GENERATE_PDB "Generate Program Database (PDB) files (will make compilation uncacheable)" OFF)
+option(TI_WITH_LTO "Enable Link Time Optimization (LTO) (affects Windows + MSVC for now)" OFF)  # wheel-tag: lto
 
 if(LINUX OR APPLE)
     if (NOT IOS)

cmake/TaichiCXXFlags.cmake

@@ -25,10 +25,14 @@ if (WIN32)
         set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS} -flto=thin")
         set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS} -flto=thin")
     elseif (MSVC)
-        set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS} /GL /Gy")
-        set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS} /GL /Gy")
-        set(CMAKE_EXE_LINKER_FLAGS_RELWITHDEBINFO "${CMAKE_EXE_LINKER_FLAGS} /LTCG")
-        set(CMAKE_EXE_LINKER_FLAGS_RELEASE "${CMAKE_EXE_LINKER_FLAGS} /LTCG")
+        set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS} /Gy")
+        set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS} /Gy")
+        if (TI_WITH_LTO)
+            set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS} /GL")
+            set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS} /GL")
+            set(CMAKE_EXE_LINKER_FLAGS_RELWITHDEBINFO "${CMAKE_EXE_LINKER_FLAGS} /LTCG")
+            set(CMAKE_EXE_LINKER_FLAGS_RELEASE "${CMAKE_EXE_LINKER_FLAGS} /LTCG")
+        endif()
     endif()
 endif()