[windows] Add /Gw to compiler flags

This is like -fdata-sections, and it's not part of /O2 by default for some reason. In the cmake build, reduces the size of clang.exe from 70,358,016 bytes to 69,982,720 bytes. clang-format.exe goes from 3,703,296 bytes to 3,331,072 bytes. Differential Revision: https://reviews.llvm.org/D74573 (cherry picked from commit 09153ab)
llvm · Feb 20, 2020 · f004359 · f004359
1 parent 5175565
commit f004359
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diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -788,6 +788,10 @@ if(NOT CYGWIN AND NOT WIN32)
     endif()
     add_flag_if_supported("-fdata-sections" FDATA_SECTIONS)
   endif()
+elseif(MSVC)
+  if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
+    append("/Gw" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  endif()
 endif()
 
 if(MSVC)

diff --git a/llvm/utils/gn/build/BUILD.gn b/llvm/utils/gn/build/BUILD.gn
@@ -60,6 +60,7 @@ config("compiler_defaults") {
     if (is_optimized) {
       cflags += [
         "/O2",
+        "/Gw",
         "/Zc:inline",
       ]
       ldflags += [