[cmake] Use -ffunction-sections and -Wl,--gc-sections on MinGW targets

[mstorsjo]

Summary

If compiling with GCC or linking with ld.bfd, these options have little
effect, but if built with Clang and linked with LLD, they provide a
quite notable size decrease - this shrinks an entire llvm-mingw
distribution package by 22%.

If building with BUILD_SHARED_LIBS with LLD, this requires a version
of LLD that contains a fix for auto exporting symbols from comdats,
2b01a417d7ccb001ccc1185ef5fdc967c9fac8d7.

@mati865, do you use such builds in MSYS2, and will this be an issue
for you, if you'd upgrade to LLVM 13 and build it with LLVM/LLD 12?
(Then again, you can set LLVM_NO_DEAD_STRIP on the first build to
get an initial build of LLVM 13.)

[rnk]

lgtm

llvm/cmake/modules/HandleLLVMOptions.cmake

898–900

To the best of my knowledge, MSVC users typically get these features by default in release builds. Consider that cl /O2 enables /Gy or function sections. From there it's just a matter of passing /OPT:REF to the linker, and I think cmake does, although I don't have a minute to check right now.

[mati865]

If compiling with GCC or linking with ld.bfd, these options have little

effect

I have observed size reduction in another project using GCC+Binutils so it's a bit surprising to me.

@mati865, do you use such builds in MSYS2, and will this be an issue

for you, if you'd upgrade to LLVM 13 and build it with LLVM/LLD 12?

We don't use BUILD_SHARED_LIBS but we do use LLVM_BUILD_LLVM_DYLIB and LLVM_LINK_LLVM_DYLIB if it matters.
I don't think this will be an issue, gc-sections fix seems important enough to backport it even on its own.

[mstorsjo]

In D101568#2727411, @mati865 wrote:

I have observed size reduction in another project using GCC+Binutils so it's a bit surprising to me.

Hmm, ok, that's odd. A user tried different combinations of this in https://github.com/lovell/sharp-libvips/pull/88#issuecomment-791326495 and saw only a size increase with those options with GCC+binutils, and I tried building LLVM (statically linked only) and compared one binary where I know -ffunction-sections -Wl,--gc-sections have got a big imapct (llvm-cvtres shrinks from ~5 MB to ~500 KB). I tried GCC+LLD, GCC+binutils, Clang+binutils and Clang+LLD, and I only saw the size decrease in the Clang+LLD configuration.

We don't use BUILD_SHARED_LIBS but we do use LLVM_BUILD_LLVM_DYLIB and LLVM_LINK_LLVM_DYLIB if it matters.
I don't think this will be an issue, gc-sections fix seems important enough to backport it even on its own.

Ah, right. FWIW I tested building with LLVM_LINK_LLVM_DYLIB and that configuration does break with -ffunction-sections -Wl,--gc-sections unless you have the fixed version of the linker. But you're right that the gains are big enough that it's worth working around that if needed, instead of delaying it by 1 release.

llvm/cmake/modules/AddLLVM.cmake

if(NOT LLVM_NO_DEAD_STRIP)
         # Support for ld -z discard-unused=sections was only added in
         # Solaris 11.4.
         include(LLVMCheckLinkerFlag)
         llvm_check_linker_flag(CXX "-Wl,-z,discard-unused=sections" LINKER_SUPPORTS_Z_DISCARD_UNUSED)
         if (LINKER_SUPPORTS_Z_DISCARD_UNUSED)
           set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                        LINK_FLAGS " -Wl,-z,discard-unused=sections")
         endif()
-      elseif(NOT WIN32 AND NOT CMAKE_SYSTEM_NAME MATCHES "OpenBSD|AIX|OS390")
+      elseif(NOT MSVC AND NOT CMAKE_SYSTEM_NAME MATCHES "OpenBSD|AIX|OS390")
         # TODO Revisit this later on z/OS.
         set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                      LINK_FLAGS " -Wl,--gc-sections")
       endif()
     else() #LLVM_NO_DEAD_STRIP
       if(${CMAKE_SYSTEM_NAME} MATCHES "AIX")
         set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                      LINK_FLAGS " -Wl,-bnogc")

llvm/cmake/modules/HandleLLVMOptions.cmake

 if (UNIX AND CMAKE_GENERATOR STREQUAL "Ninja")
   include(LLVMCheckLinkerFlag)
   llvm_check_linker_flag(CXX "-Wl,--color-diagnostics" LINKER_SUPPORTS_COLOR_DIAGNOSTICS)
   append_if(LINKER_SUPPORTS_COLOR_DIAGNOSTICS "-Wl,--color-diagnostics"
     CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 
 # Add flags for add_dead_strip().
 # FIXME: With MSVS, consider compiling with /Gy and linking with /OPT:REF?
 # But MinSizeRel seems to add that automatically, so maybe disable these
 # flags instead if LLVM_NO_DEAD_STRIP is set.
-if(NOT CYGWIN AND NOT WIN32)
+if(NOT CYGWIN AND NOT MSVC)
   if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin" AND
      NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
     check_c_compiler_flag("-Werror -fno-function-sections" C_SUPPORTS_FNO_FUNCTION_SECTIONS)
     if (C_SUPPORTS_FNO_FUNCTION_SECTIONS)
       # Don't add -ffunction-sections if it can't be disabled with -fno-function-sections.
       # Doing so will break sanitizers.
       add_flag_if_supported("-ffunction-sections" FFUNCTION_SECTIONS)
     elseif (CMAKE_CXX_COMPILER_ID MATCHES "XL")