Fix -fuse-ld to be linker flag in feature detection check

[GleasonK]

Summary

Discovered an issue working in StableHLO when attempting to build with -Wall -Werror:
https://github.com/openxla/stablehlo/pull/137

Currently, if LLVM_USE_LLD and -DCMAKE_CXX_FLAGS="${CMAKE_CXX_FLAGS} -Wall -Werror"
are both specified for build, the build will error with:

-- Performing Test CXX_SUPPORTS_CUSTOM_LINKER - Failed
CMake Error at /usr/local/google/home/gleasonk/Coding/llvm-build/lib/cmake/llvm/HandleLLVMOptions.cmake:309 (message):
  Host compiler does not support '-fuse-ld=lld'
...

$ cat <build_dir>/CMakeFiles/CMakeError.log
...
clang: error: argument unused during compilation: '-fuse-ld=lld' [-Werror,-Wunused-command-line-argument]

It looks like other repos have hit this same issue:

• https://github.com/golang/go/issues/41527 (mentioned in comment)
• https://github.com/iree-org/iree/pull/7450

This can be reproduced in llvm-project with the following build command:

# Compile command taken from https://mlir.llvm.org/getting_started/
# and modified for use case
cmake -G Ninja ../llvm \
   -DLLVM_ENABLE_PROJECTS=mlir \
   -DLLVM_BUILD_EXAMPLES=ON \
   -DLLVM_TARGETS_TO_BUILD="X86;NVPTX;AMDGPU" \
   -DCMAKE_BUILD_TYPE=Release \
   -DLLVM_ENABLE_ASSERTIONS=ON \
   -DCMAKE_CXX_COMPILER=clang++ \
   -DCMAKE_C_COMPILER=clang \
   -DLLVM_ENABLE_LLD=ON \
   -DCMAKE_CXX_FLAGS="${CMAKE_CXX_FLAGS} -Wall -Werror"

[tschuett]

I have similar issues, but I use -DLLVM_ENABLE_WERROR=ON.

[GleasonK]

In D134206#3800765, @tschuett wrote:

I have similar issues, but I use -DLLVM_ENABLE_WERROR=ON.

That is what I ended up doing in https://github.com/openxla/stablehlo/pull/137 (LLVM_ENABLE_WERROR / LLVM_ENABLE_WARNINGS), which unblocked me. Based on conversation with Mehdi in that thread I figured I would still propose this fix, since it was strange for -fuse-ld to end up in the cxx flags instead of linker flags.

It looks like check_cxx_source_compiles picks up changes to CMAKE_EXE_LINKER_FLAGS, since setting a nonsense value like -DLLVM_USE_LINKER=notalinker will still error on this line.

[mehdi_amini]

I guess you need me to land this for you?

[GleasonK]

Yes, thanks. I just tried and got 403 from the remote.

llvm/cmake/modules/HandleLLVMOptions.cmake

if( LLVM_ENABLE_LLD )
   # should be specified by CMAKE_LINKER cmake variable instead of by -fuse-ld
   # compiler option.
   if ( NOT MSVC )
     set(LLVM_USE_LINKER "lld")
   endif()
 endif()
 
 if( LLVM_USE_LINKER )
-  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fuse-ld=${LLVM_USE_LINKER}")
+  append("-fuse-ld=${LLVM_USE_LINKER}"
+    CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)
   if ( NOT CXX_SUPPORTS_CUSTOM_LINKER )
     message(FATAL_ERROR "Host compiler does not support '-fuse-ld=${LLVM_USE_LINKER}'")
   endif()
-  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
-  append("-fuse-ld=${LLVM_USE_LINKER}"
-    CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 
 if( LLVM_ENABLE_PIC )
   if( XCODE )
     # Xcode has -mdynamic-no-pic on by default, which overrides -fPIC. I don't
     # know how to disable this, so just force ENABLE_PIC off for now.
     message(WARNING "-fPIC not supported with Xcode.")
   elseif( WIN32 OR CYGWIN)