[cmake] Suppress unknown argument warning when linking with lld-link

[russell.gallop]

Summary

Linking with lld-link produced a lot of messages:
lld-link: warning: ignoring unknown argument '-fuse-ld=lld'

For example on the bot: http://lab.llvm.org:8011/builders/clang-x64-windows-msvc

This avoids adding that argument when using lld-link.

[thakis]

Hm, we don't see these warnings. We use CMAKE_LINKER=C:/b/s/w/ir/cache/builder/src/third_party/llvm-bootstrap-install/bin/lld-link.exe to make cmake use lld-link. It looks like the bot you link to explicitly sets -DLLVM_USE_LINKER=lld which I always figured translates to "pass this -fuse-ld" flag. So I'd say the bot configuration is maybe not quite right? I'd fix that instead.

Maybe we should have a "LLVM_USE_LINKER" and CMAKE_LINKER can't be set at the same time" warning instead?

But if this change helps you somehow, I don't see a problem with landing it either.

(But again, for the bot you link to, I'd argue a better fix is to make it set fewer cmake settings.)

[russell.gallop]

In D69030#1711564, @thakis wrote:

Hm, we don't see these warnings. We use CMAKE_LINKER=C:/b/s/w/ir/cache/builder/src/third_party/llvm-bootstrap-install/bin/lld-link.exe to make cmake use lld-link. It looks like the bot you link to explicitly sets -DLLVM_USE_LINKER=lld which I always figured translates to "pass this -fuse-ld" flag. So I'd say the bot configuration is maybe not quite right? I'd fix that instead.

Ah, I got the settings from the bot, but that makes sense.

Maybe we should have a "LLVM_USE_LINKER" and CMAKE_LINKER can't be set at the same time" warning instead?

Hmm, I think that makes sense. CMake defaults to using the compiler for linking. Is there a case where you'd want to specify a different compiler with CMAKE_LINKER then add LLVM_USE_LINKER?

But if this change helps you somehow, I don't see a problem with landing it either.

I think that landing this might confuse the usage (even more). I think that warning against this would help clarify the situation.

Scanned through the bot configuration and it is just:
clang-x64-windows-msvc
sanitizer-windows

that use that combination. I'll put up review to change that*, then look at adding a warning.

• https://reviews.llvm.org/D69098

[russell.gallop]

Agreed that this usage shouldn't be encouraged so abandoning review.

then look at adding a warning.

Also couldn't obviously see how to add an appropriate warning.

llvm/cmake/modules/HandleLLVMOptions.cmake

 
 if( LLVM_ENABLE_LLD )
   if ( LLVM_USE_LINKER )
     message(FATAL_ERROR "LLVM_ENABLE_LLD and LLVM_USE_LINKER can't be set at the same time")
   endif()
   set(LLVM_USE_LINKER "lld")
 endif()
 
-if( LLVM_USE_LINKER )
+if( LLVM_USE_LINKER AND NOT LINKER_IS_LLD_LINK )
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fuse-ld=${LLVM_USE_LINKER}")
   check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)
   if ( NOT CXX_SUPPORTS_CUSTOM_LINKER )
     message(FATAL_ERROR "Host compiler does not support '-fuse-ld=${LLVM_USE_LINKER}'")
   endif()
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
   append("-fuse-ld=${LLVM_USE_LINKER}"