[clang][cmake] Force CMAKE_LINKER for multistage build in case of BOOTSTRAP_LLVM_ENABLE_LLD and MSVC

[krisb]

Summary

The issue with LLVM_ENABLE_LLD is that it just passes -fuse-ld=lld
to compiler/linker options which makes sense only for those platforms
where cmake invokes a compiler driver for linking. On Windows (MSVC) cmake
invokes a linker directly and requires CMAKE_LINKER to be specified otherwise
it defaults CMAKE_LINKER to be link.exe.

This patch allows BOOTSTRAP_LLVM_ENABLE_LLD to set CMAKE_LINKER in the case
of building for host Windows. It also skips adding '-fuse-ld=lld' to make
lld-link not warning about 'unknown argument'.

[krisb]

Ping

[phosek]

clang/CMakeLists.txt

759

I don't understand the second part of this condition, can you elaborate? Why not set CMAKE_LINKER to lld-link.exe even if BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Windows"?

[krisb]

Changed MSVC -> WIN32 check and simplified the warning fix.

[krisb]

clang/CMakeLists.txt

759

The only reason that stopped me from adding BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Windows" case here is that there is no way to keep it working in the future (or, at least, I don't know one). Moreover, the build which sets BOOTSTRAP_CMAKE_SYSTEM_NAME equal to "Windows" is currently broken.
Since the same issue is exposed if to build LLVM with clang/cmake/caches/DistributionExample.cmake on Windows, I included only the changes that contribute to making this configuration buildable. I wanted to avoid making the impression that some other configurations are supported (because they are not) and also avoid adding any dead code that nobody would use and that would easily be broken.

[krisb]

Ping

[phosek]

LGTM

clang/CMakeLists.txt

759

I'd prefer to use WIN32 OR BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Windows" here even if we don't support cross-compilation to Windows now as I hope it's something we're going to support in the future and this will be one less thing we need to address. Alternatively, please at least leave a TODO so it's easier to find later.

[krisb]

Addressed the review comment.

[krisb]

@phosek thank you for reviewing this!

clang/CMakeLists.txt

759

Okay, I made the condition to be (WIN32 AND NOT BOOTSTRAP_CMAKE_SYSTEM_NAME) OR BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Windows" to avoid setting CMAKE_LINKER in case of cross-compiling with Windows host and non-Windows target.

clang/CMakeLists.txt

if (CLANG_ENABLE_BOOTSTRAP)
   endif()
 
   set(COMPILER_OPTIONS
     -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/${CXX_COMPILER}
     -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/${C_COMPILER}
     -DCMAKE_ASM_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/${C_COMPILER}
     -DCMAKE_ASM_COMPILER_ID=Clang)
 
+  # cmake requires CMAKE_LINKER to be specified if the compiler is MSVC-like,
+  # otherwise it defaults the linker to be link.exe.
+  if(BOOTSTRAP_LLVM_ENABLE_LLD)
+    if((WIN32 AND NOT BOOTSTRAP_CMAKE_SYSTEM_NAME) OR BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Windows")
+      set(${CLANG_STAGE}_LINKER -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link${CMAKE_EXECUTABLE_SUFFIX})
+    endif()
+  endif()
+
   if(BOOTSTRAP_CMAKE_SYSTEM_NAME)
     set(${CLANG_STAGE}_CONFIG -DLLVM_CONFIG_PATH=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-config)
     set(${CLANG_STAGE}_TABLEGEN
       -DLLVM_TABLEGEN=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-tblgen
       -DCLANG_TABLEGEN=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-tblgen)
     if(BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Linux")
       if(BOOTSTRAP_LLVM_ENABLE_LLD)
         set(${CLANG_STAGE}_LINKER -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/ld.lld)

llvm/cmake/modules/HandleLLVMOptions.cmake

else()
     message(WARNING "${flag} is not supported.")
   endif()
 endfunction()
 
 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()
+  # In case of MSVC cmake always invokes the linker directly, so the linker
+  # 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=${LLVM_USE_LINKER}")
   check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)
   if ( NOT CXX_SUPPORTS_CUSTOM_LINKER )
     message(FATAL_ERROR "Host compiler does not support '-fuse-ld=${LLVM_USE_LINKER}'")
   endif()