[PATCH] D80873: [clang][cmake] Force CMAKE_LINKER for multistage build in case of BOOTSTRAP_LLVM_ENABLE_LLD and MSVC

[Kristina Bessonova via Phabricator via llvm-commits]

krisb created this revision.
krisb added reviewers: phosek, thakis, russell.gallop.
Herald added subscribers: llvm-commits, cfe-commits, mgorny.
Herald added projects: clang, LLVM.

(I'm trying to get a bootstrap self-build on Windows, where lld is used as a
linker for the stage2.)

I assume BOOTSTRAP_LLVM_ENABLE_LLD works the same way as LLVM_ENABLE_LLD,
but enables to use just-built lld for the next stage.

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 cmake invokes a linker directly and requires CMAKE_LINKER
option to be specified otherwise it defaults CMAKE_LINKER to be link.exe.
Passing CMAKE_LINKER is easy for 1-stage builds, but it's notquite handy
to do for multistage builds, because it's not possible to know for sure
where to find just-built lld.

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'. The latter part is taken
from https://reviews.llvm.org/D69030 with additional checks that
CMAKE_LINKER doesn't contain a path to another compiler.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D80873

Files:
  clang/CMakeLists.txt
  llvm/cmake/modules/HandleLLVMOptions.cmake


Index: llvm/cmake/modules/HandleLLVMOptions.cmake
===================================================================
--- llvm/cmake/modules/HandleLLVMOptions.cmake
+++ llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -264,7 +264,15 @@
   set(LLVM_USE_LINKER "lld")
 endif()
 
-if( LLVM_USE_LINKER )
+# cmake defaults to invoke a linker directly on Windows, so skip adding
+# '-fuse-ld' flag in this case.
+if( LINKER_IS_LLD_LINK AND ( NOT CMAKE_LINKER OR CMAKE_LINKER MATCHES "lld-link" ))
+  set(IS_FUSE_LD_FLAG_NEEDED FALSE)
+else()
+  set(IS_FUSE_LD_FLAG_NEEDED TRUE)
+endif()
+
+if( LLVM_USE_LINKER AND IS_FUSE_LD_FLAG_NEEDED )
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fuse-ld=${LLVM_USE_LINKER}")
   check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)
Index: clang/CMakeLists.txt
===================================================================
--- clang/CMakeLists.txt
+++ clang/CMakeLists.txt
@@ -745,6 +745,14 @@
     -DCMAKE_ASM_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/${C_COMPILER}
     -DCMAKE_ASM_COMPILER_ID=Clang)
 
+  # cmake requires CMAKE_LINKER to be specified in case of MSVC or it defaults
+  # the linker to be link.exe.
+  if(BOOTSTRAP_LLVM_ENABLE_LLD)
+    if(MSVC AND NOT BOOTSTRAP_CMAKE_SYSTEM_NAME)
+      set(${CLANG_STAGE}_LINKER -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link.exe)
+    endif()
+  endif()
+
   if(BOOTSTRAP_CMAKE_SYSTEM_NAME)
     set(${CLANG_STAGE}_CONFIG -DLLVM_CONFIG_PATH=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-config)
     set(${CLANG_STAGE}_TABLEGEN


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