[llvm] f28e48f - [cmake] Don't try creating an executable when detecting the linker

[Louis Dionne via llvm-commits]

Author: Louis Dionne
Date: 2022-06-08T14:36:54-04:00
New Revision: f28e48f3ef5aad3552a21f858aa501b4139a6f69

URL: https://github.com/llvm/llvm-project/commit/f28e48f3ef5aad3552a21f858aa501b4139a6f69
DIFF: https://github.com/llvm/llvm-project/commit/f28e48f3ef5aad3552a21f858aa501b4139a6f69.diff

LOG: [cmake] Don't try creating an executable when detecting the linker

On most platforms, the linker detection command that we run ends up being
something like `clang++ -Wl,-v` or `clang++ -Wl,--version`. This usually
fails with a missing reference to `_main` because we don't have any input
file. However, when compiling for a target that is implicitly freestanding,
the invocation actually succeeds and a dummy `a.out` file is created in
the current working directory. This is extremely annoying because it
creates a `a.out` file at the root of the monorepo when running CMake
configuration from the root.

Differential Revision: https://reviews.llvm.org/D125827

Added: 
    

Modified: 
    llvm/cmake/modules/AddLLVM.cmake

Removed: 
    


################################################################################
diff  --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index 64d719f2626c9..ffc5d84227831 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -177,11 +177,17 @@ if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
     set(version_flag "-Wl,--version")
   endif()
 
+  if (CMAKE_HOST_WIN32)
+    set(DEVNULL "NUL")
+  else()
+    set(DEVNULL "/dev/null")
+  endif()
+
   if(LLVM_USE_LINKER)
-    set(command ${CMAKE_C_COMPILER} -fuse-ld=${LLVM_USE_LINKER} ${version_flag})
+    set(command ${CMAKE_C_COMPILER} -fuse-ld=${LLVM_USE_LINKER} ${version_flag} -o ${DEVNULL})
   else()
     separate_arguments(flags UNIX_COMMAND "${CMAKE_EXE_LINKER_FLAGS}")
-    set(command ${CMAKE_C_COMPILER} ${flags} ${version_flag})
+    set(command ${CMAKE_C_COMPILER} ${flags} ${version_flag} -o ${DEVNULL})
   endif()
   execute_process(
     COMMAND ${command}