[libcxx-commits] [PATCH] D145716: [CMake] Unify llvm_check_linker_flag and llvm_check_compiler_linker_flag

[Martin Storsjö via Phabricator via libcxx-commits]

mstorsjo added a comment.

I took a look at what's going wrong here; I diffed the cmake output from a cmake run before and after this change:

  @@ -17,12 +17,12 @@
   -- Performing Test LLVM_RUNTIMES_LINKING_WORKS
   -- Performing Test LLVM_RUNTIMES_LINKING_WORKS - Success
   -- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG
  --- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG - Failed
  +-- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG - Success
   -- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG
   -- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG - Failed
   -- Using Release VC++ CRT: MD
   -- Performing Test SUPPORTS_BREPRO
  --- Performing Test SUPPORTS_BREPRO - Success
  +-- Performing Test SUPPORTS_BREPRO - Failed
   -- Looking for os_signpost_interval_begin
   -- Looking for os_signpost_interval_begin - not found
   -- Found Python3: /usr/bin/python3.10 (found version "3.10.6") found components: Interpreter
  @@ -33,8 +33,6 @@
   -- Using libc++ testing configuration: /home/martin/code/llvm-project/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in
   -- Performing Test CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG
   -- Performing Test CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG - Failed
  --- Performing Test C_SUPPORTS_NODEFAULTLIBS_FLAG
  --- Performing Test C_SUPPORTS_NODEFAULTLIBS_FLAG - Failed
   -- Performing Test C_SUPPORTS_COMMENT_LIB_PRAGMA
   -- Performing Test C_SUPPORTS_COMMENT_LIB_PRAGMA - Failed
   -- Performing Test CXX_SUPPORTS_FALIGNED_ALLOCATION_FLAG
  @@ -44,71 +42,65 @@
   -- Performing Test CXX_SUPPORTS_FVISIBILITY_EQ_HIDDEN_FLAG
   -- Performing Test CXX_SUPPORTS_FVISIBILITY_EQ_HIDDEN_FLAG - Failed
   -- Performing Test CXX_SUPPORTS_W4_FLAG
  --- Performing Test CXX_SUPPORTS_W4_FLAG - Success
  +-- Performing Test CXX_SUPPORTS_W4_FLAG - Failed
   -- Performing Test CXX_SUPPORTS_WEXTRA_FLAG

I.e. this change makes `CXX_SUPPORTS_NOSTDLIBXX_FLAG` suddenly succeed where it was failing before. This causes all further checks to fail.

The reason for this is ... complex. When running a compile+link test with a non-MSVC compiler, you first compile the source file with `$CC` (where `CMAKE_C_FLAGS` and `CMAKE_REQUIRED_FLAGS` are included), then you link it with `$CC`, where `CMAKE_REQUIRED_FLAGS` and `CMAKE_EXE_LINKER_FLAGS` are included.

When doing a compile+link test with a MSVC style compiler, you compile it with `$CC` with the same flags as above. But then cmake does the linking by invoking link.exe/lld-link directly, where it only includes `CMAKE_EXE_LINKER_FLAGS`.

In the case of `-nostdlibc++`, clang-cl warns about the warning being unknown, but at least lld-link seems to accept the flag silently. Before, when we were adding the flag to `CMAKE_REQUIRED_FLAGS`, the test compile+link itself succeeded, but it was still deemed a failure due to it producing `clang-16: warning: unknown argument ignored in clang-cl: '-nostdlib++' [-Wunknown-argument]`. Now after this refactoring, the flag is only added to `CMAKE_EXE_LINKER_FLAGS`, where it doesn't produce any warning. But if this test succeeded, in `runtimes/CMakeLists.txt` we go on to add the flag to `CMAKE_REQUIRED_FLAGS`, where it later breaks every future test.

I don't have the whole picture in my mind right now, but I get the feeling that if we're planning to add `-nostdlib++` to `CMAKE_REQUIRED_FLAGS`, we also should be using a test that actually tests setting the flag in `CMAKE_REQUIRED_FLAGS`.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D145716/new/

https://reviews.llvm.org/D145716