compiler-rt fails to configure for mips 32 bit due to definitions being seen as CMake options

[DavidSpickett]

This issue was seen in https://lab.llvm.org/staging/#/builders/214/builds/5185, info copied here for reference.

Using the following configuration:

cmake -G Ninja ../llvm/llvm -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=True '-DLLVM_LIT_ARGS='"'"'-v'"'"'' -DCMAKE_INSTALL_PREFIX=../stage1.install -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_PARALLEL_LINK_JOBS=4 -DLLVM_TARGETS_TO_BUILD=Mips '-DLLVM_ENABLE_PROJECTS=llvm;clang;compiler-rt;clang-tools-extra'

compiler-rt tries to detect a multilib build of mips64 and mips32 and it does so using these flags:

    elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "mips")
      # FIXME: Ideally, we would build the N32 library too.
      if("${COMPILER_RT_MIPS_EL}" AND ("${COMPILER_RT_MIPS32R6}" OR "${COMPILER_RT_MIPS64R6}"))
        test_target_arch(mipsel "" "-mips32r6" "-mabi=32" "-D_LARGEFILE_SOURCE" "-D_FILE_OFFSET_BITS=64")
        test_target_arch(mips64el "" "-mips64r6" "-mabi=64")

This is passed down to https://cmake.org/cmake/help/v3.20/module/CheckCSourceCompiles.html which then errors like this:

-- Performing Test COMPILER_RT_HAS_mipsel_BFLOAT16
CMake Error: Parse error in command line argument: _LARGEFILE_SOURCE
 Should be: VAR:type=value
CMake Error at /usr/share/cmake-3.25/Modules/Internal/CheckSourceCompiles.cmake:94 (try_compile):
  Failed to configure test project build system.
Call Stack (most recent call first):
  /usr/share/cmake-3.25/Modules/CheckCSourceCompiles.cmake:76 (cmake_check_source_compiles)
  /var/lib/buildbot/workers/debian-tritium-mips64el/clang-mips64el-linux/llvm/compiler-rt/lib/builtins/CMakeLists.txt:810 (check_c_source_compiles)

I believe this is because definitions -D.... are passed as flags instead of using CMAKE_REQUIRED_DEFINITIONS to check_c_source_compiles.

Then somehow CMake thinks it's a CMake option not a compiler option. This may have worked in previous versions of CMake but it definitely doesn't with version 3.20.

You can reproduce this issue on another architecture by adding a -DSOMETHING to the architecture test flags.

The workaround for this is to build only for one architecture by adding the CMake options:

-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON -DCMAKE_C_COMPILER_TARGET="<your target triple>"

(https://reviews.llvm.org/D147598 improves the error reporting in that situation but does not fix this issue overall)

[llvmbot]

@llvm/issue-subscribers-backend-mips

[DavidSpickett]

Note that the configuration is not using the runtimes build, I do not know if that has the same issue.

In general the runtimes build is the "modern" way to be doing this, so prefer that if possible.