release/19.x: [builtins] Fix missing main() function in float16/bfloat16 support checks (#104478) #106843
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@lntue What do you think about merging this PR to the release branch? |
…ecks (llvm#104478) The CMake docs state that `check_c_source_compiles()` checks whether the supplied code "can be compiled as a C source file and linked as an executable (so it must contain at least a `main()` function)." https://cmake.org/cmake/help/v3.30/module/CheckCSourceCompiles.html In practice, this command is a wrapper around `try_compile()`: - https://gitlab.kitware.com/cmake/cmake/blob/2904ce00d2ed6ad5dac6d3459af62d8223e06ce0/Modules/CheckCSourceCompiles.cmake#L54 - https://gitlab.kitware.com/cmake/cmake/blob/2904ce00d2ed6ad5dac6d3459af62d8223e06ce0/Modules/Internal/CheckSourceCompiles.cmake#L101 When `CMAKE_SOURCE_DIR` is compiler-rt/lib/builtins/, `CMAKE_TRY_COMPILE_TARGET_TYPE` is set to `STATIC_LIBRARY`, so the checks for `float16` and `bfloat16` support work as intended in a Clang + compiler-rt runtime build for example, as it runs CMake recursively from that directory. However, when using llvm/ or compiler-rt/ as CMake source directory, as `CMAKE_TRY_COMPILE_TARGET_TYPE` defaults to `EXECUTABLE`, these checks will indeed fail if the code doesn't have a `main()` function. This results in LLVM using x86 SIMD registers when generating calls to builtins that, with Arch Linux's compiler-rt package for example, actually use a GPR for their argument or return value as they use `uint16_t` instead of `_Float16`. This had been caught in post-commit review: https://reviews.llvm.org/D145237#4521152. Use of the internal `CMAKE_C_COMPILER_WORKS` variable is not what hides the issue, however. PR llvm#69842 tried to fix this by unconditionally setting `CMAKE_TRY_COMPILE_TARGET_TYPE` to `STATIC_LIBRARY`, but it apparently caused other issues, so it was reverted. This PR just adds a `main()` function in the checks, as per the CMake docs. (cherry picked from commit 68d8b38)
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@overmighty (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR. |
Fixed compiler-rt builtins' CMake checks for |
Backport 68d8b38
Requested by: @overmighty