[all-commits] [llvm/llvm-project] 6d7e42: [builtins] Fix missing main() function in float16/...

[OverMighty via All-commits]

  Branch: refs/heads/release/19.x
  Home:   https://github.com/llvm/llvm-project
  Commit: 6d7e428df611861fb1f5151dea938ebfcc7b1363
      https://github.com/llvm/llvm-project/commit/6d7e428df611861fb1f5151dea938ebfcc7b1363
  Author: OverMighty <its.overmighty at gmail.com>
  Date:   2024-09-01 (Sun, 01 Sep 2024)

  Changed paths:
    M compiler-rt/lib/builtins/CMakeLists.txt

  Log Message:
  -----------
  [builtins] Fix missing main() function in float16/bfloat16 support checks (#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 #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 68d8b3846ab1e6550910f2a9a685690eee558af2)



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