[compiler-rt] [builtins] Fix missing main() function in float16/bfloat16 support checks (PR #104478)

[via llvm-commits]

https://github.com/overmighty created https://github.com/llvm/llvm-project/pull/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://github.com/Kitware/CMake/blob/2904ce00d2ed6ad5dac6d3459af62d8223e06ce0/Modules/CheckCSourceCompiles.cmake#L54
- https://github.com/Kitware/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 expected 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. On Arch
Linux, this results in Clang using x86 SIMD registers when calling
builtins that, with compiler-rt, actually use GPRs 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.


>From 120229c2bba0139432545f2e877dd603eaf7aacd Mon Sep 17 00:00:00 2001
From: OverMighty <its.overmighty at gmail.com>
Date: Mon, 12 Aug 2024 21:18:48 +0200
Subject: [PATCH] [builtins] Fix missing main() function in float16/bfloat16
 support checks

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://github.com/Kitware/CMake/blob/2904ce00d2ed6ad5dac6d3459af62d8223e06ce0/Modules/CheckCSourceCompiles.cmake#L54
- https://github.com/Kitware/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 expected 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. On Arch
Linux, this results in Clang using x86 SIMD registers when calling
builtins that, with compiler-rt, actually use GPRs 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.
---
 compiler-rt/lib/builtins/CMakeLists.txt | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/compiler-rt/lib/builtins/CMakeLists.txt b/compiler-rt/lib/builtins/CMakeLists.txt
index 13adbd6c4d57d9..2c3b0fa84a4782 100644
--- a/compiler-rt/lib/builtins/CMakeLists.txt
+++ b/compiler-rt/lib/builtins/CMakeLists.txt
@@ -868,10 +868,12 @@ else ()
           endif()
         endif()
       endif()
-      check_c_source_compiles("_Float16 foo(_Float16 x) { return x; }"
+      check_c_source_compiles("_Float16 foo(_Float16 x) { return x; }
+                               int main(void) { return 0; }"
                               COMPILER_RT_HAS_${arch}_FLOAT16)
       append_list_if(COMPILER_RT_HAS_${arch}_FLOAT16 -DCOMPILER_RT_HAS_FLOAT16 BUILTIN_CFLAGS_${arch})
-      check_c_source_compiles("__bf16 foo(__bf16 x) { return x; }"
+      check_c_source_compiles("__bf16 foo(__bf16 x) { return x; }
+                               int main(void) { return 0; }"
                               COMPILER_RT_HAS_${arch}_BFLOAT16)
       # Build BF16 files only when "__bf16" is available.
       if(COMPILER_RT_HAS_${arch}_BFLOAT16)