[compiler-rt] r352341 - [CMake] Use __libc_start_main rather than fopen when checking for C library

[Hans Wennborg via llvm-commits]

We're seeing breakages of Chromium's compiler-rt build on Mac after
this change: https://bugs.chromium.org/p/chromium/issues/detail?id=925895

I'm not entirely sure how your change would cause the error we're
seeing, but it seems clear that COMPILER_RT_HAS_LIBC was getting set
before, and is set no longer, and I'm guessing that wasn't
intentional. Do you know what might be going on here? Maybe there's
some other symbol that could be used and exists across both glibc and
Mac's libc?

On Sun, Jan 27, 2019 at 11:12 PM Petr Hosek via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
>
> Author: phosek
> Date: Sun Jan 27 20:12:54 2019
> New Revision: 352341
>
> URL: http://llvm.org/viewvc/llvm-project?rev=352341&view=rev
> Log:
> [CMake] Use __libc_start_main rather than fopen when checking for C library
>
> The check_library_exists CMake uses a custom symbol definition. This
> is a problem when checking for C library symbols because Clang
> recognizes many of them as builtins, and returns the
> -Wbuiltin-requires-header (or -Wincompatible-library-redeclaration)
> error. When building with -Werror which is the default, this causes
> the check_library_exists check fail making the build think that C
> library isn't available.
>
> To avoid this issue, we should use a symbol that isn't recognized by
> Clang and wouldn't cause the same issue. __libc_start_main seems like
> reasonable choice that fits the bill.
>
> Differential Revision: https://reviews.llvm.org/D57142
>
> Modified:
>     compiler-rt/trunk/cmake/config-ix.cmake
>
> Modified: compiler-rt/trunk/cmake/config-ix.cmake
> URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/cmake/config-ix.cmake?rev=352341&r1=352340&r2=352341&view=diff
> ==============================================================================
> --- compiler-rt/trunk/cmake/config-ix.cmake (original)
> +++ compiler-rt/trunk/cmake/config-ix.cmake Sun Jan 27 20:12:54 2019
> @@ -12,7 +12,7 @@ function(check_linker_flag flag out_var)
>    cmake_pop_check_state()
>  endfunction()
>
> -check_library_exists(c fopen "" COMPILER_RT_HAS_LIBC)
> +check_library_exists(c __libc_start_main "" COMPILER_RT_HAS_LIBC)
>  if (COMPILER_RT_USE_BUILTINS_LIBRARY)
>    include(HandleCompilerRT)
>    find_compiler_rt_library(builtins COMPILER_RT_BUILTINS_LIBRARY)
>
>
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