Meson fails to configure OpenMP when using Clang from MacPorts

[lpsinger]

Describe the bug
Meson fails to detect OpenMP with Clang from MacPorts.

To Reproduce

1. Install MacPorts.

2. Install the MacPorts version of Clang by running sudo port install clang-18.

3. Place the following in the file meson.build:

   project('example', 'c')
   dependency('openmp')

4. Configure the project, and observe the failure:

   $ CC=clang-mp-18 meson setup build
   The Meson build system
   Version: 1.4.0
   Source dir: /private/tmp
   Build dir: /private/tmp/build
   Build type: native build
   Project name: example
   Project version: undefined
   C compiler for the host machine: clang-mp-18 (clang 18.1.4 "clang version 18.1.4")
   C linker for the host machine: clang-mp-18 ld64 1053.12
   Host machine cpu family: x86_64
   Host machine cpu: x86_64
   WARNING: OpenMP found but omp.h missing.
   Run-time dependency OpenMP found: NO (tried system)
   
   meson.build:2:0: ERROR: Dependency "openmp" not found, tried system
   
   A full log can be found at /private/tmp/build/meson-logs/meson-log.txt
   

Here is the log file: meson-log.txt

The check that is failing is a test compilation of the following testfile.c, compiled with the command clang-mp-18 testfile.c -E -P -P -O0 -Werror=implicit-function-declaration, i.e., without the command line option -fopenmp:

#ifdef __has_include
  #if !__has_include("omp.h")
    #error "Header 'omp.h' could not be found"
  #endif
#else
  #include <omp.h>
#endif

This test would have succeeded if Meson had added the flag -fopenmp; in some compilers such as this one, omp.h is not findable unless the command-line flag -fopenmp is present.

Expected behavior
Meson should add the -fopenmp flag when doing this test, and the test should succeed.

system parameters

• Is this a cross build or just a plain native build (for the same computer)? native build
• what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.) macOS Sonoma
• what Python version are you using 3.11.9
• what meson --version 1.4.0
• what ninja --version if it's a Ninja build 1.11.1.git.kitware.jobserver-1

[dcbaker]

If the compiler is identifying itself as an Apple compiler (ie, if Apple is in the output of clang --version), then Meson considers OpenMP unsupported. See: #7435 for more information.

[lpsinger]

Indeed, it does:

$ clang-mp-18 --version
clang version 18.1.4
Target: x86_64-apple-darwin23.4.0
Thread model: posix
InstalledDir: /opt/local/libexec/llvm-18/bin

[dcbaker]

That looks like we're incorrectly identifying a "vanilla" clang as an "apple" clang. The latter of which really means "the Clang Apple ships with XCode", so we should fix our detection code to detect that as a vanilla clang and then OpenMP should just work.

edit: Strangly though we look for "Apple" (Case sensitive) in the output, and the output you have is reporting "apple", so OpenMP should just work