-Xclang -fopenmp errors with UnknownFlag.

[ahoereth]

Getting Can't handle UnknownFlag arguments with -Xclang when using OpenMP from brew through find_package(OpenMP REQUIRED) on macOS 14.8.3.

Expected sccache behavior:

• First compilation: Cache miss (expected)
• Second compilation: Cache hit (expected, but fails due to -Xclang flag)

Actual sccache behavior:

• Can't handle UnknownFlag arguments with -Xclang

To reproduce install brew install libomp, then run the "not working" command below. To easily verify the error run sccache --zero-stats before and sccache --show-stats after execution.

Not working:

sccache clang++ -Xclang -fopenmp -I/opt/homebrew/include -L/opt/homebrew/lib -lomp sccache_openmp.cpp -o sccache_openmp

Replacing -Xclang with -Xpreprocessor works around the problem:

sccache clang++ -Xpreprocessor -fopenmp -I/opt/homebrew/include -L/opt/homebrew/lib -lomp sccache_openmp.cpp -o sccache_openmp

sccache_openmp.cpp:

#include <iostream>
#include <omp.h>

int main() {
#pragma omp parallel
  {
    int thread_id   = omp_get_thread_num();
    int num_threads = omp_get_num_threads();

#pragma omp critical
    {
      std::cout << "Thread " << thread_id << " of " << num_threads << " threads"
                << std::endl;
    }
  }

  return 0;
}