Question

I’m just getting started experimenting adding OpenMP to some SSE code.

My first test program SOMETIMES crashes in _mm_set_ps, but works when I set the if (0).

It looks so simple I must be missing something obvious.
I’m compiling with gcc -fopenmp -g -march=core2 -pthreads

  #include <stdio.h>
  #include <stdlib.h>
  #include <immintrin.h>

  int main()
  {
  #pragma omp parallel if (1)
   {
  #pragma omp sections
       {
  #pragma omp section
           {
              __m128 x1 = _mm_set_ps ( 1.1f, 2.1f, 3.1f, 4.1f );
           }
  #pragma omp section
           {
              __m128 x2 = _mm_set_ps ( 1.2f, 2.2f, 3.2f, 4.2f );
           }
       } // end omp sections
   } //end omp parallel

  return 0;
  }

Answer 1

This is a bug in the openMP implementation. I was having the same problem in gcc on Windows (MinGW). -mstackrealign command line option solved my problem. This adds an instruction to the prolog of every function to realign the stack at the 16-byte boundary. I didn’t notice any performance penalty. You can also try to add __attribute__ ((force_align_arg_pointer)) to a function declaration, which should do the same, but only for a specific function. You might have to put the SSE code in a separate function that you then call from the function with #pragma omp, so that the stack has a chance to be realigned.

I stopped having the problem when I moved onto compiling for a 64-bit target (MinGW64, such as TDM GCC build).

I am playing with AVX instructions which require a 32-byte alignment, but GCC doesn’t support that for windows at all. This forced me to fix the produced assembly code using a python script, but it works.