OpenMP pragma translation to runtime calls - gcc

Question

I wrote a short program in C with OpenMP pragma, and I need to know to which libGOMP function a pragma is translated by GCC.
Here is my marvelous code:
#include <stdio.h>
#include "omp.h"
int main(int argc, char** argv)
{
int k = 0;
#pragma omp parallel private(k) num_threads(4)
{
k = omp_get_thread_num();
printf("Hello World from %d !\n", k);
}
return 0;
}
In order to generate intermediate language from GCC v8.2.0, I compiled this program with the following command:
gcc -fopenmp -o hello.exe hello.c -fdump-tree-ompexp
And the result is given by:
;; Function main (main, funcdef_no=0, decl_uid=2694, cgraph_uid=0, symbol_order=0)
OMP region tree
bb 2: gimple_omp_parallel
bb 3: GIMPLE_OMP_RETURN
Added new low gimple function main._omp_fn.0 to callgraph
Introduced new external node (omp_get_thread_num/2).
Introduced new external node (printf/3).
;; Function main._omp_fn.0 (main._omp_fn.0, funcdef_no=1, decl_uid=2700, cgraph_uid=1, symbol_order=1)
main._omp_fn.0 (void * .omp_data_i)
{
int k;
<bb 6> :
<bb 3> :
k = omp_get_thread_num ();
printf ("Hello World from %d !\n", k);
return;
}
;; Function main (main, funcdef_no=0, decl_uid=2694, cgraph_uid=0, symbol_order=0)
Merging blocks 2 and 7
Merging blocks 2 and 4
main (int argc, char * * argv)
{
int k;
int D.2698;
<bb 2> :
k = 0;
__builtin_GOMP_parallel (main._omp_fn.0, 0B, 4, 0);
D.2698 = 0;
<bb 3> :
<L0>:
return D.2698;
}
The function call to "__builtin_GOMP_parallel" is what it interest me. So, I looked at the source code of the libGOMP from GCC.
However, the only function calls I found was (from parallel.c file):
GOMP_parallel_start (void (*fn) (void *), void *data, unsigned num_threads)
GOMP_parallel_end (void)
So, I can imiagine that, in a certain manner, the call to "__builtin_GOMP_parallel" is transformed to GOMP_parallel_start and GOMP_parallel_end.
How can I be sure of this assumption ? How can I found the translation from the builtin function to the two other ones I found in the source code ?
Thank you

Answer 1

You almost got it. __builtin_GOMP_parallel is just a compiler alias to GOMP_parallel (defined in omp-builtins.def) which is translated very late in compilation, you can see the actual call in the assembly with gcc -S.
GOMP_parallel is similar to
GOMP_parallel_start(...);
fn(...);
GOMP_parallel_end();