Problems with gcc 7 and 8 (debian) in OpenMP offloading to nvptx - c

Question

I installed both gcc-7, gcc-8, gcc-7-offload-nvptx and gcc-8-offload-nvptx
I tried with both to compile a simple OpenMP code with offloading:
#include <omp.h>
#include <stdio.h>
int main(){
#pragma omp target
#pragma omp teams distribute parallel for
for (int i=0; i<omp_get_num_threads(); i++)
printf("%d in %d of %d\n",i,omp_get_thread_num(), omp_get_num_threads());
}
With the following line (with gcc-7 too):
gcc-8 code.c -fopenmp -foffload=nvptx-none
But it doesn't compile, giving the following error:
/tmp/ccKESWcF.o: In function "main":
teste.c:(.text+0x50): undefined reference to "GOMP_target_ext"
/tmp/cc0iOH1Y.target.o: In function "init":
ccPXyu6Y.c:(.text+0x1d): undefined reference to "GOMP_offload_register_ver"
/tmp/cc0iOH1Y.target.o: In function "fini":
ccPXyu6Y.c:(.text+0x41): undefined reference to "GOMP_offload_unregister_ver"
collect2: error: ld returned 1 exit status
some clues?

Answer 1

You code compiles and runs for me using -foffload=disable -fno-stack-protector with gcc7 and gcc-7-offload-nvptx and Ubuntu 17.10.
But on the GPU (without -foffload=disable) it fails to compile. You can't call printf from the GPU. Instead you can do this:
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
int main(){
int nthreads;
#pragma omp target teams map(tofrom:nthreads)
#pragma omp parallel
#pragma omp single
nthreads = omp_get_num_threads();
int *ithreads = malloc(sizeof *ithreads *nthreads);
#pragma omp target teams distribute parallel for map(tofrom:ithreads[0:nthreads])
for (int i=0; i<nthreads; i++) ithreads[i] = omp_get_thread_num();
for (int i=0; i<nthreads; i++)
printf("%d in %d of %d\n", i, ithreads[i], nthreads);
free(ithreads);
}
For me this outputs
0 in 0 of 8
1 in 0 of 8
2 in 0 of 8
3 in 0 of 8
4 in 0 of 8
5 in 0 of 8
6 in 0 of 8
7 in 0 of 8