Simple OpenMP program for detecting GPU and other devices does not work - gcc

Question

I'm trying to write a very simple example with OpenMP using GCC 12 as compiler and the target clause in order to detect all my devices, but it does not work. First of all, I have a Lenovo W540 running Debian Sid with an integrated Inted HD Graphics 4600 (which is the GPU the GUI -plasma- is using). Also, there is a NVIDIA GK106GLM [Quadro K2100M]. I have installed the xserver-xorg-video-nvidia-legacy-390xx, nvidia-legacy-390xx-driver, and nvidia-legacy-390xx-kernel-dkms packages, as they are the corresponding drivers for my GPU. Running the lspci -k | grep -A 2 -i "VGA" command I obtain
00:02.0 VGA compatible controller: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller (rev 06)
Subsystem: Lenovo 4th Gen Core Processor Integrated Graphics Controller
Kernel driver in use: i915
--
01:00.0 VGA compatible controller: NVIDIA Corporation GK106GLM [Quadro K2100M] (rev a1)
Subsystem: Lenovo GK106GLM [Quadro K2100M]
Kernel modules: nvidia
so I suppose that the NVDIA GPU is well configured and detected. Am I right?
Then, I borrowed this simply code from https://enccs.github.io/openmp-gpu/target/
/* Copyright (c) 2019 CSC Training */
/* Copyright (c) 2021 ENCCS */
#include <stdio.h>
#ifdef _OPENMP
#include <omp.h>
#endif
int main()
{
int num_devices = omp_get_num_devices();
printf("Number of available devices %d\n", num_devices);
#pragma omp target
{
if (omp_is_initial_device()) {
printf("Running on host\n");
} else {
int nteams= omp_get_num_teams();
int nthreads= omp_get_num_threads();
printf("Running on device with %d teams in total and %d threads in each team\n",nteams,nthreads);
}
}
}
Compilation with GCC as gcc -Wall -fopenmp example.c -o example works fine and does not produce warnings nor errors, but at execution time I obtain simply
Number of available devices 0
Running on host
so not Inter nor NVIDIA GPUs were detected. Inspecting the Debian repositories I've seen and installed the packages gcc-offload-nvptx, gcc-12-offload-nvptx, libgomp-plugin-nvptx1, and nvptx-tools. Now, when I compile again as gcc -Wall -fopenmp example.c -o example I obtain a warning:
/usr/bin/ld: /tmp/ccrWP8gn.crtoffloadtable.o: warning: relocation against `__offload_vars_end' in read-only section `.rodata'
/usr/bin/ld: warning: creating DT_TEXTREL in a PIE
When I execute the code I obtain again
Number of available devices 0
Running on host
but in this case the execution is terrubly slow (various seconds of time).
Searching the web I've seen that I must add the option -foffload=nvptx-none to the compilation order, but I obtain the same results as previously (this option only is recognized if gcc-offload-nvptx et al. are installed.
Running gcc -v I can see that GCC 12 in Debian is configured for offloading:
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
I don't know which number is my NVIDIA card, but I've tried export OFFLOAD_TARGET_DEFAULT=0 and export OFFLOAD_TARGET_DEFAULT=2 with the same wrong result of
Number of available devices 0
Running on host
So, how can I run my OpenMP code in the GPU?
Thanks