linker - c - Linking a PGI OpenACC-enabled library with gcc

Question

Briefly speaking, my question relies in between compiling/building files (using libraries) with two different compilers while exploiting OpenACC constructs in source files.

I have a C source file that has an OpenACC construct. It has only a simple function that computes total sum of an array:

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

double calculate_sum(int n, double *a) {
    double sum = 0;
    int i;

    printf("Num devices: %d
", acc_get_num_devices(acc_device_nvidia));

    #pragma acc parallel copyin(a[0:n])
    #pragma acc loop
    for(i=0;i<n;i++) {
        sum += a[i];
    }

    return sum;
}

I can easily compile it using following line:

pgcc -acc -ta=nvidia -c libmyacc.c

Then, create a static library by following line:

ar -cvq libmyacc.a libmyacc.o

To use my library, I wrote a piece of code as following:

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

#define N 1000

extern double calculate_sum(int n, double *a);

int main() {
    printf("Hello --- Start of the main.
");
    double *a = (double*) malloc(sizeof(double) * N);
    int i;
    for(i=0;i<N;i++) {
        a[i] = (i+1) * 1.0;
    }

    double sum = 0.0;
    for(i=0;i<N;i++) {
        sum += a[i];
    }
    printf("Sum: %.3f
", sum);


    double sum2 = -1;
    sum2 = calculate_sum(N, a);
    printf("Sum2: %.3f
", sum2);

    return 0;
}

Now, I can use this static library with PGI compiler itself to compile above source (f1.c):

pgcc -acc -ta=nvidia f1.c libmyacc.a

And it will execute flawlessly. However, it differs for gcc. My question relies in here. How can I built it properly with gcc?

Thanks to Jeff's comment on this question: linking pgi compiled library with gcc linker, now I can build my source file (f1.c) without errors, but the executable file emits some fatal errors.

This is what I use to compile my source file with gcc (f1.c):

gcc f1.c -L/opt/pgi/linux86-64/16.5/lib -L/usr/lib64 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5 -L. -laccapi -laccg -laccn -laccg2 -ldl -lcudadevice -lpgmp -lnuma -lpthread -lnspgc -lpgc -lm -lgcc -lc -lgcc -lmyacc

This is the error:

Num devices: 2
Accelerator Fatal Error: No CUDA device code available

---

Thanks to -v option when compiling f1.c with PGI compiler, I see that the compiler invokes so many other tools from PGI and NVidia (like pgacclnk and nvlink).

---

My questions:

1. Am I on the wrong path? Can I call functions in PGI compiled libraries from GCC and use OpenACC within those functions?
2. If answer to above is positive, can I use still link without steps (calling pgacclnk and nvlink) that PGI takes?
3. If answer to above is positive too, what should I do?

Answer 1

Add "-ta=tesla:nordc" to your pgcc compilation. By default PGI uses runtime dynamic compilation (RDC) for the GPU code. However RDC requires an extra link step (with nvlink) that gcc does not support. The "nordc" sub-option disables RDC so you'll be able to use OpenACC code in a library. However by disabling RDC you can no longer call external device routines from a compute region.

% pgcc -acc -ta=tesla:nordc -c libmyacc.c
% ar -cvq libmyacc.a libmyacc.o
a - libmyacc.o
% gcc f1.c -L/proj/pgi/linux86-64/16.5/lib -L/usr/lib64 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5 -L. -laccapi -laccg -laccn -laccg2 -ldl -lcudadevice -lpgmp -lnuma -lpthread -lnspgc -lpgc -lm -lgcc -lc -lgcc -lmyacc
% a.out
Hello --- Start of the main.
Sum: 500500.000
Num devices: 8
Sum2: 500500.000

Hope this helps, Mat