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From: "kmccarty at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/51360] New: spurious unused-but-set-variable warning for var used in OpenMP pragma
Date: Wed, 30 Nov 2011 17:20:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D51360

             Bug #: 51360
           Summary: spurious unused-but-set-variable warning for var used
                    in OpenMP pragma
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: kmccarty@gmail.com


Hi,

gcc 4.6.2 produces a spurious unused variable warning for the following cod=
e.=20
The variable 'num' is used in setting the number of threads within an OpenMP
parallel region but the compiler does not realize this.

The warning is:

% /usr/local/gcc46/bin/gcc -Wall -fopenmp omp.c
omp.c: In function =E2=80=98main=E2=80=99:
omp.c:12:12: warning: variable =E2=80=98num=E2=80=99 set but not used
[-Wunused-but-set-variable]

% cat omp.c
#include <omp.h>
#include <stdlib.h>
#include <stdio.h>

// Compile with: gcc -Wall -fopenmp omp.c
// Run with:     ./a.out <num_threads> e.g. "./a.out 4"

int main(int argc, char ** argv) {
    if (argc !=3D 2) return EXIT_FAILURE;

    int vec[20] =3D { 0, };
    int i, num =3D atoi(argv[1]);

    #pragma omp parallel for num_threads(num)
    for (i =3D 0; i < 20; ++i) {
        #pragma omp critical
        {
            printf("thread %d\n", (int)omp_get_thread_num());
            fflush(stdout);
        }

        vec[i] *=3D 5;
    }

    return 0;
}


Running the generated binary (RHEL 4 update 6 on x86_64) with a specified v=
alue
for argv[1] indicates that the value of 'num' is being used as expected.

gcc 4.2.4 and 4.4.3 do not warn with the same compiler flags.

Thanks,
- Kevin B. McCarty