[Bug libgomp/106490] New: loop counter overflow within omp

[Bug libgomp/106490] New: loop counter overflow within omp

[dxin at usc dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106490

            Bug ID: 106490
           Summary: loop counter overflow within omp
           Product: gcc
           Version: og11 (devel/omp/gcc-11)
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dxin at usc dot edu
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

Created attachment 53390
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53390&action=edit
minimum test case

gcc's omp implementation seems to use a signed integer type for the loop
counter. As a result, if the user's code use an unsigned loop variable with a
large loop count, the internal loop counter overflows and the loop body never
run. 

This is bug only happens when omp is used, i.e. the test case runs as expected
if the omp pragma is removed.

gcc (Ubuntu 11.2.0-19ubuntu1) 11.2.0

---------------------test case begin------------------------
#include <stdio.h>
#include <limits.h>
#include"omp.h"

//const unsigned int I_LIMIT=(UINT_MAX)>>1;// good, printf runs for a while 
const unsigned int I_LIMIT=UINT_MAX;// bad case, crash before any printf

int main()
{
    #pragma omp parallel for
    for(unsigned int i=0;i<=I_LIMIT;i++){
        printf("i=%u,i=%f\n",i,(float)(i));
    }
    return 0;
}
---------------------test case end------------------------
gcc test_omp_overflow.c -fopenmp
./a.out

[Bug middle-end/106490] loop counter overflow within omp

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106490

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|libgomp                     |middle-end
           Keywords|                            |openmp
            Version|og11 (devel/omp/gcc-11)     |12.1.0

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Does the OMP standard allow never ending loops?  I think OMP computes the
number of iterations but that doesn't fit an uint here.  Using UINT_MAX-1 as
upper bound works as expected.