[Bug middle-end/105345] New: [OpenMP] Wrong iteration loop count when mixing signed and unsigned

["burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105345

            Bug ID: 105345
           Summary: [OpenMP] Wrong iteration loop count when mixing signed
                    and unsigned
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: openmp, wrong-code
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: frederik at gcc dot gnu.org, jakub at gcc dot gnu.org
  Target Milestone: ---

The following program has a loop count of 0 without OpenMP (-fno-openmp)
but is executed 1010 times with -fopenmp in GCC.


The problem is that the following is not honored:

"If var has a signed integer type and the var operand of test-expr after usual
 arithmetic conversions has an unsigned integer type then the loop iteration
count
 is computed from lb, test-expr and incr using an unsigned integer type
corresponding
 to the type of var."  (OpenMP 5.2, 4.4.2 OpenMP Loop-Iteration Spaces and
Vectors)
 (Likewise: OpenMP 5.1, 2.11.1 Canonical Loop Nest Form)

The wording seems to be new since OpenMP 5.1 - and to ensure that the OpenMP
and C loop counts are the same:


int
main ()
{
  #pragma omp for
  for (int i = -1000; i < 10u; ++i) {
    __builtin_printf("%d\n", i);
    __builtin_abort ();
  } 
}