OpenMP canonical loop form

[Thomas Schwinge]

Hi!

OpenMP defines a canonical loop form (in OpenMP 4: »2.6 Canonical Loop
Form«, in OpenMP 3.1 as part of »2.5.1 Loop Construct«) that says that
the loop index variable »must not be modified during the execution of the
for-loop other than in incr-expr«.  The following code, which violates
this when modifying i in the loop body, thus isn't a conforming program,
and GCC may then exhibit unspecified behavior.  Instead of accepting it
silently, I wonder if it makes sense to have GCC detect this violation
and warn about the unspecified behavior, or even turn it into a hard
error?

    #include <omp.h>
    #include <stdio.h>

    int
    main(void)
    {
    #pragma omp parallel
    #pragma omp for
      for (int i = 0; i < 20; i += 2)
        {
          printf("%d: #%d\n", omp_get_thread_num(), i);

          /* Violation of canonical loop form.  */
          --i;
        }

      return 0;
    }

    2: #8
    2: #9
    0: #0
    0: #1
    0: #2
    0: #3
    3: #10
    3: #11
    1: #4
    1: #5
    1: #6
    1: #7
    6: #16
    6: #17
    4: #12
    4: #13
    5: #14
    5: #15
    7: #18
    7: #19


Grüße,
 Thomas

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