[Bug tree-optimization/115697] New: Miscompilation with -fgraphite-identity at -O2

[michael at orlitzky dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115697

            Bug ID: 115697
           Summary: Miscompilation with -fgraphite-identity at -O2
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michael at orlitzky dot com
  Target Milestone: ---

While debugging a test failure in the flint library
(https://github.com/flintlib/flint/issues/2029), we seem to have discovered a
miscompilation. Fredrik Johansson was able to come up with a minimal
reproducer. This may be a duplicate of bug 47048, but having more reproducible
test cases never hurts.

$ gcc-15 -O2 -Wall -Wextra main.c 
$ ./a.out 
6 4 1

$ gcc-15 -O2 -fgraphite-identity -Wall -Wextra main.c 
$ ./a.out 
3 2 1

$ cat main.c 
#include <stdio.h>

void
shift(long * c, long n)
{
    long i, j;

    if (n <= 62)
    {
        for (i = n - 2; i >= 0; i--)
            for (j = i; j < n - 1; j++)
                c[j] += c[j + 1];
    }
}

int main(void)
{
    long c[3];
    volatile long n = 3;
    c[0] = 3;
    c[1] = 2;
    c[2] = 1;
    shift(c, n);
    printf("%ld %ld %ld\n", c[0], c[1], c[2]);
}