[Bug middle-end/116751] New: GCC trunk (-O3) doesn't optimize a loop that can be folded into a constant

[Bug middle-end/116751] New: GCC trunk (-O3) doesn't optimize a loop that can be folded into a constant

[dccitaliano at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116751

            Bug ID: 116751
           Summary: GCC trunk (-O3) doesn't optimize a loop that can be
                    folded into a constant
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dccitaliano at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/qq1bbdMKn

```
#include <stdint.h>

int64_t patatino() {
    int64_t result = 0;
    for (int r = 0; r < 7; r++) {
        for (int s = 0; s < 10; s++) {
            for (int t = 0; t < 10; t++) {
                if (t + s + r == 10) {
                    result += 1;
                }
            }
        }
    }
    return result;
}
```

Changing the condition from `t + s + r == 10` to `t + s == 10` changes the
generated code into:

```
patatino():
        mov     eax, 63
        ret
```

[Bug tree-optimization/116751] GCC trunk (-O3) doesn't optimize a loop that can be folded into a constant

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116751

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |15.0
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
          Component|middle-end                  |tree-optimization
   Last reconfirmed|                            |2024-09-18
           Keywords|                            |missed-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Unroll heuristic is (too) easily confused here I guess.  Note we need to unroll
all loops completely to be able to optimize this.