[Bug tree-optimization/84648] New: Missed optimization : loop not removed.

[cassio.neri at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84648

            Bug ID: 84648
           Summary: Missed optimization : loop not removed.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cassio.neri at gmail dot com
  Target Milestone: ---

The loop below is not eliminated:

int main() {
    for (unsigned i = 0; i < (1u << 31); ++i) {
    }
    return 0;
}

Compiled with -O3:

main:
  xor eax, eax
.L2:
  add eax, 1
  jns .L2
  xor eax, eax
  ret

The loop is removed for other bounds, e.g. (1u << 31) + 1 or (1u << 31) - 1, or
when < is replaced with <=.

Allow me to make a guess of the underlying problem: The optimization that uses
jns to detect when i reaches (10...0)_2 ends up by blocking the other
optimization that eliminates the loop altoghether.

Same issue when using unsigned long long and (1ull << 63).

FWIW: clang has the same issue (in C but not in C++).