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[SCEV] Don't always add no-wrap flags to post-inc add recs

Description

[SCEV] Don't always add no-wrap flags to post-inc add recs

Fixes PR27315.

The post-inc version of an add recurrence needs to "follow the same
rules" as a normal add or subtract expression. Otherwise we miscompile
programs like

int main() {
  int a = 0;
  unsigned a_u = 0;
  volatile long last_value;
  do {
    a_u += 3;
    last_value = (long) ((int) a_u);
    if (will_add_overflow(a, 3)) {
      // Leave, and don't actually do the increment, so no UB.
      printf("last_value = %ld\n", last_value);
      exit(0);
    }
    a += 3;
  } while (a != 46);
  return 0;
}

This patch changes SCEV to put no-wrap flags on post-inc add recurrences
only when the poison from a potential overflow will go ahead to cause
undefined behavior.

To avoid regressing performance too much, I've assumed infinite loops
without side effects is undefined behavior to prove poison<->UB
equivalence in more cases. This isn't ideal, but is not new to LLVM as
a whole, and far better than the situation I'm trying to fix.

Details

Committed
sanjoyMay 28 2016, 5:32 PM
Parents
rL271150: [ValueTracking] ICmp instructions propagate poison
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