[llvm-dev] Regarding ScalarEvolution's loop backedge computation

[Chawla, Pankaj via llvm-dev]

Hi,

It looks like ScalarEvolution bails out of loop backedge computation if it cannot prove the IV stride as either positive or negative (based on loop control condition). I think this logic can be refined for signed IVs.

Consider this simple loop-

void foo(int *A, int n, int s) {

  int i;

  for(i=0; i<n; i += s) {
    A[i]++;
  }
}

The IV of this loop has this SCEV form-
{0,+,%s}<nsw><%for.body>

Can someone please clarify why it is not ok to deduce the stride to be positive based on the assumption that the IV cannot have a signed underflow due to the presence of the NSW flag otherwise the program has undefined behavior?


Thanks,
Pankaj




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