[LLVMdev] [LoopVectorizer] Missed vectorization opportunities caused by sext/zext operations

[Sanjoy Das]

For

void test0(unsigned short a, unsigned short *  in, unsigned short * out) {
  for (unsigned short w = 1; w < a - 1; w++) //this will never overflow
      out[w] = in[w+7] * 2;
}

I think it will be sufficient to add a couple of new cases to
ScalarEvolution::HowManyLessThans --

  zext(A) ult zext(B) == A ult B
  sext(A) slt sext(B) == A slt B

Currently it bails out if it sees a non-add recurrence on the LHS of
the ult/slt.

You could also teach ScalarEvolution::isImpliedCondOperands the
following:

  zext(A) ult zext(B) <=> A ult B
  sext(A) slt sext(B) <=> A slt B

to get more aggressive promotion from ext{A,+,B} to {ext A,+,ext B}.

I'll be happy to review these patches.

-- Sanjoy