[PATCH] InstCombine Check if Signed Addition Overflows

[Rafael Espíndola]

Sorry for the delay, I was stuck in the GlobalAlias patches.

Please be sure to address Jingyue Wu comment too.

+    APInt LHSKnownZero(BitWidth, 0, 1);

Since the value is 0, there is no difference in passing isSigned=true.
If you think that is better to be explicit, please write it as

APInt LHSKnownZero(BitWidth, 0, /*isSigned*/ true);

+    // TODO: Handle for Vectors.

The comment is more for the entire function, no just this particular
case. Please move it to the top.

+  if (RHSKnownOne.countPopulation() == 1)

Jingyue Wu comment....

Please don't duplicate the code to handle the symmetry. You can
probably use a small static helper function and write

if (helper(LHSKnowZero, LHSKnowOne, RHSKnownZero, RHSKnownOne))
  return true;

if (helper(RHSKnownZero, RHSKnownOne, LHSKnowZero, LHSKnowOne))
  return true;

+      int LHSZeroPosition = BitWidth - LHSKnownZero.countLeadingZeros() - 1;

Please add a FIXME about this missing the case where LHS has a zero
before the 1 in the RHS, but also has one after.

Cheers,
Rafael