[PATCH] D112298: [InstCombine] Generalize sadd.sat combine to compute sign bits.

[Dave Green via Phabricator via llvm-commits]

dmgreen added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp:2319-2322
+  unsigned NeededSignBits =
+      AddSub->getType()->getScalarSizeInBits() - NewBitWidth + 1;
+  if (ComputeNumSignBits(AddSub->getOperand(0), 0, AddSub) < NeededSignBits ||
+      ComputeNumSignBits(AddSub->getOperand(1), 0, AddSub) < NeededSignBits)
----------------
lebedev.ri wrote:
> dmgreen wrote:
> > lebedev.ri wrote:
> > > I've spent more time trying to reason about this, which means this can be improved.
> > > Please add `ComputeMinSignedBits()` wrapper next to `ComputeNumSignBits()`,
> > > and use it here. Then this becomes
> > > ```
> > >   if (ComputeMinSignedBits(AddSub->getOperand(0), 0, AddSub) > NewBitWidth ||
> > >       ComputeMinSignedBits(AddSub->getOperand(1), 0, AddSub) > NewBitWidth)
> > > ```
> > > which at least to me is immensely more readable.
> > Sounds good. Do you mean next to the ComputeNumSignBits in InstCombiner, or next to the base variant in ValueTracking?
> > 
> > I added it to InstCombiner for the moment, but can easily move it if you think that is better.
> Eh, probably both, instcombine-one should use the valuetracking-one.
OK will do.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D112298/new/

https://reviews.llvm.org/D112298