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

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp:2316-2317
 
+  // The two operands of the add/sub need to contain at least enough sign bits
+  // to be valid in the NewBitWidth. This is usually achieved via a sext from a
+  // smaller type.
----------------



================
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)
----------------
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.


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

https://reviews.llvm.org/D112298