[PATCH] D116616: [InstSimplify] use knownbits to fold more udiv/urem

Wed Jan 5 06:03:42 PST 2022

spatel marked an inline comment as done.
spatel added a comment.

In D116616#3221740 <https://reviews.llvm.org/D116616#3221740>, @foad wrote:

> It seems like you're using knownbits information to derive range information. It would be good to do this more universally, and in both directions.

Yes - we do have a combination analysis called "computeConstantRangeIncludingKnownBits", so that should be more powerful. 
But that is currently a static helper function in ValueTracking, so it would have to be made visible, and it would be good to have more tests to show where the extra logic gives us a better result. Ok to make that a TODO item?

================
Comment at: llvm/lib/Analysis/InstructionSimplify.cpp:1087-1088
+        computeKnownBits(X, Q.DL, MaxRecurse, Q.AC, Q.CxtI, Q.DT);
+    Optional<bool> IsULT = KnownBits::ult(KnownX, KnownBits::makeConstant(*C));
+    if (IsULT.hasValue() && IsULT.getValue())
+      return true;
----------------
foad wrote:
> Can't you just test `if (KnownX.getMaxValue().ult(*C))`?
Yes, that's shorter and faster. Thanks!

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D116616/new/

https://reviews.llvm.org/D116616