[PATCH] D89214: [X86] Add basic computeKnownBits support for X86ISD::BSR

Mon Oct 12 10:41:24 PDT 2020

craig.topper added a comment.

In D89214#2325139 <https://reviews.llvm.org/D89214#2325139>, @RKSimon wrote:

> Also, can BSF be handled here as well?

Technically yes, we just didn't have test coverage for it. Probably because we only use X86ISD::BSF when we immediately emit a CMOV for CTTZ and we need to connect the flag output. For CTTZ_ZERO_UNDEF we match BSF in isel patterns.

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Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:34045
+    // TODO: Bound with input known bits?
+    Known.Zero.setBitsFrom(Log2_32(BitWidth));
+    break;
----------------
RKSimon wrote:
> I'm a bit worried about not handling the src==0 undef case - how well does this work if we guarded it with a KnownNeverZero check?
The case I was trying to fix was this cross basic block case from the OPTIMIZE version of https://skanthak.homepage.t-online.de/llvm.html#case21

I doubt KnownNeverZero would work since its control flow dependent and it looks like the SelectionDAG implementation is only handles non-zero constants or an OR involving a non-zero constant.

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  https://reviews.llvm.org/D89214/new/

https://reviews.llvm.org/D89214