[PATCH] D52146: [InstCombine] foldICmpWithLowBitMaskedVal(): handle ~(-1 << y) mask

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, craig.topper, RKSimon.

Two folds are happening here:

1. https://rise4fun.com/Alive/oaFX
2. And then `foldICmpWithHighBitMask()` (https://reviews.llvm.org/D52001): https://rise4fun.com/Alive/wsP4

This change doesn't just add the handling for eq/ne predicates,
it actually builds upon the previous `foldICmpWithLowBitMaskedVal()` work,
so **all** the 16 fold variants* are immediately supported.

I'm indeed only testing these two predicates.
I do not feel like re-proving all 16 folds*, because they were already proven
for the general case of constant with all-ones in low bits. So as long as
the mask produces all-ones in low bits, i'm pretty sure the fold is valid.

But required, i can re-prove, let me know.

- eq/ult/ule/ugt/uge are commutative - 12 folds; slt/sle/sgt/sge are not commutative - 4 folds. 16 folds in total.

https://bugs.llvm.org/show_bug.cgi?id=38123
https://bugs.llvm.org/show_bug.cgi?id=38708


Repository:
  rL LLVM

https://reviews.llvm.org/D52146

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/canonicalize-low-bit-mask-v2-and-icmp-eq-to-icmp-ule.ll
  test/Transforms/InstCombine/canonicalize-low-bit-mask-v2-and-icmp-ne-to-icmp-ugt.ll

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