[PATCH] D117508: [SDAG] add demanded bits transform for bswap

[Sanjay Patel via Phabricator via llvm-commits]

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A possible codegen regression for PowerPC is noted in D117406 <https://reviews.llvm.org/D117406> because we don't recognize a pattern that demands a single byte from a bswap.

I was wondering if we have that fold in IR, and it has existed there since close to the beginning of LLVM:
https://github.com/llvm/llvm-project/blame/main/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp#L794
...so this patch copies that code as much as possible and adapts it for SDAG.

The test for PowerPC is over-reduced with undefs, so I made it valid again by passing in pointer args and renamed the values to make the logic clearer.


https://reviews.llvm.org/D117508

Files:
  llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
  llvm/test/CodeGen/AArch64/bswap-known-bits.ll
  llvm/test/CodeGen/X86/combine-bswap.ll

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