[PATCH] D128123: [SDAG] try to replace subtract-from-constant with xor

[Sanjay Patel via Phabricator via llvm-commits]

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


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Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:3766
+  // If there's no chance any bit will need to borrow from an adjacent bit:
+  // sub C, X --> xor X, C
+  if (ConstantSDNode *C0 = isConstOrConstSplat(N0)) {
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barannikov88 wrote:
> It might be more profitable to do this transformation in SimplifyDemandedBits. Some bits of the result might not be used downstream, in which case the check can be relaxed.
> 
The known-bits vs. demanded-bits transforms are two different things if I'm seeing it correctly, although there can be some overlap. Demanded bits may already be used to shrink the constants, so that could enable this transform. 

If you have an example in mind that doesn't transform as expected, would you please file an issue, so we can track the fix? If it's not working here in codegen, it may also be missing from the IR equivalent. 

I updated the IR side to match this patch with:
79bb915fb60b


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D128123/new/

https://reviews.llvm.org/D128123