[PATCH] D25221: [PPC][DAGCombine] Convert SETCC to subtract when the result is zero extended

[Ehsan Amiri via llvm-commits]

amehsan created this revision.
amehsan added reviewers: hfinkel, kbarton, nemanjai.
amehsan added a subscriber: llvm-commits.

The code here can be extended. I post this patch right now, to get some feedback before adding those extensions.

When we see a SETCC whose only users are zero extend operations, we can replace it with a subtraction. This results in doing all calculation in GPRs and avoids CR use.

Extensions that I plan to look into:

1- Check if this works for any_extend/sign extend
2- Implement similar change for signed condition codes.
3- Look into handling equal and not equal cases, using cntlz
4- More tests should be added

Please let me know if there are other ways for extending this.


https://reviews.llvm.org/D25221

Files:
  lib/Target/PowerPC/PPCISelLowering.cpp
  lib/Target/PowerPC/PPCISelLowering.h
  test/CodeGen/PowerPC/setcc-to-sub.ll

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