[LLVMdev] XOR Optimization

Bruno Cardoso Lopes bruno.cardoso at gmail.com
Thu Jul 28 14:56:22 PDT 2011

Hi Daniel,

I'm not the best person to review your patch but a couple of things:

1) Also attach a testcase as in test/Transforms/InstCombine
2) Generate your patch from svn "TOT"
3) Send patches directly to llvm-commits

2011/7/28 Daniel Nicácio <dnicacios at gmail.com>

> Hey guys,
> I still think there is no optimization doing what I want. When the loop is
> unrolled 32 times, llvm is able to identify that the loop is working on a
> whole word, it finds some constants and propagate them, resulting in the
> folded XOR instruction. However, when the loop operates on some bits of the
> word, llvm is still not able to fold those XOR, even when the operated bits
> does not overlap each other.
> Therefore, I am implementing the following optimization for folding XOR
> instructions working on bits (it still must be extended to OR and AND
> instructions). Any comments and critics are appreciated.
> Basically, I try to identify a chain of XOR instructions and fold it. The
> below image illustrate this:
> [image: XORChain.png]
> In order to do that I am adding an additional function call to "visitXor()"
> in Instruction Combining.
> My Optimization function is attached as a patch file. (the diff was made
> using the 2.9 release version of llvm).
> Thanks
> Daniel Nicacio
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Bruno Cardoso Lopes
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