[PATCH][InstCombiner] Expose opportunities to merge subtract and comparison

[Evan Cheng]

On Aug 27, 2013, at 12:34 PM, Quentin Colombet <qcolombet at apple.com> wrote:

> 
> ** Notes **
> 
> 1. Why LLVM IR for this “low-level” optimization?
> As already stated, several architectures expose such opportunities, therefore, I thought it may be best to do it as a target independent optimization. LLVM IR makes more sense for that. Doing this at MI IR level would require several additional target hooks or a specific pass for each target.

I'm not sure about this. Instcombine's primary responsibility is canonicalization. Is it possible for this to pessimize code on certain targets? Have you considered doing this at codegenprep time?

Evan

> 
> 2. What about the canonical form?
> The optimization is only performed when both operands have the same complexity.
> We might want to break the complexity assumption (operands ordered from most to less complex) but I am not sure it will bring new opportunities.
> Thus, assuming we want to do that transformation at LLVM IR level, is it desirable to break that assumption and if yes, in which pass?
> 
> Cheers,
> -Quentin
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> <InstCombineCSE.svndiff>
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