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

[Quentin Colombet]

On Aug 27, 2013, at 1:30 PM, Evan Cheng <evan.cheng at apple.com> wrote:

> 
> 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?
Not that I am aware of, but I guess it might be possible.
Do you think we should somehow guard this transformation with a target hook?

> Have you considered doing this at codegenprep time?
No, I have not, and I think you are right, it would make more sense there.

Thanks Evan.

-Quentin
> 
> 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
>> <scratch.cc>
>> <InstCombineCSE.svndiff>
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