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

[Evan Cheng]

On Aug 27, 2013, at 1:41 PM, Quentin Colombet <qcolombet at apple.com> wrote:

> 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.

Actually ignore me. After reading your original email more carefully I think instcombine is ok.

Evan

> 
> 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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