[PATCH] [InstCombineAddSub] Added the transformation - "sub (or A B) (xor A B) -> (and A B)"

David Majnemer david.majnemer at gmail.com
Sat Oct 18 17:20:06 PDT 2014


On Sat, Oct 18, 2014 at 10:07 AM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Sat, Oct 18, 2014 at 2:47 AM, David Majnemer <david.majnemer at gmail.com>
> wrote:
>
>> InstCombine is not supposed to handle every possible just because it is
>> theoretically possible case.  It's engineered to be a series of engineering
>> tradeoffs.  I don't know what the history of the similar transform is in
>> VisitAdd, it's possible that it wasn't added in a principled way.
>>
>
> IIRC I did a cursory look at the blame and it was actually you who showed
> up for the corresponding transform in VisitAdd. Or maybe you were
> committing a patch for someone? Probably worth taking a look at the log to
> see if you remember.
>

I believe it was reduced from the Linux kernel.  Frankly, I fail to see the
relevance to whether or not we should accept this patch.


>
> -- Sean Silva
>
>
>>
>> If we were to arbitrarily add patterns, we would eventually make
>> InstCombine cripplingly slow.
>>
>> http://reviews.llvm.org/D5719
>>
>>
>>
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>>
>
>
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