[llvm] r188315 - DAG: Combine (and (setne X, 0), (setne X, -1)) -> (setuge (add X, 1), 2)

[Eli Friedman]

On Tue, Aug 13, 2013 at 3:09 PM, Jim Grosbach <grosbach at apple.com> wrote:

>
> On Aug 13, 2013, at 2:39 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>
> On Tue, Aug 13, 2013 at 2:30 PM, Jim Grosbach <grosbach at apple.com> wrote:
>
>> Author: grosbach
>> Date: Tue Aug 13 16:30:58 2013
>> New Revision: 188315
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=188315&view=rev
>> Log:
>> DAG: Combine (and (setne X, 0), (setne X, -1)) -> (setuge (add X, 1), 2)
>>
>> A common idiom is to use zero and all-ones as sentinal values and to
>> check for both in a single conditional ("x != 0 && x != (unsigned)-1").
>> That generates code, for i32, like:
>>   testl %edi, %edi
>>   setne %al
>>   cmpl  $-1, %edi
>>   setne %cl
>>   andb  %al, %cl
>>
>> With this transform, we generate the simpler:
>>   incl  %edi
>>   cmpl  $1, %edi
>>   seta  %al
>>
>> Similar improvements for other integer sizes and on other platforms. In
>> general, combining the two setcc instructions into one is better.
>>
>> rdar://14689217
>>
>>
> We already have code in InstCombiner::FoldAndOfICmps to handle this sort
> of thing; it looks like it isn't catching this particular case for some
> reason, though.
>
>
> There’s already a bunch of similar checks in the DAGCombiner, too. I
> mainly put it there to that we’ll be able to catch more complicate cases
> that simplify to this one from other DAG transformations, that are exposed
> from target lowerings, etc..
>

I'm not convinced it will actually show up from lowering in practice.


> Also, the transformation depends on the wrapping behavior of the add.
> While we can express that in IR, it feels a bit more appropriate to do that
> sort of change at the DAG level.
>

I don't follow.  Addition has the same semantics in IR and SelectionDAG.


> Do either of you have a strong preference for InstCombine instead? This
> isn’t the sort of thing where we have a definitive “right place” to put the
> transform, really.
>

Considering we already have code which catches "(x  !=  0) & (x != 1)", we
should probably catch "(x != 0) & (x != -1)" there as well.

-Eli
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