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

[Eli Friedman]

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.

-Eli
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