Ping 2: [PATCH] DAGCombiner: Recognise rotates by X+C

[Richard Sandiford]

Ping
Richard Sandiford <rsandifo at linux.vnet.ibm.com> writes:
> Kay Tiong Khoo <kkhoo at perfwizard.com> writes:
>> Hi Richard,
>>
>> Could this patch be enhanced to recognize this case too:
>> http://llvm.org/bugs/show_bug.cgi?id=17332#c12
>>
>> Maybe it already does? Sorry, I can't test it myself at the moment.
>
> Hi, thanks for the pointer.  The patch didn't handle that case but I agree
> it'd be a good thing to add.
>
> Here's a version that does both.  I've split it into three patches:
>
> (1) is just a refactoring exercise.  It's easier to make changes to this
>     code if there's only one copy of it.  No behavioural change intended.
>
> (2) is my original patch adjusted for (1).  E.g. we now recognise:
>        (or (shl X, (add Y, 10)), (shr X, (sub 22, Y)))
>     as a rotate by Y + 10.
>
> (3) also handles cases where the shift amount is ANDed.  E.g. we now
>     recognise:
>        (or (shl X, Y), (shr X, (and (sub 0, Y), 31)))
>        (or (shl X, Y), (shr X, (and (sub 32, Y), 31)))
>     as rotates by Y.  We can also handle redundant ANDs for (2) too:
>        (or (shl X, (add Y, 10)), (shr X, (and (sub 22, Y), 31)))
>
> OK to commit?
>
> Thanks,
> Richard

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