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

[Kay Tiong Khoo]

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.


On Thu, Dec 19, 2013 at 10:28 AM, Richard Sandiford <
rsandifo at linux.vnet.ibm.com> wrote:

> Second ping for this patch.
>
> Richard Sandiford <rsandifo at linux.vnet.ibm.com> writes:
> > The SystemZ tests check for rotates by X+10 using:
> >
> >    %add = add i32 %amt, 10
> >    %sub = sub i32 32, %add
> >    %parta = shl i32 %a, %add
> >    %partb = lshr i32 %a, %sub
> >    %or = or i32 %parta, %partb
> >
> > But instcombine turns this into:
> >
> >    %add = add i32 %amt, 10
> >    %sub = sub i32 22, %amt
> >    %parta = shl i32 %a, %add
> >    %partb = lshr i32 %a, %sub
> >    %or = or i32 %parta, %partb
> >
> > so that the subtraction isn't dependent on the addition.  This patch
> extends DAGCombiner to handle this pattern too.
> >
> >
> > http://llvm-reviews.chandlerc.com/D2347
> >
> > Files:
> >   lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> >   test/CodeGen/SystemZ/shift-04.ll
>
> Thanks,
> Richard
>
>
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