[PATCH] Fix for wrong instcombine on vector insert/extract

[Andrea_DiBiagio at sn.scee.net]

Oops. I just noticed that my last email didn't thread correctly.
I am reattaching an updated patch against the latest revision for 
completeness.



Sorry for the spam.
Andrea  Di Biagio

Andrea DiBiagio/SN R&D/BS/UK/SCEE wrote on 03/04/2013 13:06:44:

> From: Andrea DiBiagio/SN R&D/BS/UK/SCEE
> To: llvm-commits at cs.uiuc.edu
> Date: 03/04/2013 13:06
> Subject: Re: [PATCH] Fix for wrong instcombine on vector insert/extract
> 
> Friendly Ping.
> 
> Andrea DiBiagio/SN R&D/BS/UK/SCEE wrote on 22/03/2013 19:53:00:
> 
> > From: Andrea DiBiagio/SN R&D/BS/UK/SCEE
> > To: llvm-commits at cs.uiuc.edu
> > Date: 22/03/2013 19:53
> > Subject: [PATCH] Fix for wrong instcombine on vector insert/extract
> > 
> > Hello,
> > 
> > I would like to contribute a fix for a wrong transformation performed 
> > by the Instruction Combiner on sequences of 
insertelement/extractelement 
> > instructions.
> > 
> > When trying to collapse sequences of insertelement/extractelement
> > instructions into single shuffle instructions, there is one specific
> > case where the Instruction Combiner wrongly updates the resulting
> > Mask of shuffle indexes.
> > 
> > The problem is in function CollectShuffleElments.
> > 
> > If we have a sequence of insert/extract element instructions
> > like the one below:
> > 
> >   %tmp1 = extractelement <4 x float> %LHS, i32 0
> >   %tmp2 = insertelement <4 x float> %RHS, float %tmp1, i32 1
> >   %tmp3 = extractelement <4 x float> %RHS, i32 2
> >   %tmp4 = insertelement <4 x float> %tmp2, float %tmp3, i32 3
> > 
> > Where:
> >  . %RHS will have a mask of [4,5,6,7]
> >  . %LHS will have a mask of [0,1,2,3] 
> > 
> > The Mask of shuffle indexes is wrongly computed to [4,1,6,7]
> > instead of [4,0,6,7].
> > When analyzing %tmp2 in order to compute the Mask for the 
> > resulting shuffle instruction, the algorithm forgets to update
> > the mask index at position 1 with the index associated to the
> > element extracted from %LHS by instruction %tmp1.
> > 
> > The patch in attachment fixes the problem with the wrong
> > propagation of shuffle indexes.
> > It also adds two extra cases to test 
> > Transforms/InstCombine/vec_shuffle.ll to check that now we
> > do the correct thing when collapsing sequences of insert/extract
> > element instruction in that problematic scenario.
> > 
> > Thanks!
> > Andrea Di Biagio
> > 


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