[LLVMdev] Extending Vector GEP - proposal

[Nadav Rotem]

> On Mar 2, 2015, at 11:30 PM, Demikhovsky, Elena <elena.demikhovsky at intel.com> wrote:
> 
> > This problem can be solved by sinking the broadcast instruction at codegen-prepare time. 
> I considered this option. We currently don’t have target specific optimizations in codegen-prepare time. (Or I’m wrong?)
> And it will be very X86-directed optimization. Even gather-scatter intrinsics are considered as common for all targets.
>  
> And the second reason, why I’d prefer to generate a splat-GEP, is compile-time saving.
>  I should generate 2 (or more, for each splat element) redundant instructions (broadcast is insert+shuffle), hoist them outside the loop on some stage. Then look for them  on CodeGenPreare pass, sink them back and rebuild the CFG.

Okay. I think that it’s reasonable to add support for GEP with a single base pointer and a vector of indices. 

>  
> -           Elena
>  
> From: Nadav Rotem [mailto:nrotem at apple.com <mailto:nrotem at apple.com>] 
> Sent: Monday, March 02, 2015 19:01
> To: Demikhovsky, Elena
> Cc: llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>; Duncan P. N. Exon Smith; dag at cray.com <mailto:dag at cray.com>; Philip Reames (listmail at philipreames.com <mailto:listmail at philipreames.com>); Hal Finkel (hfinkel at anl.gov <mailto:hfinkel at anl.gov>); Chandler Carruth (chandlerc at gmail.com <mailto:chandlerc at gmail.com>)
> Subject: Re: Extending Vector GEP - proposal
>  
> I don’t have a strong opinion on this. The current GEP syntax is more restrictive and the single base pointer case can be emulated using a broadcast + vector-gep, that can easily be patten matched at codegen time. The problem with the current syntax is that the ‘broadcast’ instruction can be hoisted outside of loops and this can be a problem with our "one block at a time" codegen implementation. This problem can be solved by sinking the broadcast instruction at codegen-prepare time. 
>  
> Is there a strong motivation to prefer one representation over the other?
>  
>  
> On Mar 1, 2015, at 2:10 AM, Demikhovsky, Elena <elena.demikhovsky at intel.com <mailto:elena.demikhovsky at intel.com>> wrote:
>  
> Hi,
>  
> According to the current GEP syntax, vector GEP requires that each index must be a vector with the same number of elements.
>  
> %A = getelementptr <4 x i8*> %ptrs, <4 x i64> %offsets
>  
> I propose to lessen this requirement. Let each index be or vector or scalar. All vector indices must have the same number of elements. The scalar value will mean the splat vector value.
>  
> %A = getelementptr i8* %ptr, <4 x i64> %offsets
> or
> %A = getelementptr <4 x i8*> %ptrs, i64 %offset
>  
> In this case we don’t have to add a “broadcast” before GEP. It is actually will be developer’s decision what form to choose.
> I plan to use vector GEP in gather/scatter and the “broadcasting” of the scalar value impedes to narrow this operation to the “common base, multiple indices” form in the future.
>  
> What do you think?
> Thanks.
>  
> ·         Elena
>  
>  
>  
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