[llvm-dev] [RFC] Vector Predication

[Simon Moll via llvm-dev]

On 2/2/19 1:39 AM, Luke Kenneth Casson Leighton wrote:
>
>
> On Friday, February 1, 2019, Simon Moll <moll at cs.uni-saarland.de 
> <mailto:moll at cs.uni-saarland.de>> wrote:
>
>     We could untie the mask length from the data length:
>
>       %result = call <scalable 4 x float>
>     @llvm.evl.fsub.v4f32(<scalable 4 x float> %x, <scalable 4 x float>
>     %y, <scalable 1 x i1> %M, i32 %L)
>
>     would then indicate the mask %M applies to groups of "4 / 1" float
>     elements.
>
>
> That would provide the greatest flexibility, as a 1:1 ratio could mean 
> 1 bit per element, covering the normal case.
>
> Question: are there any circumstances under which it is desirable to 
> underspecify or overspecify the number of bits in the predicate?
>
> ie to deliberately have a FP vector of length 11 and a mask of length 
> 9 or 13?

You are referring to the sub-vector sizes, if i am understanding 
correctly. I'd assume that the mask sub-vector length always has to be 
either 1 or the same as the data sub-vector length. For example, this is ok:

    %result = call <scalable 3 x float> @llvm.evl.fsub.v4f32(<scalable 3 
x float> %x, <scalable 3 x float> %y, <scalable 1 x i1> %M, i32 %L)

    %result = call <scalable 5 x float> @llvm.evl.fsub.v4f32(<scalable 5 
x float> %x, <scalable 5 x float> %y, <scalable 1 x i1> %M, i32 %L)

    %result = call <16 x float> @llvm.evl.fsub.v4f32(<16 x float> %x, <4 
x float> %y, <4 x i1> %M, i32 %L)

This is invalid IR:

    %result = call <scalable 4 x float> @llvm.evl.fsub.v4f32(<scalable 4 
x float> %x, <scalable 4 x float> %y, <scalable 2 x i1> %M, i32 %L)

    %result = call <scalable 11 x float> @llvm.evl.fsub.v4f32(<scalable 
11 x float> %x, <scalable 11 x float> %y, <scalable 9 x i1> %M, i32 %L)

    %result = call <5 x float> @llvm.evl.fsub.v4f32(<5 x float> %x, <5 x 
float> %y, <7 x i1> %M, i32 %L)


In case you are talking about the dynamic vector length (eg what happens 
if the dynamic length's don't match at runtime), i think the key here is 
to regard the vector length parameter "vlen %L" as a contract: the 
semantics of the EVL operation is undefined if the runtime lengths of 
the vectors are shorter than indicated by %L. That is the mask has a 
minimum element count of %L * mask sub-vector length, the data has a 
minimum element count of %L * data sub-vector length.

- Simon

>
> Or, is that just a runtime error.
>
> L.
>
>
> -- 
> ---
> crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
>
-- 

Simon Moll
Researcher / PhD Student

Compiler Design Lab (Prof. Hack)
Saarland University, Computer Science
Building E1.3, Room 4.31

Tel. +49 (0)681 302-57521 : moll at cs.uni-saarland.de
Fax. +49 (0)681 302-3065  : http://compilers.cs.uni-saarland.de/people/moll

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