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On Friday, February 1, 2019, Simon Moll <moll@cs.uni-saarland.de> wrote:

We could untie the mask length from the data length:

%result = call @llvm.evl.fsub.v4f32( %x, %y, %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 @llvm.evl.fsub.v4f32( %x, %y, %M, i32 %L)

%result = call @llvm.evl.fsub.v4f32( %x, %y, %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 @llvm.evl.fsub.v4f32( %x, %y, %M, i32 %L)

%result = call @llvm.evl.fsub.v4f32( %x, %y, %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.

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Simon Moll
Researcher / PhD Student
Compiler Design Lab (Prof. Hack)
Saarland University, Computer Science
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Tel. +49 (0)681 302-57521 : moll@cs.uni-saarland.de
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