[cfe-dev] Proposed changes to vectorize_width #pragma (original) (raw)

David Sherwood via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 1 05:46:44 PST 2020

Hi,

So by adding support for scalable vectorisation widths we are effectively updating the pragma to mirror the existing VectorType class in LLVM, which is defined by a ElementCount and an element Type. The ElementCount is a tuple consisting of a minimum number of elements and a scalable flag. The meaning of 'scalable' as used in the vectorize_width pragma as identical to that of ElementCount. Using one of my examples in the initial proposal then this pragma

#pragma clang loop vectorize_width(4, scalable)

would mean the same in LLVM as a VectorType like this:

<vscale x 4 x eltty>

where eltty depends upon the types used in the loop. The 'vscale' parameter is defined by the target - it is at least 1 and does not have to be a power of 2.

Kind Regards, David.

-----Original Message----- From: Michael Kruse <llvm at meinersbur.de> Sent: 30 November 2020 18:01 To: Sjoerd Meijer <Sjoerd.Meijer at arm.com> Cc: David Sherwood <David.Sherwood at arm.com>; cfe-dev at lists.llvm.org; Sander De Smalen <Sander.DeSmalen at arm.com> Subject: Re: [cfe-dev] Proposed changes to vectorize_width #pragma

Am Mi., 25. Nov. 2020 um 10:15 Uhr schrieb Sjoerd Meijer via cfe-dev <cfe-dev at lists.llvm.org>:

I think that leaves us with 2 options that can express the same things, i.e. change or introduce:

1) vectorizewidth(VF, fixed|scalable) vectorizewidth(fixed|scalable) vectorizewidth(VF) 2) vectorizestyle(fixed|scalable)

Another proposal:

  1. vectorize_width(VF) // For fixed vector width. vectorize_width_at_least(MinVF) // For SVE; alternatives: vectorize_dynamic, vectorize_scalable.

What are the intended semantics? Does scalable mean "width of MinVF or more", "any multiple of MinVF", "power-of-2 multiple of MinVF", "any width of at least MinVF allowed by ARM's SVE"?

Michael

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