[llvm-dev] [RFC][SVE] Supporting SIMD instruction sets with variable vector lengths

[David A. Greene via llvm-dev]

Graham Hunter <Graham.Hunter at arm.com> writes:

> ========
> 1. Types
> ========
>
> To represent a vector of unknown length a boolean `Scalable` property has been
> added to the `VectorType` class, which indicates that the number of elements in
> the vector is a runtime-determined integer multiple of the `NumElements` field.
> Most code that deals with vectors doesn't need to know the exact length, but
> does need to know relative lengths -- e.g. get a vector with the same number of
> elements but a different element type, or with half or double the number of
> elements.
>
> In order to allow code to transparently support scalable vectors, we introduce
> an `ElementCount` class with two members:
>
> - `unsigned Min`: the minimum number of elements.
> - `bool Scalable`: is the element count an unknown multiple of `Min`?
>
> For non-scalable vectors (``Scalable=false``) the scale is considered to be
> equal to one and thus `Min` represents the exact number of elements in the
> vector.
>
> The intent for code working with vectors is to use convenience methods and avoid
> directly dealing with the number of elements. If needed, calling
> `getElementCount` on a vector type instead of `getVectorNumElements` can be used
> to obtain the (potentially scalable) number of elements. Overloaded division and
> multiplication operators allow an ElementCount instance to be used in much the
> same manner as an integer for most cases.
>
> This mixture of compile-time and runtime quantities allow us to reason about the
> relationship between different scalable vector types without knowing their
> exact length.

How does this work in practice?  Let's say I populate a vector with a
splat.  Presumably, that gives me a "full length" vector.  Let's say the
type is <scalable 2 x double>.  How do I split the vector and get
something half the width?  What is its type?  How do I split it again
and get something a quarter of the width?  What is its type?  How do I
use half- and quarter-width vectors?  Must I resort to predication?

Ths split question comes into play for backward compatibility.  How
would one take a scalable vector and pass it into a NEON library?  It is
likely that some math functions, for example, will not have SVE versions
available.

Is there a way to represent "double width" vectors?  In mixed-data-size
loops it is sometimes convenient to reason about double-width vectors
rather than having to split them (to legalize for the target
architecture) and keep track of their parts early on.  I guess the more
fundamental question is about how such loops should be handled.

What do insertelement and extractelement mean for scalable vectors?
Your examples showed insertelement at index zero.  How would I, say,
insertelement into the upper half of the vector?  Or any arbitrary
place?  Does insertelement at index 10 of a <scalable 2 x double> work,
assuming vscale is large enough?  It is sometimes useful to constitute a
vector out of various scalar pieces and insertelement is a convenient
way to do it.

Thanks for your patience.  :)

                          -David