[llvm-dev] [RFC] Vector Predication

[Jacob Lifshay via llvm-dev]

On Thu, Feb 7, 2019, 09:21 Simon Moll <moll at cs.uni-saarland.de> wrote:

>
> On 2/7/19 6:08 PM, David Greene wrote:
> > Jacob Lifshay <programmerjake at gmail.com> writes:
> >
> >> So it would be handy for the vector length on evl intrinsics to be in
> >> units of the mask length so we don't have to pattern match a division
> >> in the backend. We could have 2 variants of the vector length
> >> argument, one in terms of the data vector and one in terms of the mask
> >> vector. we could legalize the mask vector variant for those
> >> architectures that need it by pulling the multiplication out and
> >> switching to the data vector variants.
> > Would it make sense to have two different intrinsics?
> >
> > # "Normal" form, L is in terms of flat vector length.
> > <scalable 2 x float> evl.fsub(<scalable 2 x float> %x,
> >                                <scalable 2 x float> %y,
> >                                <scalable 2 x i1> %M, i32 %L)
> >
> > # "Sub-vector" form, L is in terms of sub-vectors elements.
> > <scalable 1 x <2 x float>> evl.fsub(<scalable 1 x <2 x float>> %x,
> >                                      <scalable 1 x <2 x float>> %y,
> >                                      <scalable 1 x <2 x i1>> %M, i32 %L
> >
> > Overloading types to mean two very different things is confusing to me.
> >
> >                             -David
>
> Allowing vector types as vector elements would solve the vlen
> interpretation issue in an elegant way.
>
This seems like the best solution, though maybe out of scope of the current
evl proposal. This would also create a decent IR representation of a matrix
or other 2D vector: matMxN would be <N x <M x float>>.
While we're adding this, it may be a good idea to go all in and just lift
the restriction on vectors containing other non-scalable vectors, so you
could do something like: <scalable 1 x <4 x <4 x float>>> to be a scalable
vector of 4x4 matrices.
all the standard arithmetic operations (fadd, fmul, etc.) would still be
the standard operations (so fmul would be element-wise, not matrix
multiplication), we would, as a separate proposal, add matrix
multiplication as a separate intrinsic.

Most operations would treat a <scalable I x <J x <K x <L x ... type ...>>>>
as a <scalable I x <(J * K * L * ...) x type>> for everything but result
type.

We could have methods on Type that return the next level down vector type,
the scalar type under the vector type nest (probably getScalarType), and
the number of total scalar elements (getVectorNumElementsRecursive)
assuming VL is the first valid non-zero integer: so <scalable I x <J x <K x
<L x ... type ...>>>> returns I * J * K * L * ...
We will probably want to cache the innermost scalar type and the number of
total scalar elements. Since we want to avoid adding another field to Type,
we could have vector types store the getVectorNumElementsRecursive  instead
of the getVectorNumElements value, for getVectorNumElements we would divide
the stored value by the stored value of the next level down.

There would be a pass to convert vector kinds for different targets.

>
> - Simon
>
> --
>
> 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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