[llvm-dev] [RFC] Matrix support (take 2)

[David Greene via llvm-dev]

Simon Moll <moll at cs.uni-saarland.de> writes:

>> How will existing passes be taught about the new intrinsics?  For
>> example, what would have to be done to instcombine to teach it about
>> these intrinsics?  Let's suppose every existing operation had an
>> equivalent masked intrinsic.  Would it be easier to teach all of the
>> passes about them or would it be easier to teach the passes about a mask
>> operand on the existing Instructions?  Would it be easier to teach isel
>> about all the intrinsics or would it be easier to teach isel about a
>> mask operand?
>
> Consider that over night we introduce optional mask parameters to
> vector instructions. Then, since you can not safely ignore the mask,
> every transformation and analysis that is somehow concerned with
> vector instructions is potentially broken and needs to be fixed.

True, but is there a way we could do this incrementally?  Even if we
start with intrinsics and then migrate to first-class support, at some
point passes are going to be broken with respect to masks on
Instructions.

> If you go with masking intrinsics, and set the attributes right, it is
> clear that transformations won't break your code and you will need to
> teach InstCombine, DAGCombiner, etc that a `masked.fadd` is just an
> fadd` with a mask. However, this gives you the opportunity to
> "re-enable" one optimization add a time each time making sure that the
> mask is handled correctly. In case of InstCombine, the vector
> instruction patterns transfer to mask intrinsics: if all mask
> intrinsics in the pattern have the same mask parameter you can apply
> the transformation, the resulting mask intrinsics will again take the
> same mask parameter.

Right.

> Also, this need not be a hard transition from vector instructions to
> masking intrinsics.. you can add new types of masking intrinsics in
> batches along with the required transformations. Masking intrinsics
> and vector instruction can live side by side (as they do today,
> anyway).

Of course.


>> I honestly don't know the answers to these questions.  But I think they
>> are important to consider, especially if intrinsics are seen as a bridge
>> to first-class IR support for masking.
>
> I think its sensible to use masking intrinsics (or EVL
> https://reviews.llvm.org/D53613) on IR level and masked SD nodes in
> the backend. However, i agree that intrinsics should just be a bridge
> to native support mid term.

The biggest question I have is how such a transition would happen.
Let's say we have a full set of masking intrinsics.  Now we want to take
one IR-level operation, say fadd, and add mask support to it.  How do we
do that?  Is it any easier because we have all of the intrinsics, or
does all of the work on masking intrinsics get thrown away at some
point?

I'm reminded of this now decade-old thread on gather/scatter and masking
from Don Gohman, which I also mentioned in an SVE thread earlier this
year:

https://lists.llvm.org/pipermail/llvm-dev/2008-August/016284.html

The applymask idea got worked through a bit and IIRC at some later point
someone found issues with it that need to be addressed, but it's an
interesting idea to consider.  I wasn't too hot on it at the time but it
may be a way forward.

In that thread, Tim Foley posted a summary of options for mask support,
one of which was adding intrinsics:

https://lists.llvm.org/pipermail/llvm-dev/2008-August/016371.html

                                -David