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

[Robin Kruppe via llvm-dev]

Hi Graham,
Hi David,

glad to hear other people are thinking about RVV codegen!

On 7 June 2018 at 18:10, Graham Hunter <Graham.Hunter at arm.com> wrote:
>
> Hi,
>
> > On 6 Jun 2018, at 17:36, David A. Greene <dag at cray.com> wrote:
> >
> > Graham Hunter via llvm-dev <llvm-dev at lists.llvm.org> writes:
> >>> Is there anything about vscale or a scalable vector that requires a
> >>> minimum bit width?  For example, is this legal?
> >>>
> >>> <scalable 1 x double>
> >>>
> >>> I know it won't map to an SVE type.  I'm simply curious because
> >>> traditionally Cray machines defined vectors in terms of
> >>> machine-dependent "maxvl" with an element type, so with the above vscale
> >>> would == maxvl.  Not that we make any such things anymore.  But maybe
> >>> someone else does?
> >>
> >> That's legal in IR, yes, and we believe it should be usable to represent the vectors for
> >> RISC-V's 'V' extension. The main problem there is that they have a dynamic vector
> >> length within the loop so that they can perform the last iterations of a loop within vector
> >> registers when there's less than a full register worth of data remaining. SVE uses
> >> predication (masking) to achieve the same effect.


Yes, <scalable 1 x T> should be allowed in the IR type system (even <1
x T> is currently allowed and unlike the scalable variant that's not
even useful) and it would be the sole legal vector types in the RISCV
backend.

>
> >> For the 'V' extension, vscale would indeed correspond to 'maxvl', and I'm hoping that a
> >> 'setvl' intrinsic that provides a predicate will avoid the need for modelling a change in
> >> dynamic vector length -- reducing the vector length is effectively equivalent to an implied
> >> predicate on all operations. This avoids needing to add a token operand to all existing
> >> instructions that work on vector types.

Yes, vscale would be the *maximum* vector length (how many elements
fit into each register), not the *active* vector length (how many
elements are operated on in the current loop iteration).

This has nothing to do with tokens, though. The tokens I proposed were
to encode the fact that even 'maxvl' varies on a function by function
basis. This RFC approaches the same issue differently, but it's still
there -- in terms of this RFC, operations on scalable vectors depend
on `vscale`, which is "not necessarily [constant] across functions".
That implies, for example, that an unmasked <scalable 4 x i32> load or
store (which accesses vscale * 16 bytes of memory) can't generally be
moved from one function to another unless it's somehow ensured that
both functions will have the same vscale. For that matter, the same
restriction applies to calls to `vscale` itself.

The evolution of the active vector length is a separate problem and
one that doesn't really impact the IR type system (nor one that can
easily be solved by tokens).

> >
> > Right.  In that way the RISC V method is very much like what the old
> > Cray machines did with the Vector Length register.
> >
> > So in LLVM IR you would have "setvl" return a predicate and then apply
> > that predicate to operations using the current select method?  How does
> > instruction selection map that back onto a simple setvl + unpredicated
> > vector instructions?
> >
> > For conditional code both vector length and masking must be taken into
> > account.  If "setvl" returns a predicate then that predicate would have
> > to be combined in some way with the conditional predicate (typically via
> > an AND operation in an IR that directly supports predicates).  Since
> > LLVM IR doesn't have predicates _per_se_, would it turn into nested
> > selects or something?  Untangling that in instruction selection seems
> > difficult but perhaps I'm missing something.
>
> My idea is for the RISC-V backend to recognize when a setvl intrinsic has
> been used, and replace the use of its value in AND operations with an
> all-true value (with constant folding to remove unnecessary ANDs) then
> replace any masked instructions (generally loads, stores, anything else
> that might generate an exception or modify state that it shouldn't) with
> target-specific nodes that understand the dynamic vlen.

I am not quite so sure about turning the active vector length into
just another mask. It's true that the effects on arithmetic, load,
stores, etc. are the same as if everything executed under a mask like
<1, 1, ..., 1, 0, 0, ..., 0> with the number of ones equal to the
active vector length. However, actually materializing the masks in the
IR means the RISCV backend has to reverse-engineer what it must do
with the vl register for any given (masked or unmasked) vector
operation. The stakes for that are rather high, because (1) it applies
to pretty much every single vector operation ever, and (2) when it
fails, the codegen impact is incredibly bad.

(1) The vl register affects not only loads, stores and other
operations with side effects, but all vector instructions, even pure
computation (and reg-reg moves, but that's not relevant for IR). An
integer vector add, for example, only computes src1[i] + src2[i] for 0
<= i < vl and the remaining elements of the destination register (from
vl upwards) are zeroed. This is very natural for strip-mined loops
(you'll never need those elements), but it means an unmasked IR level
vector add is a really bad fit for the RISC-V 'vadd' instruction.
Unless the backend can prove that only the first vl elements of the
result will ever be observed, it will have to temporarily set vl to
MAXVL so that the RVV instruction will actually compute the "full"
result. Establishing that seems like it will require at least some
custom data flow analysis, and it's unclear how robust it can be made.

(2) Failing to properly use vl for some vector operation is worse than
e.g. materializing a mask you wouldn't otherwise need. It requires
that too (if the operation is masked), but more importantly it needs
to save vl, change it to MAXVL, and finally restore the old value.
That's quite expensive: besides the ca. 3 extra instructions and the
scratch GPR required, this save/restore dance can have other nasty
effects depending on uarch style. I'd have to consult the hardware
people to be sure, but from my understanding risks include pipeline
stalls and expensive roundtrips between decoupled vector and scalar
units.

To be clear: I have not yet experimented with any of this, so I'm not
saying this is a deal breaker. A well-engineered "demanded elements"
analysis may very well be good enough in practice. But since we
broached the subject, I wanted to mention this challenge. (I'm
currently side stepping it by not using built-in vector instructions
but instead intrinsics that treat vl as magic extra state.)

> This could be part of lowering, or maybe a separate IR pass, rather than ISel.
> I *think* this will work, but if someone can come up with some IR where it
> wouldn't work then please let me know (e.g. global-state-changing instructions
> that could move out of blocks where one setvl predicate is used and into one
> where another is used).

There are some operations that use vl for things other than simple
masking. To give one example, "speculative" loads (which silencing
some exceptions to safely permit vectorization of some loops with
data-dependent exits, such as strlen) can shrink vl as a side effect.
I believe this can be handled by modelling all relevant operations
(including setvl itself) as intrinsics that have side effects or
read/write inaccessible memory. However, if you want to have the
"current" vl (or equivalent mask) around as SSA value, you need to
"reload" it after any operation that updates vl. That seems like it
could get a bit complex if you want to do it efficiently (in the
limit, it seems equivalent to SSA construction).

>
>
> Unfortunately, I can't find a description of the instructions included in
> the 'V' extension in the online manual (other than setvl or configuring
> registers), so I can't tell if there's something I'm missing.

I'm very sorry for that, I know how frustrating it can be. I hope the
above gives a clearer picture of the constraints involved. Exact
instructions, let alone encodings, are still in flux as Bruce said.


Cheers,
Robin