[llvm-dev] [RFC] Vector Predication

Fri Feb 1 03:57:38 PST 2019

On Fri, 1 Feb 2019 at 01:04, Philip Reames <listmail at philipreames.com>
wrote:

>
> On 1/31/19 1:14 PM, Robin Kruppe wrote:
>
>
>
> On Thu, 31 Jan 2019 at 20:17, Philip Reames via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>> On 1/31/19 11:03 AM, David Greene wrote:
>> > Philip Reames <listmail at philipreames.com> writes:
>> >
>> >> Question 1 - Why do we need separate mask and lengths? Can't the
>> >> length be easily folded into the mask operand?
>> >>
>> >> e.g. newmask = (<4 x i1>)((i4)%y & (1 << %L -1))
>> >> and then pattern matched in the backend if needed
>> > I'm a little concerned about how difficult it will be to maintain enough
>> > information throughout compilation to be able to match this on a machine
>> > with an explicit vector length value.
>> Does the hardware *also* have a mask register?  If so, this is a likely
>> minor code quality issue which can be incrementally refined on.  If it
>> doesn't, then I can see your concern.
>>
>
> Masking/predication is supported nearly universally, but I don't think the
> code quality issue is minor. It would be on a typical packed-SIMD machine
> with 128/256/512 bit registers, but the processors with a vector length
> register are usually built with much larger registers files and without a
> corresponding increase in the number of functional units. For example, 4096
> bit per vector register is really quite modest for this kind of machine,
> while the data path can reasonable be "only" 128 or 256 bit.
>
> This changes the calculus quite a bit: vector lengths much shorter or
> minimally larger than one full register are suddenly reasonable common (in
> application code, not so much in HPC kernels) and because each vector
> instruction is split into many data-path-sized uops, it's trivial and very
> rewarding to cut processing short halfway through a vector. The efficiency
> of "short vector code" then depends on the ability to finish each operation
> on those short vectors relatively quickly rather than padding everything to
> a full vector register.
>
> For example, if a loop with a trip count of 20 is vectorized on a machine
> with 64 elements per vector (that's 64b elements in a 4096b register, so
> this is lowballing it!), using only masks and not the vector length
> register makes your vector unit do about three times more work than it
> would have to if you set the vector length register to 20. That keeps the
> register file and functional units busy for no good reason. Some
> microarchitectures take on the burden of determining when a whole chunk of
> the vector is masked out and can then skip over it quickly, but many others
> don't. So you're likely burning a whole bunch of power and quite possibly
> taking up cycles that could be filled with useful work from other
> instructions instead.
>
> Thank you for the explanation.
>
> Do such architectures frequently have arithmetic operations on the mask
> registers?  (i.e. can I reasonable compute a conservative length given a
> mask register value)  If I can, then having a mask as the canonical form
> and re-deriving the length register from a mask for a sequence of
> instructions which share a predicate seems fairly reasonable.
>
A mask is frequently too large to reasonably treat it as an integer, but
"find the index of the first mask bit that is 0" or some variant of it is a
common instruction. However, using it in the way you suggest can be very
expensive. Long vector operations have very high latency (easily tens of
cycles) until all result lanes are available (this doesn't stall later
lane-wise operations thanks to chaining). In the code pattern you describe,
all following vector operations have to stall until the "find first zero in
the mask" operation is complete, plus roundrip latency from the vector unit
to the scalar control logic and back. If the vector length is large, this
means having to stall until the entire mask is computed (and then some),
and so you stall for tens of cycles.

And it might be just as bad for short vector lengths. It's easy to imagine
that "find first zero in mask" instruction short-circuits and finishes as
soon as any 0 bit is encountered, but my understanding is that this would
require extra control logic, so I can also easily imagine hardware
designers not going there (because they might expect it to not matter on
the workloads they care about). But this is a microarchitectural detail
that I can't make sweeping statements about, I would have to poll designers
to give an answer even just about RISC-V vector units currently in
development.

> The only case where the combination of a CKB and dynamic mask->length
> fallback wouldn't handle reliably is when we have a mask loaded from an
> external source (memory, function call boundary, etc...) and a short
> sequence of vector ops.  Are such really common enough that it needs to be
> a first class element of the design?
>
As Hideki said, masks passed in as parameters are everywhere when "whole
functions" are vectorized rather than just loops in a leaf function, and
while I don't have any hard data, I see no good reason why short functions
should be less common in that setting than in other code. Although I
suppose one could pass the active vector length separately as an integer
and re-compute length->mask in the function body.

> p.s. To make sure my tone is coming across correctly, let me spell out
> that I'm not convinced, but I'm not actively objecting.  I'm playing devils
> advocate for the purposes of fleshing out a design, but if folks more
> knowledgeable than I strongly believe the right design requires both masks
> and lengths, I'm happy to defer on that point.
>
Gotcha. I think it's important to make a good case for this and not just
assert on authority what is good codegen for these architecture.

Cheers,
Robin

>
>
> Cheers,
> Robin
>
> >> Question 2 - Have you explored using selects instead? What practical
>> >> problems do you run into which make you believe explicit predication
>> >> is required?
>> >>
>> >> e.g. %sub = fsub <4 x float> %x, %y
>> >> %result = select <4 x i1> %M, <4 x float> %sub, undef
>> > That is semantically incorrect.  According to IR semantics, the fsub is
>> > fully evaluated before the select comes along.  It could trap for
>> > elements where %M is 0, whereas a masked intrinsic conveys the proper
>> > semantics of masking traps for masked-out elements.  We need intrinsics
>> > and eventually (IMHO) fully first-class predication to make this work
>> > properly.
>>
>> If you want specific trap behavior, you need to use the constrained
>> family of intrinsics instead.  In IR, fsub is expected not to trap.
>>
>> We have an existing solution for modeling FP environment aspects such as
>> rounding and trapping.  The proposed signatures for your EVL proposal do
>> not appear to subsume those, and you've not proposed their retirement.
>> We definitely don't want *two* ways of describing FP trapping.
>>
>> In other words, I don't find this reason compelling since my example can
>> simply be rewritten using the appropriate constrained intrinsic.
>>
>>
>> >
>> >> My context for these questions is that my experience recently w/o
>> >> existing masked intrinsics shows us missing fairly basic
>> >> optimizations, precisely because they weren't able to reuse all of the
>> >> existing infrastructure. (I've been working on
>> >> SimplifyDemandedVectorElts recently for exactly this reason.) My
>> >> concern is that your EVL proposal will end up in the same state.
>> > I think that's just the nature of the beast.  We need IR-level support
>> > for masking and we have to teach LLVM about it.
>> I'm solidly of the opinion that we already *have* IR support for
>> explicit masking in the form of gather/scatter/etc...  Until someone has
>> taken the effort to make masking in this context *actually work well*,
>> I'm unconvinced that we should greatly expand the usage in the IR.
>> >
>> >                             -David
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>
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