[llvm-dev] [RFC] Vector Predication

[Philip Reames via llvm-dev]

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 <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>
>     On 1/31/19 11:03 AM, David Greene wrote:
>     > Philip Reames <listmail at philipreames.com
>     <mailto: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.  Note that 
I'm assuming this as a fallback, and that the common case is handled via 
the equivalent of ComputeKnownBits on the mask itself at compile time.

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?


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.


>
> 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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