[llvm-dev] [RFC] Vector Predication

Thu Jan 31 13:14:03 PST 2019

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.

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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