[llvm-dev] RFC: Generic IR reductions

[Demikhovsky, Elena via llvm-dev]

Constant propagation:

%sum = add <N x float> %a, %b
@llvm.reduce(ext <N x double>  %sum)

if %a and %b are vector of constants, the %sum also becomes a vector of constants.
At this point you have @llvm.reduce(ext <N x double>  %sum) and don't know what kind of reduction do you need.

-  Elena


-----Original Message-----
From: Renato Golin [mailto:renato.golin at linaro.org] 
Sent: Wednesday, February 01, 2017 14:40
To: Demikhovsky, Elena <elena.demikhovsky at intel.com>
Cc: Amara Emerson <amara.emerson at gmail.com>; Amara Emerson <Amara.Emerson at arm.com>; llvm-dev at lists.llvm.org; nd <nd at arm.com>; Simon Pilgrim <llvm-dev at redking.me.uk>
Subject: Re: [llvm-dev] RFC: Generic IR reductions

On 1 February 2017 at 11:59, Demikhovsky, Elena <elena.demikhovsky at intel.com> wrote:
>   > @llvm.reduce(ext <N x double> ( add <N x float> %a, %b))
>
> And if we don't have %b? We just want to sum all elements of %a? 
> Something like @llvm.reduce(ext <N x double> ( add <N x float> %a, 
> zeroinitializer))

Hum, that's a good point. My examples were actually wrong, as they weren't related to simple reductions. Your zeroinit is the thing I was looking for.


> Don't we have a problem with constant propagation in this approach?

I'm not sure. Can you expand this?


> I proposed a "generic" intrinsic approach on the BOF (Nov, 2016), like
>     %scalar = @llvm.reduce(OPCODE, %vector_input) - OPCODE may be a string, integer or metadata.

I wouldn't use metadata. Integer would be cumbersome and lead to eventual ABI breakages, and "text" would be the same as:

  %scalar = @llvm.reduce.add(%vector)

which is the same thing Amara proposed.

I'm not saying it is wrong, I'm just worried that, by mandating the encoding of the reduction into an intrinsic, we'll force the middle-end to convert high-level code patterns to the intrinsic or the target will ignore it completely.

There is a pattern already for reductions, and the back-ends already match it. This should not change, unless there is a serious flaw in it
- for the targets that *already* support it. This is an orthogonal discussion.

SVE has more restrictions, for instance, one cannot know how many shuffles to do because the vector size is unknown, so the current representation is insufficient, in which case, we need the intrinsic.

But replace everything else with intrinsics just because one target can't cope with it doesn't work.

On thing that does happen is that code optimisations expose patterns that would otherwise not be apparent. This includes potential reduction or fusion patterns and can lead to massively smaller code or even eliding the whole block. If you convert a block to an intrinsic too early you may lose the ability to merge it back again later, as we're doing today.

These are all hypothetical wrt SVE, but they did happen in NEON in the past and were the reason why we only have a handful of NEON intrinsics. Everything else are encoded with sequences of instructions.

cheers,
--renato
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