It’s been a few years now since I added some intrinsics for doing vector reductions. We’ve been using them exclusively on AArch64, and I’ve seen some traffic a while ago on list for other targets too. Sander did some work last year to refine the semantics after some discussion.
Are we at the point where we can drop the “experimental” from the name? IMO all target should begin to transition to using these as the preferred representation for reductions. But for now, I’m only proposing the naming change.
Cheers,
Amara
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Can you provide a pointer to the docs that describe these?
-Chris
Hi,
It’s been a few years now since I added some intrinsics for doing vector reductions. We’ve been using them exclusively on AArch64, and I’ve seen some traffic a while ago on list for other targets too. Sander did some work last year to refine the semantics after some discussion.
Are we at the point where we can drop the “experimental” from the name? IMO all target should begin to transition to using these as the preferred representation for reductions. But for now, I’m only proposing the naming change.
On Apr 8, 2020, at 9:54 AM, Nikita Popov <nikit...@gmail.com> wrote:On Wed, Apr 8, 2020 at 6:59 AM Amara Emerson via llvm-dev <llvm...@lists.llvm.org> wrote:Hi,
It’s been a few years now since I added some intrinsics for doing vector reductions. We’ve been using them exclusively on AArch64, and I’ve seen some traffic a while ago on list for other targets too. Sander did some work last year to refine the semantics after some discussion.
Are we at the point where we can drop the “experimental” from the name? IMO all target should begin to transition to using these as the preferred representation for reductions. But for now, I’m only proposing the naming change.There's still a couple of open issues that I'm aware of:1. fmin/fmax reductions without nnan flag do not work. IR expansion code assumes that these always use FMF. It's also under-documented what their exact behavior is, though I assume it should match llvm.minnum/llvm.maxnum semantics to be most useful.
As one recent example, consider
https://bugs.llvm.org/show_bug.cgi?id=45378. (There's nothing special
about this one other than it was recent.)
I'm not necessarily arguing they can't be promoted from experimental,
but it would be a much easier case if the code gen was routinely as good
or better than the scalar forms. Or to say that a bit differently, if
we could canonicalize to them in the IR without major regression.
Having two ways to represent something in the IR without any agreed upon
canonical form is always sub-optimal.
Philip
We (ARM / MVE) would also at some point want to add the concept of masked reductions. This could either be done by extending the existing intrinsics much like how a masked gather/scatter works, or with an entirely new set of intrinsics. I'm not sure which way is better in the long run, but the masked variants probably have a superset of the functionality of the non-masked ones, considering the mask can be all ones.
I believe other vector architectures such as SVE and the VE target might make use of these too, but perhaps with slightly different semantics around active vector lengths.
Dave
From: llvm-dev <llvm-dev...@lists.llvm.org> on behalf of Nikita Popov via llvm-dev <llvm...@lists.llvm.org>
Sent: 08 April 2020 17:54
To: Amara Emerson <aeme...@apple.com>
Cc: LLVM Developers' List <llvm...@lists.llvm.org>
Subject: Re: [llvm-dev] RFC: Promoting experimental reduction intrinsics to first class intrinsics
Hi,
Regards,
Nikita
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Yes, we absolutely would want masked reductions. Those are already
planned for VP intrinsics https://reviews.llvm.org/D57504
I'd very much appreciate if we could cooperate on this and implement
masked reduction intrinsics in the llvm.vp.* namespace following the
schema of the VP intrinsics that are already upstream
(https://llvm.org/docs/LangRef.html#vector-predication-intrinsics).
Btw, i am working on the VP to non-VP expansion pass. If
`llvm.vp.reduce.*` intrinsics should replace the existing ones then
that's the pass where their expansion would be implemented as well.
Cheers
- Simon
A minor point, but I think we need to more explicitly describe
the order of floating point operations in the LangRef as well:
"If the intrinsic call has the ‘reassoc’ or ‘fast’ flags set, then the reduction will not preserve the associativity of an equivalent scalarized counterpart. Otherwise the reduction will be ordered, thus implying that the operation respects the associativity of a scalarized reduction."
Please could we add some pseudocode to show exactly how the
intrinsic will be re-expanded for ordered cases?
[re-sending to list with new email]Ping. As far as I can tell the remaining issues are solved?If so, we’ll need to duplicate the intrinsics without the “experimental”, and add support in the IR autoupgrader and fix up the tests.Amara
On Sep 9, 2020, at 9:37 AM, Sanjay Patel <spa...@rotateright.com> wrote: