[llvm-dev] [RFC] Lanai backend

Tue Feb 9 22:24:10 PST 2016

You've raised an important point here Pete, and while I disagree pretty
strongly with it (regardless of whether Lanai makes sense or not), I'm glad
that you've surfaced it where we can clearly look at the issue.

The idea of "it really should have users outside of just the people who
have access to the HW" I think is deeply problematic for the project as a
whole. Where does it stop?

While I may have the theoretical ability to get access to an AVR, Hexagon,
MSP430, SystemZ, or XCore processor... It is a practical impossibility.
There is no way that I, or I suspect 95% of LLVM contributors, will be able
to run code for all these platforms. And for some of them, I suspect it is
already the case that their only users have access to specialized, quite
hard to acquire hardware (both Hexagon[1] and SystemZ come to mind).

But I really don't think we want to kick any of these backends out of LLVM,
or that they were a mistake to include in the first place. So the question
really is whether the lack of *theoretical* access matters substantially
more than lack of *practical* access.

This policy would also set an extremely alarming precedent to me at least,
maybe one that you're not even intending: what's the difference between
hardware and an operating system? I don't think we want to limit the
operating system support of Clang to those which are readily accessible.
Lots of folks don't have a practical ability to use Apple's operating
system because they don't own any of Apple's hardware. And the reverse is
of course true -- I think if you work at Apple and only own Apple hardware,
you probably shoudn't be expected to boot Windows or Linux and debug
something.

We have a growing history of support diverse platforms, both hardware and
software. And when we do this, we rely on the community contributors who
care deeply about those platforms to bridge the gap and support those
developers who don't have ready access to the platform. I don't want to see
this change, as I think it has been absolutely critical to the community
growing and thriving over the past few years. My best memory of this is
with AMDGPU (which is why I brought it up, I agree with others that MSP430
or XCore are perhaps better examples here). I was making a change to a DAG
combine of some form and it ended up regressing the AMDGPU (R600 at the
time) regression tests. I was completely lost. While in theory, I could go
pick up such a GPU and install it in a machine to test things out, in
practice I'm never going to do that. If that were the requirement, I would
just have to abandon such changes. But instead, Tom and Matt bent over
backwards to help me out, looked at all the test case updates I sent them
and helped verify that indeed, the changes were harmless or even helpful. I
didn't need hardware, an emulator, or even an ISA spec. And I think this is
*awesome*.

So regardless of what we do with Lanai, I feel really strongly that we
don't want to use "have other users" or "can run the code" as the bar for
supporting platforms.

But all that really doesn't mean we should necessarily adopt Lanai. I think
this is probably the nearest tradeoff the community has looked at (that's
actually one reason I worked internally to see if we could contribute the
backend). And it is definitely is a tradeoff.

IMO, it is worth the tradeoff, just like it was for BPF, AMDGPU, and
others. Our goal should be to get more upstream contributors and LLVM
developers, and to the extent that we can do that by encouraging people
working on new (to the community) platforms, whether software or hardware,
to work on them upstream and in-tree, I think we should.

Personally, I'm not concerned about how encumbered in any sense SystemZ is
or isn't (I have no idea about such things). I don't care that I'm
completely unlikely to be able to get a SystemZ system, or even an
emulator. I'm probably not going to spend a whole lot of time trying to
read the entire ISA spec. To me, while those things would *help* motivate
the issue, they're not the operative factor in my decision. Instead, the
fact that IBM contributed SystemZ, maintained it, and was able to do more
work in-tree was justification enough.

My 2 cents, sorry I took so many words to express them. =]
-Chandler

[1] Yes, I know Hexagon chips are present all over Android devices, but I
have no idea how one would go about actually programming most of them.
Maybe it's actually much easier than I imagine, if so, apologize. My
impression has just been that Qualcomm (the producer of the Hexagon chips)
is by far the largest user of the backend if not the exclusive user. None
of this is a criticism of course! =]

On Tue, Feb 9, 2016 at 9:35 PM Pete Cooper via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Feb 9, 2016, at 9:15 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
> ----- Original Message -----
>
> From: "Pete Cooper via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Sean Silva" <chisophugis at gmail.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Tuesday, February 9, 2016 10:59:58 PM
> Subject: Re: [llvm-dev] [RFC] Lanai backend
>
>
> Hi Sean
>
>
> I think you’ve summed it up really well here.
>
>
> Personally I don’t think we should accept backends for which there is
> no way to run the code. The burden (however small) on the community
> to having an in-tree backend they can’t use is too high IMO.
>
>
> As you point out ‘no way to run the code’ may mean not having access
> to HW, or having HW but no API.
>
>
> Out of curiosity, would the existence of some kind of open-source emulator
> affect your opinion on this? Or does it need to be actual hardware?
>
> I guess hardware isn’t even the correct term given we have BPF which I
> think is a virtual ISA of some kind.
>
> I’d be ok with targeting an emulator, but with the caveat that people are
> actually using the emulator.  An emulator which exists purely to get this
> backend in to LLVM isn’t useful, but one which people do use for education
> or otherwise is useful.  But it really should have users outside of just
> the people who have access to the real HW.
>
> Perhaps this can be an experimental backend to give time for an emulator
> and community to grow around it, and then we can decide later whether to
> promote to a first class backend?  Then we still have options, including
> just continuing as an experimental backend for longer if thats best.
>
> As a small digression, I would say that this has raised a very valid
> discussion about the code that ends up in LLVM and is built as standard.  I
> don’t know, nor am I going to look, but I think the same could (should?) be
> applied to other code like passes.  If a pass isn’t used by anyone in tree
> then should it be in tree?
>
> Pete
>
>
> -Hal
>
>
> NVPTX is a good example. Now you can take the output from LLVM and
> run it on HW. It may or may not be how Nvidia do it in their code,
> but that doesn’t matter, you can do it. Same for AMDGPU.
>
>
> So -1 from me to having backends we can’t make use of.
>
>
> Finally, one option is to have perpetually experimental backends.
> Then all the code is in tree but no-one in tree should ever be
> expected to update it. That does have the big advantage that all of
> the code is there to discuss and the maintainers can make
> contributions to common code and gain/provide help in the community.
> They can also be involved in discussions which impact them such as
> changes to common code.
>
>
> Cheers,
> Pete
>
>
>
>
> On Feb 9, 2016, at 4:18 PM, Sean Silva via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
>
>
> One data point (IIRC) is that the NVPTX backend sat in tree for a
> long time without a way to actually use them. But lately this has
> been opening up (e.g. http://llvm.org/docs/CompileCudaWithLLVM.html
> ). However, the obstacle for NVPTX was mostly a software
> proprietary-ness (no way to plug it into the driver stack really,
> except via nvidia's own proprietary software), whereas the actual
> hardware was available. For the Lanai stuff, it seems like the
> hardware is fundamentally not available for purchase.
>
>
> The reverse situation is with e.g. Apple's GPU backends, where the
> devices are readily available, but (AFAIK) even if the backend were
> open-source you couldn't run the code produced by the open-source
> compiler.
>
>
> Or to put it in matrix form (this is all heavily prefixed by "AFAIK";
> corrections welcome):
>
>
> AMDGPU: InTree:Yes DevicesAvailable:Yes CanIRunTheCode:Yes
> NVPTX: InTree:Yes DevicesAvailable:Yes CanIRunTheCode :Yes
> Lanai: InTree:? DevicesAvailable:No CanIRunTheCode :No
> Apple GPU's: InTree:No DevicesAvailable:Yes CanIRunTheCode :No
>
> I couldn't come up with a good name for "Can I Run The Code" column.
> Basically it means: "assuming the backend were in open source, could
> I actually run the code produced by the open source backend
> somehow?".
>
>
> I had a quick look at lib/Target and it seems like every backend we
> have has "CanIRunTheCode:Yes" in theory.
> IIRC, the NVPTX stuff used to actually be "No" though?
>
>
> Anyway, just a random thought. Not sure what the conclusion is.
>
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>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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