[llvm-dev] [SPIR-V] SPIR-V in LLVM

[Neil Hickey via llvm-dev]

I agree with Neil and Tom's comments that going through the whole backend process of LLVM is too heavyweight and loses too much information. I'm very interested in seeing, and contributing to, development of the SPIR-V<->LLVM translation library and we should continue to discuss whether this can be upstreamed into LLVM in some way, and exactly what form it should take.

I think if we allow lib/Target backends to not use SelectionDAG, this would be a way to integrate it in.

Neil

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of via
> llvm-dev
> Sent: 08 May 2017 13:47
> To: tstellar at redhat.com
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] [SPIR-V] SPIR-V in LLVM
> 
> I'd like to add my weight to this statement from Tom Stellard:
> 
> > This goes back to something I mentioned in my original email, but I
> > really think the best thing to do for this project right now is to
> > keep it separate from LLVM, clean up the code, and try to get people
> > using it.  It's going to be much easier to get this upstream  in LLVM
> > or even convince people that the answer to question #1 should be 'no'
> > if we have a code base that is mature, well supported, and has a
> > healthy userbase.
> 
> I think SPIR-V should go into tip LLVM eventually, I think it should not use
> SelectionDAG/GlobalISel because it doesn't make sense in my opinion.
> But the most important issue is getting LLVM based languages be able to
> target SPIR-V.
> 
> I think it would be highly beneficial if any SPIR-V target was used to make
> external targets in LLVM/Clang work much more nicely. Being blunt - external
> targets in LLVM suck, you're forced to patch the source code.
> If a SPIR-V target could be used to:
> 
> - Allow experimental targets to be discovered outwith the LLVM tree (EG.
> the target doesn't have to be in lib/Target)
> - Move target triple information into the lib/Target/* folders (or at least allow
> a target to register a triple and information about the target in the target
> folder)
> 
> That'll get us a huge step towards being able to use LLVM with external
> targets, and also allow a bunch of developers to use a SPIR-V backend as if it
> was in tip LLVM, thus demonstrating its viability for upstream.
> 
> Cheers,
> -Neil.
> 
> On 2017-05-03 20:04, Tom Stellard via llvm-dev wrote:
> > On 05/03/2017 11:19 AM, Nicholas Wilson wrote:
> >>> Right, what I was trying to say is that there are more benefits from
> >>
> >>> having this not be a target than there is from having it be a target.
> >>
> >> Please enumerate them, I have seen none posted so far . The implied
> >> “it is what all the the other backends do” w.r.t ISel/MC is at
> >> best(worst?) an implementation detail, and I’m still not quite sure
> >> why Chandler was so adamant about that. He seemed to imply that
> >> generating straight from the IR (as opposed to post legalisation?)
> >> introduces a direct dependance in the IR that the rest of LLVM would
> >> then be required to not break? I agree that the SPIRV backend should
> >> be insulated from changes the IR, although I’m not sure how to
> >> achieve that property. I’m also not sure how much, if at all, it
> >> would be susceptible to that to begin with. Deletions of
> >> instructions/attributes would obviously cause breakage and additions
> >> may cause unhandled and/or invalid combinations. I still don’t get
> >> the severity if this though, insight appreciated.
> >>
> >
> > So there are really two questions here:
> > 1. Should targets be required to use SelectionDAG/GlobalISEL ?
> > 2. Should SPIR-V use SelectionDAG/GlobalISel?
> >
> > In my opinion, regardless of the answer to question #1, the answer to
> > question #2 is no, SPIR-V should not use SelectionDAG/GlobalISel.
> >
> > I touched on this before in previous emails, but the main problem is
> > that SelectionDAG (and GlobalISel to a lesser extent) plus the whole
> > MachineInstr layer is a much lower-level representation than SPIR-V,
> > so you will need to do a lot of extra work and/or modifications to
> > existing infrastructure in order to get a working target, and even
> > then you may be limited to emitting poor quality SPIR-V that other
> > backends will have a hard time optimizing.
> >
> > With all this work, what advantages are you getting?  If the only
> > reason to do it this way is so you can use intrinsics, or
> > TargetLibraryInfo, or easier integration with other tools, I think it
> > would be better to try to save the effort and try to solve those
> > problems in some other way.
> >
> > LLVM IR -> SPIR-V directly will give you better code, lower compile
> > times.  It will be more simple and easier to maintain, and you will be
> > able to re-use existing SPIR-V parsers/writers that exist in
> > SPIRV-Tools.
> >
> > This goes back to something I mentioned in my original email, but I
> > really think the best thing to do for this project right now is to
> > keep it separate from LLVM, clean up the code, and try to get people
> > using it.  It's going to be much easier to get this upstream  in LLVM
> > or even convince people that the answer to question #1 should be 'no'
> > if we have a code base that is mature, well supported, and has a
> > healthy userbase.
> >
> >
> >>
> >>> What you are proposing is a lot of work, and even if it does help to
> >>> avoid some duplicate work (which to be honest, I still don't quite
> >>> understand what duplication there would be if it weren't a target),
> >>> I don't think that is enough to justify the effort required.
> >>
> >> My point is that (modulo metadata, which I am still  investigating
> >> better alternatives, and calling conventions) if SPIRV is a target,
> >> then a producer need not change their compilation pipeline /at all/
> >> to target SPIRV. There should be no effort required, it would come as
> >> a property of being a target. I think we are confusing each other
> >> again :(
> >>
> >> Leaving that aside for a moment, there are a number of
> >> advantages/requirements that, correct me if I’m wrong, would be
> >> impossible without a proper target.
> >>
> >> * Most critically: Intrinsics. I am almost certain that you would not
> >> accept the current mangling hacks, and if I am to support windows
> >> neither can I. Any solution would therefore need to be able to
> >> register intrinsics and I believe this is impossible without a target
> >> (and even if it is, it makes less sense than a target that doesn’t
> >> use ISel/MC). Not being able to use intrinsics is a complete deal breaker.
> >>
> >
> > You don't need to register intrinsics to be able to use them, and it's
> > also possible to register them without a backend, but this has not
> > been done before.
> >
> >> *Basic optimisations (basic CSE,DCE,inlining): requires a
> >> TargetLibraryInfoImpl(?) which I believe requires a target. While not
> >> strictly necessary it would improve the readability of the resulting
> >> IR/SPIRV. All of the more complex optimisations would be done “post
> >> ingestion” of the SPIRV and with a different target triple so are
> >> unaffected by any decision made. See my reply to Hongbin for an
> >> approach.
> >
> > You don't need a target for this.  TargeLibraryInfo is constructed
> > based on the triple.
> >
> > -Tom
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev