[llvm-dev] InlineSpiller - hoists leave virtual registers without live intervals

[Wei Mi via llvm-dev]

On Mon, Nov 4, 2019 at 6:41 PM <alex.davies at iinet.net.au> wrote:

> From: Wei Mi <wmi at google.com>
> > I havn't noticed before that TII.storeRegToStackSlot is allowed to
> create new virtual registers. I am surprised to know it is allowed to do
> that. I am mostly working on x86 and TII.storeRegToStackSlot for x86
> doesn't create new register. As you said, if TII.storeRegToStackSlot is
> allowed to create new virtual registers, that could be a problem. It is not
> exposed maybe because of some later pass cover the problem -- maybe
> RegScavenger?
>
> > Are you aware of any other architecture other than AMDGPU on which
> TII.storeRegToStackSlot creates new virtual registers? Could you explain in
> which case new virtual registers will be created?
>
> It is quite possible it's a largely forgotten feature from before
> RegScavenger was fully featured, however being able to storeRegToStackSlot
> did solve a major problem for me.
>
>
I see. You expect that TII.storeRegToStackSlot could allow new temporary to
be generated to handle the irregular spill easily and the patch
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20130812/184331.html
said
it did some preparation for that, but looks like in reality no one actually
let TII.storeRegToStackSlot generate new temporary yet. I find RegScavenger
calls TII.storeRegToStackSlot as well when spill is needed to scavenge a
register. If TII.storeRegToStackSlot allows to generate new temporary, that
will be a problem for RegScavenger as well (Looks like RegScavenger doesn't
work in an iterative way)

So I guess currently most implementation in llvm assumes
TII.storeRegToStackSlot doesn't generate new virtual register usage?


> On the architecture I'm on (and many DSPs like it, I suspect), there's
> many special function registers that control various aspects of the CPU.
> These registers can be pushed, popped, and moved to/from GPRs. The issue is
> that the architecture has no "move from SFR to stack + offset" command,
> which LLVM requires for stack-frame store/loads. They must go through a GPR
> intermediate. Even solving the frame address would not avoid this issue, as
> there is no "move from SFR to [address]" either, they must either be
> directly pushed/popped from stack, or go through an intermediate.
>
> Implementing "getLargestLegalSuperClass" to return a RegisterClass of
> (GPRs + SFRs) goes a long way towards avoiding these difficult spills, as
> they'll often be assigned a GPR over call-clobbering etc, but unfortunately
> AFAIK there's no way to tell the register allocator that spills must be
> from the GPR side of that union. Sometimes, it will request that the SFR
> gets spilled, and so in those instances you ultimately need a temporary in
> storeRegToStackSlot. One option would be a pseudo, followed by a pass using
> RegScavenger to provide the intermediate - but I would expect the codegen
> to be worse than generating the intermediate in storeRegToStackSlot, if
> supported.
>
> Really though, this is all just a workaround to a problem. What's really
> desired/wanted, is just a way to say that SFRs must be spilled to GPRs, a
> "getSpillRegClass" if you will. This seems a missing piece/problem in
> general, whilst trying to find out how to address the problem I came across
> at least one question to llvm-dev on how to do it, unanswered iirc.
>
> AMDGPU also seemingly has an extensive/complex workaround in
> "EnableSpillVGPRToAGPR", implemented in SILowerSGPRSpills, which if I
> understand it correctly, attempts to simplify similar two-stage spills
> post-regalloc by eliminating unnecessary frame indices. I'm not familiar
> with the architecture and may be wrong on the constraints they're working
> around, but on glancing it over, it seems to me that if a hypothetical
> "spillRegClass()" could be expressed, AMDGPU too would not need a temporary
> to be allocated in storeRegToStackSlot, and could also do away with much of
> SILowerSGPRSpills as well.
>
>         /// Special case of eliminateFrameIndex. Returns true if the SGPR
> was spilled to
>         /// a VGPR and the stack slot can be safely eliminated when all
> other users are
>         /// handled.
>         bool SIRegisterInfo::eliminateSGPRToVGPRSpillFrameIndex
>
> Please do let me know if I've missed a better way at addressing this,
> perhaps there is just the function I'm looking for already.
>

Yes, EnableSpillVGPRToAGPR seems to work from your description. But I am
not familiar with GPU register allocation and have no experience about how
this kind of irregular spill is handled in llvm now generally.

Quentin, do you have suggestion about how such irregular spill should be
handled?

I added Mattias and Matthew to the thread as well because they are very
experienced on GPU register allocation.

Thanks,
Wei.


>
> Thank you,
> Alex
>
>
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