[llvm-dev] How to handle ISD::STORE when both operands are FrameIndex?

[Gleb Popov via llvm-dev]

On Wed, Jun 26, 2019 at 1:37 PM Tim Northover <t.p.northover at gmail.com>
wrote:

> On Wed, 26 Jun 2019 at 10:27, Gleb Popov <6yearold at gmail.com> wrote:
> > For the sake of simplicity I decided not to bother with stack register
> yet and instead I just emit FrameIndex as immediate:
> >
> > bool MyBackendDAGToDAGISel::SelectAddrFI(SDValue &N, SDValue &R) {
> >   if (N.getOpcode() != ISD::FrameIndex)
> >     return false;
> >   MachineFrameInfo &MFI = MF->getFrameInfo();
> >   int FX = cast<FrameIndexSDNode>(N)->getIndex();
> >   R = CurDAG->getTargetFrameIndex(FX, MVT::i32);
> >   return true;
> > }
> >
> > This way I end up with
> >
> > store %r1, [1]
> >
> > and handle it in my CPU emulator accordingly.
> >
> > So, instead of matching that FrameIndex in store, I really want to emit
> a load first and then use a register in the store instruction.
>
> As you say, SelectAddrFI will only be used when looking into a store,
> and I think that's fine: it's a specialized addressing-mode
> implemented in just the right place. You need a completely separate
> "case" in MyBackendDAGToDAGISel::Select to handle a bare
> ISD::FrameIndex.
>

Thanks for clearing this up! I was thinking the same, but wasn't sure.


> Since you don't use SP, your implementation of FrameIndex will
> probably just be a MOV-immediate instruction. Final output would be
> something like:
>
>     mov %r1, 2     // Produced by matching the ISD::FrameIndex
>     store %r1, [1]
>
> I don't think load is going to be involved anywhere unless you meant
> it in the broader sense where (e.g.) Z80 referred to its move
> instructions as loads.
>

Silly me -_\
Why was I talking about a load all this time? You're right, I needed a
simple mov there. I have added the following code into
MyBackendDAGToDAGISel::Select() and it worked:

if(FrameIndexSDNode *FIN = dyn_cast<FrameIndexSDNode>(N))
  {
      auto Addr = CurDAG->getTargetFrameIndex(
                    FIN->getIndex(),
TLI->getPointerTy(CurDAG->getDataLayout()));
      SDNode *MOV = CurDAG->getMachineNode(MYBACK::MovRI, dl, MVT::i32,
Addr);
      CurDAG->ReplaceAllUsesWith(N, MOV);
      return;
  }

I'm eternally grateful for helping me with this, thank you again.


> Cheers.
>
> Tim.
>
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