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

[Tim Northover via llvm-dev]

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.

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.

Cheers.

Tim.