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

[Gleb Popov via llvm-dev]

On Wed, Jun 26, 2019 at 12:38 PM Tim Northover <t.p.northover at gmail.com>
wrote:

> Hi Gleb,
>
> On Wed, 26 Jun 2019 at 07:28, Gleb Popov <6yearold at gmail.com> wrote:
> > def StoreStackF : InstRI<2, (outs), (ins IntRegs:$reg, i32imm:$i),
> >                     "storestackf $reg, [$i]", [(store_stack i32:$reg,
> AddrFI:$i)]>;
> >
> > I'm puzzled why despite having "IntRegs:$reg" in ins list, it still
> matches FrameIndex:
>
> A register-class will match anything with the appropriate type (i32
> here). The idea is that any value of that type can be put into a
> register somehow, and matching whatever it happens to be (a FrameIndex
> in this case) is exactly what's needed to do that.
>

That's a useful explanation, thanks.


> > ISEL: Starting selection on root node: t11: i32 = FrameIndex<2>
> > ISEL: Starting pattern match
> >   Initial Opcode index to 0
> >   Match failed at index 0
> > LLVM ERROR: Cannot select: t11: i32 = FrameIndex<2>
>
> Ah good, this is what I thought was probably happening so it's nothing
> exotic. You need to be able to match a FrameIndex somehow. Not just
> for this, but for something like:
>
>     void foo() {
>       int arr[5];
>       bar(&arr[0]);
>     }
>
> It's pretty simple to implement matching, you generally just need to
> convert it to some add instruction that can do SP+imm.


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.
Can you, please, advise me how to implement that?

See for example
> lib/Target/AArch64/AArch64ISelDAGToDAG.cpp:2916.
>
> Cheers.
>
> Tim.
>
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