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

[Tim Northover via llvm-dev]

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.

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

Cheers.

Tim.