[llvm-dev] [WebAssembly] Lower type in new DAG node

[Paulo Matos via llvm-dev]

Hi all,

Through my work on implementing the reference types WebAssembly proposal
in LLVM IR, I have hit a blocker and would be keen to have suggestions
on how to proceed or similar cases from which to draw inspiration. The
current WIP patch is at https://reviews.llvm.org/D95425

The current design to represent the externref type is to use LLVM type
opaque in address space 1. Therefore an externref looks like:

%extern = type opaque
%externref = type %extern addrspace(1)* ;; addrspace 1 is nonintegral

To represent storing and loading of externref from globals, we use store
and load from LLVM globals. So, a store of externref would look like:

@externref_global = local_unnamed_addr addrspace(1) global %externref undef
define void @set_externref_global(%externref %g) {
  ;; this generates a global.set of @externref.global
  store %externref %g, %externref addrspace(1)* @externref_global
  ret void
}

What's currently happening is that we lower the store into a new DAG
node GLOBAL_SET and then pattern match it into a GLOBAL_SET_EXTERNREF
that generates the proper global.set Wasm instruction.

// Global SET
def wasm_global_set_t : SDTypeProfile<0, 2, [SDTCisPtrTy<1>]>;
def wasm_global_set : SDNode<"WebAssemblyISD::GLOBAL_SET", wasm_global_set_t,
                        [SDNPHasChain, SDNPMayStore, SDNPMemOperand]>;
def : Pat<(wasm_global_set externref:$v, (WebAssemblywrapper tglobaladdr:$addr)),
          (GLOBAL_SET_EXTERNREF global_op:$addr, EXTERNREF:$v)>,
      Requires<[HasReferenceTypes]>;

The problem I am finding is that the pattern matcher is not being able
to match the new node with (wasm_global_set externref:$v,
(WebAssemblywrapper tglobaladdr:$addr))

Further analysis shows that the problem is in matching externref:$v.

The issue is that in IR, $v has type opaque addrspace(1)*, but the MVT
we re trying to match it to is externref. Initially I assumed, we could
somehow lower the type and instruct LLVM to lower all type opaque
addrspace(1)* to MVT::externref but I was not able to achieve that. I
welcome any suggestions/comments on the approach and ways to proceed.

Regards,

-- 
Paulo Matos