WebAssembly: handle `ret void`.

[jfb]

Summary

Use -1 as numoperands for the return SDTypeProfile, denoting that return is variadic. Note that the patterns in InstrControl.td still need to match the inputs, so this ins't an "anything goes" variadic on ret!

The next step will be to handle other local types (not just int32).

[sunfish]

lgtm

lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

SDValue WebAssemblyTargetLowering::LowerReturn(
   assert(Outs.size() <= 1 && "WebAssembly can only return up to one value");
   if (CallConv != CallingConv::C)
     fail(DL, DAG, "WebAssembly doesn't support non-C calling conventions");
   if (IsVarArg)
     fail(DL, DAG, "WebAssembly doesn't support varargs yet");
 
   SmallVector<SDValue, 4> RetOps(1, Chain);
   RetOps.append(OutVals.begin(), OutVals.end());
-  const SDValue Ops[] = {Chain, OutVals.front()};
-  Chain = DAG.getNode(WebAssemblyISD::RETURN, DL, MVT::Other, Ops);
+  Chain = DAG.getNode(WebAssemblyISD::RETURN, DL, MVT::Other, RetOps);
 
   return Chain;
 }
 
 SDValue WebAssemblyTargetLowering::LowerFormalArguments(
     SDValue Chain, CallingConv::ID CallConv, bool IsVarArg,
     const SmallVectorImpl<ISD::InputArg> &Ins, SDLoc DL, SelectionDAG &DAG,
     SmallVectorImpl<SDValue> &InVals) const {

lib/Target/WebAssembly/WebAssemblyInstrControl.td

  * break: break to end from nested loop or block
  * switch: switch statement with fallthrough
  */
 
 let hasSideEffects = 1, isReturn = 1, isTerminator = 1, hasCtrlDep = 1,
     isBarrier = 1 in {
 //FIXME return more than just int32.
 def RETURN : I<(outs), (ins Int32:$val), [(WebAssemblyreturn Int32:$val)]>;
+def RETURN_VOID : I<(outs), (ins), [(WebAssemblyreturn)]>;
 } // hasSideEffects = 1, isReturn = 1, isTerminator = 1, hasCtrlDep = 1,
   // isBarrier = 1

lib/Target/WebAssembly/WebAssemblyInstrInfo.td

 def HasSIMD128 : Predicate<"Subtarget->hasSIMD128()">,
                            AssemblerPredicate<"FeatureSIMD128", "simd128">;
 
 //===----------------------------------------------------------------------===//
 // WebAssembly-specific DAG Node Types.
 //===----------------------------------------------------------------------===//
 
 def SDT_WebAssemblyArgument : SDTypeProfile<1, 1, [SDTCisVT<1, i32>]>;
-def SDT_WebAssemblyReturn   : SDTypeProfile<0, 1, []>;
+def SDT_WebAssemblyReturn   : SDTypeProfile<0, -1, []>;
 
 //===----------------------------------------------------------------------===//
 // WebAssembly-specific DAG Nodes.
 //===----------------------------------------------------------------------===//
 
 def WebAssemblyargument : SDNode<"WebAssemblyISD::ARGUMENT",
                                  SDT_WebAssemblyArgument>;
 def WebAssemblyreturn   : SDNode<"WebAssemblyISD::RETURN",

test/CodeGen/WebAssembly/return-void.ll

+; RUN: llc < %s -asm-verbose=false | FileCheck %s
+
+target datalayout = "e-p:32:32-i64:64-v128:8:128-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+; CHECK-LABEL: return_void:
+; CHECK-NEXT: (RETURN_VOID)
+define void @return_void() {
+  ret void
+}