[WebAssembly] Fix call_indirect on funcrefs

[pmatos]

Summary

The currently implementation of funcrefs is broken since it is putting
the funcref itself on the stack before the call_indirect. Instead what
should be on the stack is the constant 0, which is the index at which
we store the funcref in __funcref_call_table.

[tlively]

Ah, yes, this fix makes sense. LGTM modulo comments.

llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

571

It would be good to add a comment about why we are adding this zero operand here.

578

Could we use CallParams.addOperand here for consistency with the other branch?

[pmatos]

llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

571

Sure.

578

I don't think so. You cannot create MachineOperands from scratch. The only reason we can use the addOperand before is because we did not build the operand FnPtr, we extracted it. MachineOperands are built as part of the MachineInstrBuilder, which is what I used.

llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

BuildMI(*BB, CallResults.getIterator(), DL,
         .addReg(FnPtr.getReg());
     FnPtr.setReg(Reg32);
   }
 
   // Move the function pointer to the end of the arguments for indirect calls
   if (IsIndirect) {
     auto FnPtr = CallParams.getOperand(0);
     CallParams.RemoveOperand(0);
+
+    // For funcrefs, call_indirect is done through __funcref_call_table and the
+    // funcref is always installed in slot 0 of the table, therefore instead of having
+    // the function pointer added at the end of the params list, a zero (the index in
+    // __funcref_call_table is added).
+    if (IsFuncrefCall) {
+      Register RegZero =
+          MF.getRegInfo().createVirtualRegister(&WebAssembly::I32RegClass);
+      MachineInstrBuilder MIBC0 =
+          BuildMI(MF, DL, TII.get(WebAssembly::CONST_I32), RegZero).addImm(0);
+
+      BB->insert(CallResults.getIterator(), MIBC0);
+      MachineInstrBuilder(MF, CallParams).addReg(RegZero);
+    } else
       CallParams.addOperand(FnPtr);
   }
 
   for (auto Def : CallResults.defs())
     MIB.add(Def);
 
   if (IsIndirect) {
     // Placeholder for the type index.
     MIB.addImm(0);

llvm/test/CodeGen/WebAssembly/funcref-call.ll

 
 ; CHECK: .tabletype __funcref_call_table, funcref, 1
 
 ; CHECK-LABEL: call_funcref:
 ; CHECK-NEXT: functype       call_funcref (funcref) -> ()
 ; CHECK-NEXT: i32.const 0
 ; CHECK-NEXT: local.get 0
 ; CHECK-NEXT: table.set __funcref_call_table
-; CHECK-NEXT: local.get 0
+; CHECK-NEXT: i32.const 0
 ; CHECK-NEXT: call_indirect __funcref_call_table, () -> ()
 ; CHECK-NEXT: i32.const 0
 ; CHECK-NEXT: ref.null func
 ; CHECK-NEXT: table.set __funcref_call_table
 ; CHECK-NEXT: end_function