WebAssembly: handle unused function arguments.

lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

for (const ISD::InputArg &In : Ins) {
     if (In.Flags.isInConsecutiveRegs())
       fail(DL, DAG, "WebAssembly hasn't implemented cons regs arguments");
     if (In.Flags.isInConsecutiveRegsLast())
       fail(DL, DAG, "WebAssembly hasn't implemented cons regs last arguments");
     if (In.Flags.isSplit())
       fail(DL, DAG, "WebAssembly hasn't implemented split arguments");
     if (In.VT != MVT::i32)
       fail(DL, DAG, "WebAssembly hasn't implemented non-i32 arguments");
-    if (!In.Used)
-      fail(DL, DAG, "WebAssembly hasn't implemented unused arguments");
     // FIXME Do something with In.getOrigAlign()?
-    InVals.push_back(DAG.getNode(WebAssemblyISD::ARGUMENT, DL, In.VT,
-                                 DAG.getTargetConstant(ArgNo++, DL, MVT::i32)));
+    InVals.push_back(
+        In.Used
+            ? DAG.getNode(WebAssemblyISD::ARGUMENT, DL, In.VT,
+                          DAG.getTargetConstant(ArgNo, DL, MVT::i32))
+            : DAG.getNode(ISD::UNDEF, DL, In.VT));
+    ++ArgNo;
   }
 
   return Chain;
 }
 
 //===----------------------------------------------------------------------===//
 //  Other Lowering Code
 //===----------------------------------------------------------------------===//

test/CodeGen/WebAssembly/unused-argument.ll

+; RUN: llc < %s -asm-verbose=false | FileCheck %s
+
+; Make sure that argument offsets are correct even if some arguments are unused.
+
+target datalayout = "e-p:32:32-i64:64-v128:8:128-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+; CHECK-LABEL: unused_first:
+; CHECK-NEXT: (setlocal @0 (argument 1))
+; CHECK-NEXT: (RETURN @0)
+define i32 @unused_first(i32 %x, i32 %y) {
+  ret i32 %y
+}
+
+; CHECK-LABEL: unused_second:
+; CHECK-NEXT: (setlocal @0 (argument 0))
+; CHECK-NEXT: (RETURN @0)
+define i32 @unused_second(i32 %x, i32 %y) {
+  ret i32 %x
+}