[wasm] Consider void returns in the int64-lowering

The int64-lowering lowers return nodes which return one int64 value into
a return node which returns two int32 values. For this lowering it has
to adjust the input count of the return operator.

The existing code assumed that if the signature of a function said that
the return type is int64, then all return nodes have int64 inputs.
However, with a recent CL we also introduced void returns. With this CL
I check if the number of inputs of a return node changes with the
DefaultLowering, and only if the number of inputs changes, then I check
if I also have to change the operator of the return node.

R=mstarzinger@chromium.org
TEST=mjsunit/regress/wasm/regression-6164
BUG=v8:6164

Change-Id: I004ab1b4be942cc045719f306705d95b48707a1c
Reviewed-on: https://chromium-review.googlesource.com/461941Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44232}

src/compiler/int64-lowering.cc

@@ -278,10 +278,13 @@ void Int64Lowering::LowerNode(Node* node) {
       break;
     }
     case IrOpcode::kReturn: {
+      int input_count = node->InputCount();
       DefaultLowering(node);
-      int new_return_count = GetReturnCountAfterLowering(signature());
-      if (static_cast<int>(signature()->return_count()) != new_return_count) {
-        NodeProperties::ChangeOp(node, common()->Return(new_return_count));
+      if (input_count != node->InputCount()) {
+        int new_return_count = GetReturnCountAfterLowering(signature());
+        if (static_cast<int>(signature()->return_count()) != new_return_count) {
+          NodeProperties::ChangeOp(node, common()->Return(new_return_count));
+        }
       }
       break;
     }

test/mjsunit/regress/wasm/regression-6164.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+load('test/mjsunit/wasm/wasm-constants.js');
+load('test/mjsunit/wasm/wasm-module-builder.js');
+
+(function() {
+  var builder = new WasmModuleBuilder();
+  builder.addMemory(31, 31, false);
+  builder.addFunction('test', kSig_l_v)
+      .addBodyWithEnd([
+// body:
+kExprUnreachable,
+kExprEnd,   // @374
+      ])
+      .exportFunc();
+  var module = builder.instantiate();
+})();