[wasm] Preliminary support for "except_ref" globals.

This adds basic support and tests for having global variables of type
"except_ref" that are default initialized to "null". The functionality
is part of the exception handling proposal and solely enabled by the
corresponding feature flag.

R=ahaas@chromium.org
TEST=mjsunit/wasm/exceptions-global
BUG=v8:8091

Change-Id: I581bc942fbe6688a5c58790a842fb024de22d924
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1528439
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60317}

src/compiler/wasm-compiler.cc

@@ -4988,6 +4988,7 @@ class WasmWrapperGraphBuilder : public WasmGraphBuilder {
         return BuildChangeFloat64ToTagged(node);
       case wasm::kWasmAnyRef:
       case wasm::kWasmAnyFunc:
+      case wasm::kWasmExceptRef:
         return node;
       default:
         UNREACHABLE();

src/wasm/module-decoder.cc

@@ -1533,7 +1533,7 @@ class ModuleDecoderImpl : public Decoder {
         break;
       }
       case kExprRefNull: {
-        if (enabled_features_.anyref) {
+        if (enabled_features_.anyref || enabled_features_.eh) {
           expr.kind = WasmInitExpr::kRefNullConst;
           len = 0;
           break;
@@ -1590,6 +1590,9 @@ class ModuleDecoderImpl : public Decoder {
             case kLocalAnyRef:
               if (enabled_features_.anyref) return kWasmAnyRef;
               break;
+            case kLocalExceptRef:
+              if (enabled_features_.eh) return kWasmExceptRef;
+              break;
             default:
               break;
           }

src/wasm/module-instantiate.cc

@@ -1194,7 +1194,7 @@ void InstanceBuilder::InitGlobals() {
                                        global.init.val.f64_const);
         break;
       case WasmInitExpr::kRefNullConst:
-        DCHECK(enabled_.anyref);
+        DCHECK(enabled_.anyref || enabled_.eh);
         if (global.imported) break;  // We already initialized imported globals.
 
         tagged_globals_->set(global.offset,

src/wasm/value-type.h

@@ -182,7 +182,12 @@ class V8_EXPORT_PRIVATE ValueTypes {
     return (expected == actual) ||
            (expected == kWasmAnyRef && actual == kWasmNullRef) ||
            (expected == kWasmAnyRef && actual == kWasmAnyFunc) ||
-           (expected == kWasmAnyFunc && actual == kWasmNullRef);
+           (expected == kWasmAnyRef && actual == kWasmExceptRef) ||
+           (expected == kWasmAnyFunc && actual == kWasmNullRef) ||
+           // TODO(mstarzinger): For now we treat "null_ref" as a sub-type of
+           // "except_ref", which is correct but might change. See here:
+           // https://github.com/WebAssembly/exception-handling/issues/55
+           (expected == kWasmExceptRef && actual == kWasmNullRef);
   }
 
   static inline bool IsReferenceType(ValueType type) {
@@ -190,7 +195,8 @@ class V8_EXPORT_PRIVATE ValueTypes {
     // {kWasmNullRef}. If this assumption is wrong, it should be added to the
     // result calculation below.
     DCHECK_NE(type, kWasmNullRef);
-    return type == kWasmAnyRef || type == kWasmAnyFunc;
+    return type == kWasmAnyRef || type == kWasmAnyFunc ||
+           type == kWasmExceptRef;
   }
 
   static byte MemSize(MachineType type) {

test/mjsunit/wasm/exceptions-global.js

@@ -0,0 +1,44 @@
+// Copyright 2019 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.
+
+// Flags: --expose-wasm --experimental-wasm-eh --allow-natives-syntax
+
+// Note that this test does not pass --experimental-wasm-anyref on purpose so
+// that we make sure the two flags can be controlled separately/independently.
+
+load("test/mjsunit/wasm/wasm-module-builder.js");
+
+// First we just test that "except_ref" global variables are allowed.
+(function TestGlobalExceptRefSupported() {
+  print(arguments.callee.name);
+  let builder = new WasmModuleBuilder();
+  let g = builder.addGlobal(kWasmExceptRef);
+  builder.addFunction("push_and_drop_except_ref", kSig_v_v)
+      .addBody([
+        kExprGetGlobal, g.index,
+        kExprDrop,
+      ]).exportFunc();
+  let instance = builder.instantiate();
+
+  assertDoesNotThrow(instance.exports.push_and_drop_except_ref);
+})();
+
+// Test default value that global "except_ref" variables are initialized with.
+(function TestGlobalExceptRefDefaultValue() {
+  print(arguments.callee.name);
+  let builder = new WasmModuleBuilder();
+  let g = builder.addGlobal(kWasmExceptRef);
+  builder.addFunction('push_and_return_except_ref', kSig_e_v)
+      .addBody([kExprGetGlobal, g.index])
+      .exportFunc();
+  let instance = builder.instantiate();
+
+  assertEquals(null, instance.exports.push_and_return_except_ref());
+})();
+
+// TODO(mstarzinger): Add test coverage for the following:
+//   - Catching exception in wasm and storing into global.
+//   - Taking "except_ref" parameter and storing into global.
+//   - Rethrowing "except_ref" from global (or parameter).
+//   - Importing a global "except_ref" during instantiation.

test/mjsunit/wasm/wasm-module-builder.js

@@ -158,6 +158,7 @@ let kSig_v_aa = makeSig([kWasmAnyFunc, kWasmAnyFunc], []);
 let kSig_r_v = makeSig([], [kWasmAnyRef]);
 let kSig_a_v = makeSig([], [kWasmAnyFunc]);
 let kSig_a_i = makeSig([kWasmI32], [kWasmAnyFunc]);
+let kSig_e_v = makeSig([], [kWasmExceptRef]);
 
 function makeSig(params, results) {
   return {params: params, results: results};
@@ -1078,6 +1079,7 @@ class WasmModuleBuilder {
               break;
             case kWasmAnyRef:
             case kWasmAnyFunc:
+            case kWasmExceptRef:
               section.emit_u8(kExprRefNull);
               break;
             }