[wasm] Add support for 'except_ref' value type.

This adds experimental support for an 'except_ref' value type for caught
exceptions as per the exception handling proposal. In the current for it
is only allowed to have such types in the stack or in a local, support
for having it as part of any signature was left out.

The default value for a local of type 'except_ref' is the 'ref_null'
value for now. Since this value cannot escape a wasm function, the
concrete value is not actually observable.

R=ahaas@chromium.org
TEST=unittests/LocalDeclDecoderTest.ExceptRef,mjsunit/wasm/exceptions
BUG=v8:8091

Change-Id: I7bd65274327a833262f8749cbe0e24e737f6e0c1
Reviewed-on: https://chromium-review.googlesource.com/1196510Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55526}

src/wasm/function-body-decoder-impl.h

@@ -737,6 +737,13 @@ class WasmDecoder : public Decoder {
           }
           decoder->error(decoder->pc() - 1, "invalid local type");
           return false;
+        case kLocalExceptRef:
+          if (enabled.eh) {
+            type = kWasmExceptRef;
+            break;
+          }
+          decoder->error(decoder->pc() - 1, "invalid local type");
+          return false;
         case kLocalS128:
           if (enabled.simd) {
             type = kWasmS128;

src/wasm/function-body-decoder.cc

@@ -99,6 +99,12 @@ class WasmGraphBuildingInterface {
     // instance parameter.
     TFNode* start = builder_->Start(
         static_cast<int>(decoder->sig_->parameter_count() + 1 + 1));
+    ssa_env->effect = start;
+    ssa_env->control = start;
+    // Initialize effect and control before initializing the locals default
+    // values (which might require instance loads) or loading the context.
+    builder_->set_effect_ptr(&ssa_env->effect);
+    builder_->set_control_ptr(&ssa_env->control);
     // Initialize the instance parameter (index 0).
     builder_->set_instance_node(builder_->Param(kWasmInstanceParameterIndex));
     // Initialize local variables. Parameters are shifted by 1 because of the
@@ -115,11 +121,6 @@ class WasmGraphBuildingInterface {
         ssa_env->locals[index++] = node;
       }
     }
-    ssa_env->effect = start;
-    ssa_env->control = start;
-    // Initialize effect and control before loading the context.
-    builder_->set_effect_ptr(&ssa_env->effect);
-    builder_->set_control_ptr(&ssa_env->control);
     LoadContextIntoSsa(ssa_env);
     SetEnv(ssa_env);
   }
@@ -596,6 +597,8 @@ class WasmGraphBuildingInterface {
         return builder_->Float64Constant(0);
       case kWasmS128:
         return builder_->S128Zero();
+      case kWasmExceptRef:
+        return builder_->RefNull();
       default:
         UNREACHABLE();
     }

src/wasm/value-type.h

@@ -21,6 +21,7 @@ enum ValueType : uint8_t {
   kWasmS128,
   kWasmAnyRef,
   kWasmAnyFunc,
+  kWasmExceptRef,
   kWasmVar,
 };
 
@@ -220,6 +221,8 @@ class V8_EXPORT_PRIVATE ValueTypes {
         return kLocalS128;
       case kWasmAnyRef:
         return kLocalAnyRef;
+      case kWasmExceptRef:
+        return kLocalExceptRef;
       case kWasmStmt:
         return kLocalVoid;
       default:

src/wasm/wasm-constants.h

@@ -25,7 +25,8 @@ enum ValueTypeCode : uint8_t {
   kLocalF64 = 0x7c,
   kLocalS128 = 0x7b,
   kLocalAnyFunc = 0x70,
-  kLocalAnyRef = 0x6f
+  kLocalAnyRef = 0x6f,
+  kLocalExceptRef = 0x68,
 };
 // Binary encoding of other types.
 constexpr uint8_t kWasmFunctionTypeCode = 0x60;

test/mjsunit/wasm/exceptions.js

@@ -7,6 +7,19 @@
 load("test/mjsunit/wasm/wasm-constants.js");
 load("test/mjsunit/wasm/wasm-module-builder.js");
 
+// First we just test that "except_ref" local variables are allowed.
+(function TestLocalExceptRef() {
+  let builder = new WasmModuleBuilder();
+  builder.addFunction("push_and_drop_except_ref", kSig_v_v)
+      .addBody([
+        kExprGetLocal, 0,
+        kExprDrop,
+      ]).addLocals({except_count: 1}).exportFunc();
+  let instance = builder.instantiate();
+
+  assertDoesNotThrow(instance.exports.push_and_drop_except_ref);
+})();
+
 // The following method doesn't attempt to catch an raised exception.
 (function TestThrowSimple() {
   let builder = new WasmModuleBuilder();

test/mjsunit/wasm/wasm-constants.js

@@ -91,6 +91,7 @@ let kWasmF32 = 0x7d;
 let kWasmF64 = 0x7c;
 let kWasmS128  = 0x7b;
 let kWasmAnyRef = 0x6f;
+let kWasmExceptRef = 0x68;
 
 let kExternalFunction = 0;
 let kExternalTable = 1;

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

@@ -569,6 +569,9 @@ class WasmModuleBuilder {
             if (l.s128_count > 0) {
               local_decls.push({count: l.s128_count, type: kWasmS128});
             }
+            if (l.except_count > 0) {
+              local_decls.push({count: l.except_count, type: kWasmExceptRef});
+            }
           }
 
           let header = new Binary;

test/unittests/wasm/function-body-decoder-unittest.cc

@@ -3127,6 +3127,20 @@ TEST_F(LocalDeclDecoderTest, UseEncoder) {
   pos = ExpectRun(map, pos, kWasmI64, 212);
 }
 
+TEST_F(LocalDeclDecoderTest, ExceptRef) {
+  WASM_FEATURE_SCOPE(eh);
+  ValueType type = kWasmExceptRef;
+  const byte data[] = {1, 1,
+                       static_cast<byte>(ValueTypes::ValueTypeCodeFor(type))};
+  BodyLocalDecls decls(zone());
+  bool result = DecodeLocalDecls(&decls, data, data + sizeof(data));
+  EXPECT_TRUE(result);
+  EXPECT_EQ(1u, decls.type_list.size());
+
+  TypesOfLocals map = decls.type_list;
+  EXPECT_EQ(type, map[0]);
+}
+
 class BytecodeIteratorTest : public TestWithZone {};
 
 TEST_F(BytecodeIteratorTest, SimpleForeach) {