[wasm] add experimental string/Wasm GC array conversion fast-path

This CL adds two experimental JS builtins to convert between
i16 Wasm GC and JS strings. This is a non-standard experimental
feature only available with the flag --wasm-gc-js-interop.

WebAssembly.experimentalConvertArrayToString(array, start, count)
Convert the `count`-many WTF-16 code units starting at index `start`
into a JS string. Throws a TypeError if `array` is not an i16 array,
or if `start` and `count` are not numbers or not in range.

WebAssembly.experimentalConvertStringToArray(string, sampleArray)
Convert `string` to an i16 array. The `sampleArray` parameter needs
to be an arbitrary i16 array, which is only used to extract the rtt.
Throws a TypeError if `string` is not a string or `sampleArray` is not
an i16 array.

Change-Id: I7ac2f6bd89b8f638427f61da1bb01ccba90d735b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3642301
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80505}

src/builtins/torque-internal.tq

@@ -200,6 +200,10 @@ struct SliceIterator<T: type, Reference: type> {
     return *this.NextReference() otherwise NoMore;
   }
 
+  macro NextNotEmpty(): T {
+    return *this.NextReferenceNotEmpty();
+  }
+
   macro NextReference(): Reference labels NoMore {
     if (this.Empty()) {
       goto NoMore;
@@ -210,6 +214,13 @@ struct SliceIterator<T: type, Reference: type> {
     }
   }
 
+  macro NextReferenceNotEmpty(): Reference {
+    dcheck(!this.Empty());
+    const result = unsafe::NewReference<T>(this.object, this.start);
+    this.start += %SizeOf<T>();
+    return result;
+  }
+
   object: HeapObject|TaggedZeroPattern;
   start: intptr;
   end: intptr;

src/builtins/wasm.tq

@@ -315,7 +315,7 @@ builtin WasmAllocateStructWithRtt(rtt: Map): HeapObject {
 
 macro WasmAllocateArray(
     rtt: Map, length: uint32, elementSize: uint32,
-    initializationMode: constexpr InitializationMode): HeapObject {
+    initializationMode: constexpr InitializationMode): WasmArray {
   // instanceSize = RoundUp(elementSize * length, kObjectAlignment)
   //              + WasmArray::kHeaderSize
   const instanceSize: intptr =
@@ -328,7 +328,7 @@ macro WasmAllocateArray(
   // TODO(ishell): consider removing properties_or_hash field from WasmObjects.
   %RawDownCast<WasmArray>(result).properties_or_hash = kEmptyFixedArray;
   %RawDownCast<WasmArray>(result).length = length;
-  return result;
+  return %RawDownCast<WasmArray>(result);
 }
 
 builtin WasmAllocateArray_Uninitialized(
@@ -663,4 +663,105 @@ builtin ThrowWasmTrapArrayOutOfBounds(): JSAny {
 builtin ThrowWasmTrapArrayTooLarge(): JSAny {
   tail WasmTrap(SmiConstant(MessageTemplate::kWasmTrapArrayTooLarge));
 }
+
+macro TryNumberToIntptr(value: JSAny): intptr labels Failure {
+  typeswitch (value) {
+    case (s: Smi): {
+      return Convert<intptr>(s);
+    }
+    case (hn: HeapNumber): deferred {
+      let value = Convert<float64>(hn);
+      if (Float64IsNaN(value)) goto Failure;
+      value = math::Float64Trunc(value);
+      return ChangeFloat64ToIntPtr(value);
+    }
+    case (JSAny): deferred {
+      goto Failure;
+    }
+  }
+}
+
+macro WasmTypeInfo(map: Map): WasmTypeInfo {
+  return UnsafeCast<WasmTypeInfo>(
+      map.constructor_or_back_pointer_or_native_context);
+}
+
+const kWasmI16ValueType: constexpr int32
+    generates 'wasm::ValueType::Primitive(wasm::kI16).raw_bit_field()';
+
+const kWasmArrayTypeRepOffset:
+    constexpr intptr generates 'wasm::ArrayType::kRepOffset';
+const kWasmValueTypeBitFieldOffset:
+    constexpr intptr generates 'wasm::ValueType::kBitFieldOffset';
+
+macro IsWord16WasmArrayMap(map: Map): bool {
+  const arrayTypePtr: RawPtr<int32> = %RawDownCast<RawPtr<int32>>(
+      WasmTypeInfo(map).foreign_address_ptr + kWasmArrayTypeRepOffset +
+      kWasmValueTypeBitFieldOffset);
+  const arrayTypeRef: &int32 =
+      torque_internal::unsafe::NewOffHeapReference(arrayTypePtr);
+  return *arrayTypeRef == kWasmI16ValueType;
+}
+
+// Non-standard experimental feature.
+transitioning javascript builtin ExperimentalWasmConvertArrayToString(
+    js-implicit context: NativeContext)(
+    array: JSAny, start: JSAny, count: JSAny): String {
+  try {
+    const start = TryNumberToIntptr(start) otherwise goto InvalidArgument;
+    const count = TryNumberToIntptr(count) otherwise goto InvalidArgument;
+
+    const array = Cast<WasmArray>(array) otherwise goto InvalidArgument;
+    if (!IsWord16WasmArrayMap(array.map)) goto InvalidArgument;
+    const arrayContent = torque_internal::unsafe::NewConstSlice<char16>(
+        array, kWasmArrayHeaderSize, Convert<intptr>(array.length));
+
+    return AllocateSeqTwoByteString(
+        Convert<uint32>(Unsigned(count)),
+        (Subslice(arrayContent, start, count) otherwise goto InvalidArgument)
+            .Iterator());
+  } label InvalidArgument deferred {
+    ThrowTypeError(MessageTemplate::kInvalidArgument);
+  }
+}
+
+// Non-standard experimental feature.
+transitioning javascript builtin ExperimentalWasmConvertStringToArray(
+    js-implicit context: NativeContext)(
+    string: JSAny, sampleArray: JSAny): WasmArray {
+  try {
+    const sampleArray =
+        Cast<WasmArray>(sampleArray) otherwise goto InvalidArgument;
+    const arrayMap = sampleArray.map;
+    if (!IsWord16WasmArrayMap(arrayMap)) goto InvalidArgument;
+
+    const string = Cast<String>(string) otherwise goto InvalidArgument;
+    const length = string.length;
+
+    const result = WasmAllocateArray(
+        arrayMap, Unsigned(length), 2, InitializationMode::kUninitialized);
+    const arrayContent = torque_internal::unsafe::NewMutableSlice<char16>(
+        result, kWasmArrayHeaderSize, Convert<intptr>(length));
+    try {
+      StringToSlice(string) otherwise OneByte, TwoByte;
+    } label OneByte(slice: ConstSlice<char8>) {
+      let fromIt = slice.Iterator();
+      let toIt = arrayContent.Iterator();
+      while (true) {
+        let toRef = toIt.NextReference() otherwise break;
+        *toRef = %RawDownCast<char16>(Convert<uint16>(fromIt.NextNotEmpty()));
+      }
+    } label TwoByte(slice: ConstSlice<char16>) {
+      let fromIt = slice.Iterator();
+      let toIt = arrayContent.Iterator();
+      while (true) {
+        let toRef = toIt.NextReference() otherwise break;
+        *toRef = fromIt.NextNotEmpty();
+      }
+    }
+    return result;
+  } label InvalidArgument deferred {
+    ThrowTypeError(MessageTemplate::kInvalidArgument);
+  }
+}
 }

src/init/bootstrapper.cc

@@ -572,19 +572,6 @@ V8_NOINLINE Handle<JSFunction> InstallFunctionWithBuiltinId(
   return fun;
 }
 
-V8_NOINLINE Handle<JSFunction> SimpleInstallFunction(
-    Isolate* isolate, Handle<JSObject> base, const char* name, Builtin call,
-    int len, bool adapt, PropertyAttributes attrs = DONT_ENUM) {
-  // Although function name does not have to be internalized the property name
-  // will be internalized during property addition anyway, so do it here now.
-  Handle<String> internalized_name =
-      isolate->factory()->InternalizeUtf8String(name);
-  Handle<JSFunction> fun =
-      SimpleCreateFunction(isolate, internalized_name, call, len, adapt);
-  JSObject::AddProperty(isolate, base, internalized_name, fun, attrs);
-  return fun;
-}
-
 V8_NOINLINE Handle<JSFunction> InstallFunctionAtSymbol(
     Isolate* isolate, Handle<JSObject> base, Handle<Symbol> symbol,
     const char* symbol_string, Builtin call, int len, bool adapt,
@@ -908,6 +895,21 @@ Handle<Map> CreateNonConstructorMap(Isolate* isolate, Handle<Map> source_map,
 
 }  // namespace
 
+Handle<JSFunction> SimpleInstallFunction(Isolate* isolate,
+                                         Handle<JSObject> base,
+                                         const char* name, Builtin call,
+                                         int len, bool adapt,
+                                         PropertyAttributes attrs) {
+  // Although function name does not have to be internalized the property name
+  // will be internalized during property addition anyway, so do it here now.
+  Handle<String> internalized_name =
+      isolate->factory()->InternalizeUtf8String(name);
+  Handle<JSFunction> fun =
+      SimpleCreateFunction(isolate, internalized_name, call, len, adapt);
+  JSObject::AddProperty(isolate, base, internalized_name, fun, attrs);
+  return fun;
+}
+
 void Genesis::CreateIteratorMaps(Handle<JSFunction> empty) {
   // Create iterator-related meta-objects.
   Handle<JSObject> iterator_prototype = factory()->NewJSObject(

src/init/bootstrapper.h

@@ -129,6 +129,10 @@ class BootstrapperActive final {
   Bootstrapper* bootstrapper_;
 };
 
+V8_NOINLINE Handle<JSFunction> SimpleInstallFunction(
+    Isolate* isolate, Handle<JSObject> base, const char* name, Builtin call,
+    int len, bool adapt, PropertyAttributes attrs = DONT_ENUM);
+
 }  // namespace internal
 }  // namespace v8
 

src/wasm/struct-types.h

@@ -139,11 +139,15 @@ class ArrayType : public ZoneObject {
     return rep_ != other.rep_ || mutability_ != other.mutability_;
   }
 
+  static const intptr_t kRepOffset;
+
  private:
   const ValueType rep_;
   const bool mutability_;
 };
 
+inline constexpr intptr_t ArrayType::kRepOffset = offsetof(ArrayType, rep_);
+
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8

src/wasm/value-type.h

@@ -550,6 +550,8 @@ class ValueType {
   static constexpr int kKindBits = 5;
   static constexpr int kHeapTypeBits = 20;
 
+  static const intptr_t kBitFieldOffset;
+
  private:
   // {hash_value} directly reads {bit_field_}.
   friend size_t hash_value(ValueType type);
@@ -576,6 +578,9 @@ class ValueType {
   uint32_t bit_field_;
 };
 
+inline constexpr intptr_t ValueType::kBitFieldOffset =
+    offsetof(ValueType, bit_field_);
+
 static_assert(sizeof(ValueType) <= kUInt32Size,
               "ValueType is small and can be passed by value");
 static_assert(ValueType::kLastUsedBit < 8 * sizeof(ValueType) - kSmiTagSize,

src/wasm/wasm-js.cc

@@ -2797,6 +2797,17 @@ void WasmJs::Install(Isolate* isolate, bool exposed_on_global_object) {
   InstallFunc(isolate, webassembly, "validate", WebAssemblyValidate, 1);
   InstallFunc(isolate, webassembly, "instantiate", WebAssemblyInstantiate, 1);
 
+  // TODO(tebbi): Put this behind its own flag once --wasm-gc-js-interop gets
+  // closer to shipping.
+  if (FLAG_wasm_gc_js_interop) {
+    SimpleInstallFunction(
+        isolate, webassembly, "experimentalConvertArrayToString",
+        Builtin::kExperimentalWasmConvertArrayToString, 0, true);
+    SimpleInstallFunction(
+        isolate, webassembly, "experimentalConvertStringToArray",
+        Builtin::kExperimentalWasmConvertStringToArray, 0, true);
+  }
+
   if (FLAG_wasm_test_streaming) {
     isolate->set_wasm_streaming_callback(WasmStreamingCallbackForTesting);
   }

test/mjsunit/wasm/gc-experimental-string-conversions.js

@@ -0,0 +1,47 @@
+// Copyright 2022 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: --experimental-wasm-gc --wasm-gc-js-interop
+
+d8.file.execute('test/mjsunit/wasm/wasm-module-builder.js');
+
+var builder = new WasmModuleBuilder();
+
+let i16Array = builder.addArray(kWasmI16, true);
+
+builder.addFunction('getHelloArray', makeSig([], [kWasmAnyRef]))
+    .addBody([
+      ...wasmI32Const(72), ...wasmI32Const(69), ...wasmI32Const(76),
+      ...wasmI32Const(76), ...wasmI32Const(79), kGCPrefix, kExprArrayInitStatic,
+      i16Array, 5
+    ])
+    .exportFunc();
+
+builder.addFunction('getChar', makeSig([kWasmAnyRef, kWasmI32], [kWasmI32]))
+    .addBody([
+      kExprLocalGet, 0, kGCPrefix, kExprRefAsData, kGCPrefix,
+      kExprRefCastStatic, i16Array, kExprLocalGet, 1, kGCPrefix, kExprArrayGetS,
+      i16Array
+    ])
+    .exportFunc();
+
+const instance = builder.instantiate();
+const getHelloArray = instance.exports.getHelloArray;
+const getChar = instance.exports.getChar;
+
+assertEquals(
+    WebAssembly.experimentalConvertArrayToString(getHelloArray(), 0, 5),
+    'HELLO');
+assertEquals(
+    WebAssembly.experimentalConvertArrayToString(getHelloArray(), 1, 4),
+    'ELLO');
+assertEquals(
+    WebAssembly.experimentalConvertArrayToString(getHelloArray(), 0, 3), 'HEL');
+
+const string = 'foobar'
+const array =
+    WebAssembly.experimentalConvertStringToArray('foobar', getHelloArray());
+for (let i = 0; i < string.length; ++i) {
+  assertEquals(getChar(array, i), string.charCodeAt(i));
+}