syscall/js: use stable references to JavaScript values

This commit changes how JavaScript values are referenced by Go code.
After this change, a JavaScript value is always represented by the same
ref, even if passed multiple times from JavaScript to Go. This allows
Go's == operator to work as expected on js.Value (strict equality).
Additionally, the performance of some operations of the syscall/js
package got improved by saving additional roundtrips to JavaScript code.

Fixes #25802.

Change-Id: Ide6ffe66c6aa1caf5327a2d3ddbe48fe7c180461
Reviewed-on: https://go-review.googlesource.com/120561
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/wasm/wasm_exec.js

@@ -31,7 +31,7 @@
 
 		let outputBuf = "";
 		global.fs = {
-			constants: {},
+			constants: { O_WRONLY: -1, O_RDWR: -1, O_CREAT: -1, O_TRUNC: -1, O_APPEND: -1, O_EXCL: -1, O_NONBLOCK: -1, O_SYNC: -1 }, // unused
 			writeSync(fd, buf) {
 				outputBuf += decoder.decode(buf);
 				const nl = outputBuf.lastIndexOf("\n");
@@ -81,21 +81,72 @@
 			}
 
 			const loadValue = (addr) => {
+				const f = mem().getFloat64(addr, true);
+				if (!isNaN(f)) {
+					return f;
+				}
+
 				const id = mem().getUint32(addr, true);
 				return this._values[id];
 			}
 
 			const storeValue = (addr, v) => {
-				if (v === undefined) {
+				if (typeof v === "number") {
-					mem().setUint32(addr, 0, true);
+					if (isNaN(v)) {
+						mem().setUint32(addr + 4, 0x7FF80000, true); // NaN
+						mem().setUint32(addr, 0, true);
+						return;
+					}
+					mem().setFloat64(addr, v, true);
 					return;
 				}
-				if (v === null) {
+
-					mem().setUint32(addr, 1, true);
+				mem().setUint32(addr + 4, 0x7FF80000, true); // NaN
+
+				switch (v) {
+					case undefined:
+						mem().setUint32(addr, 1, true);
+						return;
+					case null:
+						mem().setUint32(addr, 2, true);
+						return;
+					case true:
+						mem().setUint32(addr, 3, true);
+						return;
+					case false:
+						mem().setUint32(addr, 4, true);
+						return;
+				}
+
+				if (typeof v === "string") {
+					let ref = this._stringRefs.get(v);
+					if (ref === undefined) {
+						ref = this._values.length;
+						this._values.push(v);
+						this._stringRefs.set(v, ref);
+					}
+					mem().setUint32(addr, ref, true);
 					return;
 				}
-				this._values.push(v);
+
-				mem().setUint32(addr, this._values.length - 1, true);
+				if (typeof v === "symbol") {
+					let ref = this._symbolRefs.get(v);
+					if (ref === undefined) {
+						ref = this._values.length;
+						this._values.push(v);
+						this._symbolRefs.set(v, ref);
+					}
+					mem().setUint32(addr, ref, true);
+					return;
+				}
+
+				let ref = v[this._refProp];
+				if (ref === undefined) {
+					ref = this._values.length;
+					this._values.push(v);
+					v[this._refProp] = ref;
+				}
+				mem().setUint32(addr, ref, true);
 			}
 
 			const loadSlice = (addr) => {
@@ -109,8 +160,7 @@
 				const len = getInt64(addr + 8);
 				const a = new Array(len);
 				for (let i = 0; i < len; i++) {
-					const id = mem().getUint32(array + i * 4, true);
+					a[i] = loadValue(array + i * 8);
-					a[i] = this._values[id];
 				}
 				return a;
 			}
@@ -173,21 +223,6 @@
 						crypto.getRandomValues(loadSlice(sp + 8));
 					},
 
-					// func boolVal(value bool) ref
-					"syscall/js.boolVal": (sp) => {
-						storeValue(sp + 16, mem().getUint8(sp + 8) !== 0);
-					},
-
-					// func intVal(value int) ref
-					"syscall/js.intVal": (sp) => {
-						storeValue(sp + 16, getInt64(sp + 8));
-					},
-
-					// func floatVal(value float64) ref
-					"syscall/js.floatVal": (sp) => {
-						storeValue(sp + 16, mem().getFloat64(sp + 8, true));
-					},
-
 					// func stringVal(value string) ref
 					"syscall/js.stringVal": (sp) => {
 						storeValue(sp + 24, loadString(sp + 8));
@@ -220,10 +255,10 @@
 							const m = Reflect.get(v, loadString(sp + 16));
 							const args = loadSliceOfValues(sp + 32);
 							storeValue(sp + 56, Reflect.apply(m, v, args));
-							mem().setUint8(sp + 60, 1);
+							mem().setUint8(sp + 64, 1);
 						} catch (err) {
 							storeValue(sp + 56, err);
-							mem().setUint8(sp + 60, 0);
+							mem().setUint8(sp + 64, 0);
 						}
 					},
 
@@ -233,10 +268,10 @@
 							const v = loadValue(sp + 8);
 							const args = loadSliceOfValues(sp + 16);
 							storeValue(sp + 40, Reflect.apply(v, undefined, args));
-							mem().setUint8(sp + 44, 1);
+							mem().setUint8(sp + 48, 1);
 						} catch (err) {
 							storeValue(sp + 40, err);
-							mem().setUint8(sp + 44, 0);
+							mem().setUint8(sp + 48, 0);
 						}
 					},
 
@@ -246,28 +281,13 @@
 							const v = loadValue(sp + 8);
 							const args = loadSliceOfValues(sp + 16);
 							storeValue(sp + 40, Reflect.construct(v, args));
-							mem().setUint8(sp + 44, 1);
+							mem().setUint8(sp + 48, 1);
 						} catch (err) {
 							storeValue(sp + 40, err);
-							mem().setUint8(sp + 44, 0);
+							mem().setUint8(sp + 48, 0);
 						}
 					},
 
-					// func valueFloat(v ref) float64
-					"syscall/js.valueFloat": (sp) => {
-						mem().setFloat64(sp + 16, parseFloat(loadValue(sp + 8)), true);
-					},
-
-					// func valueInt(v ref) int
-					"syscall/js.valueInt": (sp) => {
-						setInt64(sp + 16, parseInt(loadValue(sp + 8)));
-					},
-
-					// func valueBool(v ref) bool
-					"syscall/js.valueBool": (sp) => {
-						mem().setUint8(sp + 16, !!loadValue(sp + 8));
-					},
-
 					// func valueLength(v ref) int
 					"syscall/js.valueLength": (sp) => {
 						setInt64(sp + 16, parseInt(loadValue(sp + 8).length));
@@ -288,7 +308,7 @@
 
 					// func valueInstanceOf(v ref, t ref) bool
 					"syscall/js.valueInstanceOf": (sp) => {
-						mem().setUint8(sp + 16, loadValue(sp + 8) instanceof loadValue(sp + 12));
+						mem().setUint8(sp + 24, loadValue(sp + 8) instanceof loadValue(sp + 16));
 					},
 
 					"debug": (value) => {
@@ -301,8 +321,11 @@
 		async run(instance) {
 			this._inst = instance;
 			this._values = [ // TODO: garbage collection
+				NaN,
 				undefined,
 				null,
+				true,
+				false,
 				global,
 				this._inst.exports.mem,
 				() => { // resolveCallbackPromise
@@ -312,6 +335,9 @@
 					setTimeout(this._resolveCallbackPromise, 0); // make sure it is asynchronous
 				},
 			];
+			this._stringRefs = new Map();
+			this._symbolRefs = new Map();
+			this._refProp = Symbol();
 			this.exited = false;
 
 			const mem = new DataView(this._inst.exports.mem.buffer)

src/cmd/vet/all/whitelist/wasm.txt

@@ -17,18 +17,12 @@ runtime/asm_wasm.s: [wasm] rt0_go: use of 8(SP) points beyond argument frame
 // Calling WebAssembly import. No write from Go assembly.
 runtime/sys_wasm.s: [wasm] nanotime: RET without writing to 8-byte ret+0(FP)
 runtime/sys_wasm.s: [wasm] scheduleCallback: RET without writing to 4-byte ret+8(FP)
-syscall/js/js_js.s: [wasm] boolVal: RET without writing to 4-byte ret+8(FP)
+syscall/js/js_js.s: [wasm] stringVal: RET without writing to 8-byte ret+16(FP)
-syscall/js/js_js.s: [wasm] intVal: RET without writing to 4-byte ret+8(FP)
+syscall/js/js_js.s: [wasm] valueGet: RET without writing to 8-byte ret+24(FP)
-syscall/js/js_js.s: [wasm] floatVal: RET without writing to 4-byte ret+8(FP)
+syscall/js/js_js.s: [wasm] valueIndex: RET without writing to 8-byte ret+16(FP)
-syscall/js/js_js.s: [wasm] stringVal: RET without writing to 4-byte ret+16(FP)
+syscall/js/js_js.s: [wasm] valueCall: RET without writing to 8-byte ret+48(FP)
-syscall/js/js_js.s: [wasm] valueGet: RET without writing to 4-byte ret+24(FP)
+syscall/js/js_js.s: [wasm] valueInvoke: RET without writing to 8-byte ret+32(FP)
-syscall/js/js_js.s: [wasm] valueIndex: RET without writing to 4-byte ret+16(FP)
+syscall/js/js_js.s: [wasm] valueNew: RET without writing to 8-byte ret+32(FP)
-syscall/js/js_js.s: [wasm] valueCall: RET without writing to 4-byte ret+48(FP)
-syscall/js/js_js.s: [wasm] valueInvoke: RET without writing to 4-byte ret+32(FP)
-syscall/js/js_js.s: [wasm] valueNew: RET without writing to 4-byte ret+32(FP)
-syscall/js/js_js.s: [wasm] valueFloat: RET without writing to 8-byte ret+8(FP)
-syscall/js/js_js.s: [wasm] valueInt: RET without writing to 8-byte ret+8(FP)
-syscall/js/js_js.s: [wasm] valueBool: RET without writing to 1-byte ret+8(FP)
 syscall/js/js_js.s: [wasm] valueLength: RET without writing to 8-byte ret+8(FP)
-syscall/js/js_js.s: [wasm] valuePrepareString: RET without writing to 4-byte ret+8(FP)
+syscall/js/js_js.s: [wasm] valuePrepareString: RET without writing to 8-byte ret+8(FP)
-syscall/js/js_js.s: [wasm] valueInstanceOf: RET without writing to 1-byte ret+8(FP)
+syscall/js/js_js.s: [wasm] valueInstanceOf: RET without writing to 1-byte ret+16(FP)

src/syscall/js/js.go

@@ -15,8 +15,14 @@ import (
 	"unsafe"
 )
 
-// ref is used to identify a JavaScript value, since the value itself can not be passed to WebAssembly itself.
+// ref is used to identify a JavaScript value, since the value itself can not be passed to WebAssembly.
-type ref uint32
+// A JavaScript number (64-bit float, except NaN) is represented by its IEEE 754 binary representation.
+// All other values are represented as an IEEE 754 binary representation of NaN with the low 32 bits
+// used as an ID.
+type ref uint64
+
+// nanHead are the upper 32 bits of a ref if the value is not a JavaScript number or NaN itself.
+const nanHead = 0x7FF80000
 
 // Value represents a JavaScript value.
 type Value struct {
@@ -27,6 +33,17 @@ func makeValue(v ref) Value {
 	return Value{ref: v}
 }
 
+func predefValue(id uint32) Value {
+	return Value{ref: nanHead<<32 | ref(id)}
+}
+
+func floatValue(f float64) Value {
+	if f != f {
+		return valueNaN
+	}
+	return Value{ref: *(*ref)(unsafe.Pointer(&f))}
+}
+
 // Error wraps a JavaScript error.
 type Error struct {
 	// Value is the underlying JavaScript error value.
@@ -39,11 +56,14 @@ func (e Error) Error() string {
 }
 
 var (
-	valueUndefined         = makeValue(0)
+	valueNaN               = predefValue(0)
-	valueNull              = makeValue(1)
+	valueUndefined         = predefValue(1)
-	valueGlobal            = makeValue(2)
+	valueNull              = predefValue(2)
-	memory                 = makeValue(3) // WebAssembly linear memory
+	valueTrue              = predefValue(3)
-	resolveCallbackPromise = makeValue(4) // function that the callback helper uses to resume the execution of Go's WebAssembly code
+	valueFalse             = predefValue(4)
+	valueGlobal            = predefValue(5)
+	memory                 = predefValue(6) // WebAssembly linear memory
+	resolveCallbackPromise = predefValue(7) // function that the callback helper uses to resume the execution of Go's WebAssembly code
 )
 
 // Undefined returns the JavaScript value "undefined".
@@ -73,35 +93,39 @@ func ValueOf(x interface{}) Value {
 	case nil:
 		return valueNull
 	case bool:
-		return makeValue(boolVal(x))
+		if x {
+			return valueTrue
+		} else {
+			return valueFalse
+		}
 	case int:
-		return makeValue(intVal(x))
+		return floatValue(float64(x))
 	case int8:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case int16:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case int32:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case int64:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case uint:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case uint8:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case uint16:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case uint32:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case uint64:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case uintptr:
-		return makeValue(intVal(int(x)))
+		return floatValue(float64(x))
 	case unsafe.Pointer:
-		return makeValue(intVal(int(uintptr(x))))
+		return floatValue(float64(uintptr(x)))
 	case float32:
-		return makeValue(floatVal(float64(x)))
+		return floatValue(float64(x))
 	case float64:
-		return makeValue(floatVal(x))
+		return floatValue(x)
 	case string:
 		return makeValue(stringVal(x))
 	case []byte:
@@ -114,12 +138,6 @@ func ValueOf(x interface{}) Value {
 	}
 }
 
-func boolVal(x bool) ref
-
-func intVal(x int) ref
-
-func floatVal(x float64) ref
-
 func stringVal(x string) ref
 
 // Get returns the JavaScript property p of value v.
@@ -201,27 +219,35 @@ func (v Value) New(args ...interface{}) Value {
 
 func valueNew(v ref, args []ref) (ref, bool)
 
-// Float returns the value v converted to float64 according to JavaScript type conversions (parseFloat).
+func (v Value) isNumber() bool {
+	return v.ref>>32 != nanHead || v.ref == valueNaN.ref
+}
+
+// Float returns the value v as a float64. It panics if v is not a JavaScript number.
 func (v Value) Float() float64 {
-	return valueFloat(v.ref)
+	if !v.isNumber() {
+		panic("syscall/js: not a number")
+	}
+	return *(*float64)(unsafe.Pointer(&v.ref))
 }
 
-func valueFloat(v ref) float64
+// Int returns the value v truncated to an int. It panics if v is not a JavaScript number.
-
-// Int returns the value v converted to int according to JavaScript type conversions (parseInt).
 func (v Value) Int() int {
-	return valueInt(v.ref)
+	return int(v.Float())
 }
 
-func valueInt(v ref) int
+// Bool returns the value v as a bool. It panics if v is not a JavaScript boolean.
-
-// Bool returns the value v converted to bool according to JavaScript type conversions.
 func (v Value) Bool() bool {
-	return valueBool(v.ref)
+	switch v.ref {
+	case valueTrue.ref:
+		return true
+	case valueFalse.ref:
+		return false
+	default:
+		panic("syscall/js: not a boolean")
+	}
 }
 
-func valueBool(v ref) bool
-
 // String returns the value v converted to string according to JavaScript type conversions.
 func (v Value) String() string {
 	str, length := valuePrepareString(v.ref)

src/syscall/js/js_js.s

@@ -4,18 +4,6 @@
 
 #include "textflag.h"
 
-TEXT ·boolVal(SB), NOSPLIT, $0
-  CallImport
-  RET
-
-TEXT ·intVal(SB), NOSPLIT, $0
-  CallImport
-  RET
-
-TEXT ·floatVal(SB), NOSPLIT, $0
-  CallImport
-  RET
-
 TEXT ·stringVal(SB), NOSPLIT, $0
   CallImport
   RET
@@ -48,18 +36,6 @@ TEXT ·valueNew(SB), NOSPLIT, $0
   CallImport
   RET
 
-TEXT ·valueFloat(SB), NOSPLIT, $0
-  CallImport
-  RET
-
-TEXT ·valueInt(SB), NOSPLIT, $0
-  CallImport
-  RET
-
-TEXT ·valueBool(SB), NOSPLIT, $0
-  CallImport
-  RET
-
 TEXT ·valueLength(SB), NOSPLIT, $0
   CallImport
   RET

src/syscall/js/js_test.go

@@ -8,6 +8,7 @@ package js_test
 
 import (
 	"fmt"
+	"math"
 	"syscall/js"
 	"testing"
 )
@@ -21,6 +22,7 @@ var dummys = js.Global().Call("eval", `({
 	add: function(a, b) {
 		return a + b;
 	},
+	NaN: NaN,
 })`)
 
 func TestBool(t *testing.T) {
@@ -33,6 +35,9 @@ func TestBool(t *testing.T) {
 	if got := dummys.Get("otherBool").Bool(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
+	if dummys.Get("someBool") != dummys.Get("someBool") {
+		t.Errorf("same value not equal")
+	}
 }
 
 func TestString(t *testing.T) {
@@ -45,6 +50,9 @@ func TestString(t *testing.T) {
 	if got := dummys.Get("otherString").String(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
+	if dummys.Get("someString") != dummys.Get("someString") {
+		t.Errorf("same value not equal")
+	}
 }
 
 func TestInt(t *testing.T) {
@@ -57,6 +65,9 @@ func TestInt(t *testing.T) {
 	if got := dummys.Get("otherInt").Int(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
+	if dummys.Get("someInt") != dummys.Get("someInt") {
+		t.Errorf("same value not equal")
+	}
 }
 
 func TestIntConversion(t *testing.T) {
@@ -87,6 +98,23 @@ func TestFloat(t *testing.T) {
 	if got := dummys.Get("otherFloat").Float(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
+	if dummys.Get("someFloat") != dummys.Get("someFloat") {
+		t.Errorf("same value not equal")
+	}
+}
+
+func TestObject(t *testing.T) {
+	if dummys.Get("someArray") != dummys.Get("someArray") {
+		t.Errorf("same value not equal")
+	}
+}
+
+func TestNaN(t *testing.T) {
+	want := js.ValueOf(math.NaN())
+	got := dummys.Get("NaN")
+	if got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
 }
 
 func TestUndefined(t *testing.T) {