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>

3 files changed, 94 insertions, 64 deletions

diff --git a/src/syscall/js/js.go b/src/syscall/js/js.go
index 93c3965246..8217c24c5e 100644
--- a/src/syscall/js/js.go
+++ b/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.
-type ref uint32
+// ref is used to identify a JavaScript value, since the value itself can not be passed to WebAssembly.
+// 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)
-	valueNull              = makeValue(1)
-	valueGlobal            = makeValue(2)
-	memory                 = makeValue(3) // WebAssembly linear memory
-	resolveCallbackPromise = makeValue(4) // function that the callback helper uses to resume the execution of Go's WebAssembly code
+	valueNaN               = predefValue(0)
+	valueUndefined         = predefValue(1)
+	valueNull              = predefValue(2)
+	valueTrue              = predefValue(3)
+	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) Float() float64 {
-	return valueFloat(v.ref)
+func (v Value) isNumber() bool {
+	return v.ref>>32 != nanHead || v.ref == valueNaN.ref
 }
 
-func valueFloat(v ref) float64
+// Float returns the value v as a float64. It panics if v is not a JavaScript number.
+func (v Value) Float() float64 {
+	if !v.isNumber() {
+		panic("syscall/js: not a number")
+	}
+	return *(*float64)(unsafe.Pointer(&v.ref))
+}
 
-// Int returns the value v converted to int according to JavaScript type conversions (parseInt).
+// Int returns the value v truncated to an int. It panics if v is not a JavaScript number.
 func (v Value) Int() int {
-	return valueInt(v.ref)
+	return int(v.Float())
 }
 
-func valueInt(v ref) int
-
-// Bool returns the value v converted to bool according to JavaScript type conversions.
+// Bool returns the value v as a bool. It panics if v is not a JavaScript boolean.
 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)
diff --git a/src/syscall/js/js_js.s b/src/syscall/js/js_js.s
index cb90d88a6a..0ec164d5cb 100644
--- a/src/syscall/js/js_js.s
+++ b/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
diff --git a/src/syscall/js/js_test.go b/src/syscall/js/js_test.go
index e5e950f3a3..c96ad82850 100644
--- a/src/syscall/js/js_test.go
+++ b/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) {