reflect: allocate memory in TypeAssert[I] only when the assertion succeeds

goos: linux
goarch: amd64
pkg: reflect
cpu: 11th Gen Intel(R) Core(TM) i7-1185G7 @ 3.00GHz
                                                                        │ /tmp/before │              /tmp/after               │
                                                                        │   sec/op    │   sec/op     vs base                  │
TypeAssert/TypeAssert[int](int)-8                                         2.599n ± 1%   2.558n ± 0%  -1.56% (p=0.000 n=30)
TypeAssert/TypeAssert[uint8](int)-8                                       2.506n ± 1%   2.579n ± 2%  +2.93% (p=0.008 n=30)
TypeAssert/TypeAssert[fmt.Stringer](reflect_test.testTypeWithMethod)-8    7.449n ± 2%   7.776n ± 2%  +4.39% (p=0.000 n=30)
TypeAssert/TypeAssert[fmt.Stringer](*reflect_test.testTypeWithMethod)-8   7.220n ± 2%   7.439n ± 1%  +3.04% (p=0.000 n=30)
TypeAssert/TypeAssert[interface_{}](int)-8                                4.015n ± 1%   4.207n ± 1%  +4.79% (p=0.000 n=30)
TypeAssert/TypeAssert[interface_{}](reflect_test.testTypeWithMethod)-8    4.003n ± 1%   4.190n ± 2%  +4.66% (p=0.000 n=30)
TypeAssert/TypeAssert[time.Time](time.Time)-8                             2.933n ± 1%   2.942n ± 1%       ~ (p=0.327 n=20+30)
TypeAssert/TypeAssert[func()_string](func()_string)-8                                   111.5n ± 1%
geomean                                                                   4.004n        6.208n       +2.62%

Change-Id: I6a6a6964d6f9c794adc15dc5ff3dc8634b30df89
Reviewed-on: https://go-review.googlesource.com/c/go/+/705255
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/abi/iface.go

@@ -31,3 +31,11 @@ type NonEmptyInterface struct {
 	ITab *ITab
 	Data unsafe.Pointer
 }
+
+// CommonInterface describes the layout of both [EmptyInterface] and [NonEmptyInterface].
+type CommonInterface struct {
+	// Either an *ITab or a *Type, unexported to avoid accidental use.
+	_ unsafe.Pointer
+
+	Data unsafe.Pointer
+}

src/reflect/all_test.go

@@ -8783,6 +8783,9 @@ func TestTypeAssertAllocs(t *testing.T) {
 
 	typeAssertAllocs[time.Time](t, ValueOf(new(time.Time)).Elem(), 0)
 	typeAssertAllocs[time.Time](t, ValueOf(*new(time.Time)), 0)
+
+	type I interface{ foo() }
+	typeAssertAllocs[I](t, ValueOf(new(string)).Elem(), 0) // assert fail doesn't alloc
 }
 
 func typeAssertAllocs[T any](t *testing.T, val Value, wantAllocs int) {

src/reflect/value.go

@@ -120,10 +120,16 @@ func (v Value) pointer() unsafe.Pointer {
 
 // packEface converts v to the empty interface.
 func packEface(v Value) any {
+	return *(*any)(unsafe.Pointer(&abi.EmptyInterface{
+		Type: v.typ(),
+		Data: packEfaceData(v),
+	}))
+}
+
+// packEfaceData is a helper that packs the Data part of an interface,
+// if v were to be stored in an interface.
+func packEfaceData(v Value) unsafe.Pointer {
 	t := v.typ()
-	// Declare e as a struct (and not pointer to struct) to help escape analysis.
-	e := abi.EmptyInterface{}
-	// First, fill in the data portion of the interface.
 	switch {
 	case !t.IsDirectIface():
 		if v.flag&flagIndir == 0 {
@@ -136,18 +142,15 @@ func packEface(v Value) any {
 			typedmemmove(t, c, ptr)
 			ptr = c
 		}
-		e.Data = ptr
+		return ptr
 	case v.flag&flagIndir != 0:
 		// Value is indirect, but interface is direct. We need
 		// to load the data at v.ptr into the interface data word.
-		e.Data = *(*unsafe.Pointer)(v.ptr)
+		return *(*unsafe.Pointer)(v.ptr)
 	default:
 		// Value is direct, and so is the interface.
-		e.Data = v.ptr
+		return v.ptr
 	}
-	// Now, fill in the type portion.
-	e.Type = t
-	return *(*any)(unsafe.Pointer(&e))
 }
 
 // unpackEface converts the empty interface i to a Value.
@@ -1544,8 +1547,18 @@ func TypeAssert[T any](v Value) (T, bool) {
 	//	TypeAssert[any](ValueOf(1)) == ValueOf(1).Interface().(any)
 	//	TypeAssert[error](ValueOf(&someError{})) == ValueOf(&someError{}).Interface().(error)
 	if typ.Kind() == abi.Interface {
-		v, ok := packEface(v).(T)
-		return v, ok
+		// To avoid allocating memory, in case the type assertion fails,
+		// first do the type assertion with a nil Data pointer.
+		iface := *(*any)(unsafe.Pointer(&abi.EmptyInterface{Type: v.typ(), Data: nil}))
+		if out, ok := iface.(T); ok {
+			// Now populate the Data field properly, we update the Data ptr
+			// directly to avoid an additional type asertion. We can re-use the
+			// itab we already got from the runtime (through the previous type assertion).
+			(*abi.CommonInterface)(unsafe.Pointer(&out)).Data = packEfaceData(v)
+			return out, true
+		}
+		var zero T
+		return zero, false
 	}
 
 	// Both v and T must be concrete types.