reflect,runtime: add Value.Clear

Fixes #55002

Change-Id: I7d0f14cc54f67f2769b51d2efafc4ae3714f0e3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/457895
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: cuonglm

api/next/55002.txt

@@ -0,0 +1 @@
+pkg reflect, method (Value) Clear() #55002

src/reflect/all_test.go

@@ -8363,3 +8363,48 @@ func TestInitFuncTypes(t *testing.T) {
 	}
 	wg.Wait()
 }
+
+func TestClear(t *testing.T) {
+	m := make(map[string]any, len(valueTests))
+	for _, tt := range valueTests {
+		m[tt.s] = tt.i
+	}
+	mapTestFn := func(v Value) bool { v.Clear(); return v.Len() == 0 }
+
+	s := make([]*pair, len(valueTests))
+	for i := range s {
+		s[i] = &valueTests[i]
+	}
+	sliceTestFn := func(v Value) bool {
+		v.Clear()
+		for i := 0; i < v.Len(); i++ {
+			if !v.Index(i).IsZero() {
+				return false
+			}
+		}
+		return true
+	}
+
+	panicTestFn := func(v Value) bool { shouldPanic("reflect.Value.Clear", func() { v.Clear() }); return true }
+
+	tests := []struct {
+		name     string
+		value    Value
+		testFunc func(v Value) bool
+	}{
+		{"map", ValueOf(m), mapTestFn},
+		{"slice no pointer", ValueOf([]int{1, 2, 3, 4, 5}), sliceTestFn},
+		{"slice has pointer", ValueOf(s), sliceTestFn},
+		{"non-map/slice", ValueOf(1), panicTestFn},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			if !tc.testFunc(tc.value) {
+				t.Errorf("unexpected result for value.Clear(): %value", tc.value)
+			}
+		})
+	}
+}

src/reflect/value.go

@@ -2827,6 +2827,22 @@ func (v Value) extendSlice(n int) Value {
 	return v
 }
 
+// Clear clears the contents of a map or zeros the contents of a slice.
+//
+// It panics if v's Kind is not Map or Slice.
+func (v Value) Clear() {
+	switch v.Kind() {
+	case Slice:
+		sh := *(*unsafeheader.Slice)(v.ptr)
+		st := (*sliceType)(unsafe.Pointer(v.typ))
+		typedarrayclear(st.elem, sh.Data, sh.Len)
+	case Map:
+		mapclear(v.typ, v.pointer())
+	default:
+		panic(&ValueError{"reflect.Value.Clear", v.Kind()})
+	}
+}
+
 // Append appends the values x to a slice s and returns the resulting slice.
 // As in Go, each x's value must be assignable to the slice's element type.
 func Append(s Value, x ...Value) Value {
@@ -3774,6 +3790,8 @@ func mapiternext(it *hiter)
 //go:noescape
 func maplen(m unsafe.Pointer) int
 
+func mapclear(t *rtype, m unsafe.Pointer)
+
 // call calls fn with "stackArgsSize" bytes of stack arguments laid out
 // at stackArgs and register arguments laid out in regArgs. frameSize is
 // the total amount of stack space that will be reserved by call, so this
@@ -3837,6 +3855,12 @@ func typedmemclrpartial(t *rtype, ptr unsafe.Pointer, off, size uintptr)
 //go:noescape
 func typedslicecopy(elemType *rtype, dst, src unsafeheader.Slice) int
 
+// typedarrayclear zeroes the value at ptr of an array of elemType,
+// only clears len elem.
+//
+//go:noescape
+func typedarrayclear(elemType *rtype, ptr unsafe.Pointer, len int)
+
 //go:noescape
 func typehash(t *rtype, p unsafe.Pointer, h uintptr) uintptr
 

src/runtime/map.go

@@ -1403,6 +1403,11 @@ func reflect_maplen(h *hmap) int {
 	return h.count
 }
 
+//go:linkname reflect_mapclear reflect.mapclear
+func reflect_mapclear(t *maptype, h *hmap) {
+	mapclear(t, h)
+}
+
 //go:linkname reflectlite_maplen internal/reflectlite.maplen
 func reflectlite_maplen(h *hmap) int {
 	if h == nil {

src/runtime/mbarrier.go

@@ -334,6 +334,15 @@ func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintpt
 	memclrNoHeapPointers(ptr, size)
 }
 
+//go:linkname reflect_typedarrayclear reflect.typedarrayclear
+func reflect_typedarrayclear(typ *_type, ptr unsafe.Pointer, len int) {
+	size := typ.size * uintptr(len)
+	if writeBarrier.needed && typ.ptrdata != 0 {
+		bulkBarrierPreWrite(uintptr(ptr), 0, size)
+	}
+	memclrNoHeapPointers(ptr, size)
+}
+
 // memclrHasPointers clears n bytes of typed memory starting at ptr.
 // The caller must ensure that the type of the object at ptr has
 // pointers, usually by checking typ.ptrdata. However, ptr