reflect: add Value.Grow

The Grow method is like the proposed slices.Grow function
in that it ensures that the slice has enough capacity to append
n elements without allocating.

The implementation of Grow is a thin wrapper over runtime.growslice.
This also changes Append and AppendSlice to use growslice under the hood.

Fixes #48000

Change-Id: I992a58584a2ff1448c1c2bc0877fe76073609111
Reviewed-on: https://go-review.googlesource.com/c/go/+/389635
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

api/next/48000.txt

@@ -0,0 +1 @@
+pkg reflect, method (Value) Grow(int) #48000

src/reflect/all_test.go

@@ -739,25 +739,88 @@ func TestFunctionValue(t *testing.T) {
 	assert(t, v.Type().String(), "func()")
 }
 
+func TestGrow(t *testing.T) {
+	v := ValueOf([]int(nil))
+	shouldPanic("reflect.Value.Grow using unaddressable value", func() { v.Grow(0) })
+	v = ValueOf(new([]int)).Elem()
+	v.Grow(0)
+	if !v.IsNil() {
+		t.Errorf("v.Grow(0) should still be nil")
+	}
+	v.Grow(1)
+	if v.Cap() == 0 {
+		t.Errorf("v.Cap = %v, want non-zero", v.Cap())
+	}
+	want := v.UnsafePointer()
+	v.Grow(1)
+	got := v.UnsafePointer()
+	if got != want {
+		t.Errorf("noop v.Grow should not change pointers")
+	}
+
+	t.Run("Append", func(t *testing.T) {
+		var got, want []T
+		v := ValueOf(&got).Elem()
+		appendValue := func(vt T) {
+			v.Grow(1)
+			v.SetLen(v.Len() + 1)
+			v.Index(v.Len() - 1).Set(ValueOf(vt))
+		}
+		for i := 0; i < 10; i++ {
+			vt := T{i, float64(i), strconv.Itoa(i), &i}
+			appendValue(vt)
+			want = append(want, vt)
+		}
+		if !DeepEqual(got, want) {
+			t.Errorf("value mismatch:\ngot  %v\nwant %v", got, want)
+		}
+	})
+
+	t.Run("Rate", func(t *testing.T) {
+		var b []byte
+		v := ValueOf(new([]byte)).Elem()
+		for i := 0; i < 10; i++ {
+			b = append(b[:cap(b)], make([]byte, 1)...)
+			v.SetLen(v.Cap())
+			v.Grow(1)
+			if v.Cap() != cap(b) {
+				t.Errorf("v.Cap = %v, want %v", v.Cap(), cap(b))
+			}
+		}
+	})
+
+	t.Run("ZeroCapacity", func(t *testing.T) {
+		for i := 0; i < 10; i++ {
+			v := ValueOf(new([]byte)).Elem()
+			v.Grow(61)
+			b := v.Bytes()
+			b = b[:cap(b)]
+			for i, c := range b {
+				if c != 0 {
+					t.Fatalf("Value.Bytes[%d] = 0x%02x, want 0x00", i, c)
+				}
+				b[i] = 0xff
+			}
+			runtime.GC()
+		}
+	})
+}
+
 var appendTests = []struct {
 	orig, extra []int
 }{
+	{nil, nil},
+	{[]int{}, nil},
+	{nil, []int{}},
+	{[]int{}, []int{}},
+	{nil, []int{22}},
+	{[]int{}, []int{22}},
+	{make([]int, 2, 4), nil},
+	{make([]int, 2, 4), []int{}},
 	{make([]int, 2, 4), []int{22}},
 	{make([]int, 2, 4), []int{22, 33, 44}},
 }
 
-func sameInts(x, y []int) bool {
-	if len(x) != len(y) {
-		return false
-	}
-	for i, xx := range x {
-		if xx != y[i] {
-			return false
-		}
-	}
-	return true
-}
-
 func TestAppend(t *testing.T) {
 	for i, test := range appendTests {
 		origLen, extraLen := len(test.orig), len(test.extra)
@@ -769,32 +832,51 @@ func TestAppend(t *testing.T) {
 		}
 		// Convert extra from []int to *SliceValue.
 		e1 := ValueOf(test.extra)
+
 		// Test Append.
-		a0 := ValueOf(test.orig)
-		have0 := Append(a0, e0...).Interface().([]int)
-		if !sameInts(have0, want) {
-			t.Errorf("Append #%d: have %v, want %v (%p %p)", i, have0, want, test.orig, have0)
+		a0 := ValueOf(&test.orig).Elem()
+		have0 := Append(a0, e0...)
+		if have0.CanAddr() {
+			t.Errorf("Append #%d: have slice should not be addressable", i)
+		}
+		if !DeepEqual(have0.Interface(), want) {
+			t.Errorf("Append #%d: have %v, want %v (%p %p)", i, have0, want, test.orig, have0.Interface())
 		}
 		// Check that the orig and extra slices were not modified.
+		if a0.Len() != len(test.orig) {
+			t.Errorf("Append #%d: a0.Len: have %d, want %d", i, a0.Len(), origLen)
+		}
 		if len(test.orig) != origLen {
 			t.Errorf("Append #%d origLen: have %v, want %v", i, len(test.orig), origLen)
 		}
 		if len(test.extra) != extraLen {
 			t.Errorf("Append #%d extraLen: have %v, want %v", i, len(test.extra), extraLen)
 		}
+
 		// Test AppendSlice.
-		a1 := ValueOf(test.orig)
-		have1 := AppendSlice(a1, e1).Interface().([]int)
-		if !sameInts(have1, want) {
+		a1 := ValueOf(&test.orig).Elem()
+		have1 := AppendSlice(a1, e1)
+		if have1.CanAddr() {
+			t.Errorf("AppendSlice #%d: have slice should not be addressable", i)
+		}
+		if !DeepEqual(have1.Interface(), want) {
 			t.Errorf("AppendSlice #%d: have %v, want %v", i, have1, want)
 		}
 		// Check that the orig and extra slices were not modified.
+		if a1.Len() != len(test.orig) {
+			t.Errorf("AppendSlice #%d: a1.Len: have %d, want %d", i, a0.Len(), origLen)
+		}
 		if len(test.orig) != origLen {
 			t.Errorf("AppendSlice #%d origLen: have %v, want %v", i, len(test.orig), origLen)
 		}
 		if len(test.extra) != extraLen {
 			t.Errorf("AppendSlice #%d extraLen: have %v, want %v", i, len(test.extra), extraLen)
 		}
+
+		// Test Append and AppendSlice with unexported value.
+		ax := ValueOf(struct{ x []int }{test.orig}).Field(0)
+		shouldPanic("using unexported field", func() { Append(ax, e0...) })
+		shouldPanic("using unexported field", func() { AppendSlice(ax, e1) })
 	}
 }
 

src/reflect/value.go

@@ -2780,42 +2780,61 @@ func arrayAt(p unsafe.Pointer, i int, eltSize uintptr, whySafe string) unsafe.Po
 	return add(p, uintptr(i)*eltSize, "i < len")
 }
 
-// grow grows the slice s so that it can hold extra more values, allocating
-// more capacity if needed. It also returns the old and new slice lengths.
-func grow(s Value, extra int) (Value, int, int) {
-	i0 := s.Len()
-	i1 := i0 + extra
-	if i1 < i0 {
-		panic("reflect.Append: slice overflow")
+// Grow increases the slice's capacity, if necessary, to guarantee space for
+// another n elements. After Grow(n), at least n elements can be appended
+// to the slice without another allocation.
+//
+// It panics if v's Kind is not a Slice or if n is negative or too large to
+// allocate the memory.
+func (v Value) Grow(n int) {
+	v.mustBeAssignable()
+	v.mustBe(Slice)
+	v.grow(n)
+}
+
+// grow is identical to Grow but does not check for assignability.
+func (v Value) grow(n int) {
+	p := (*unsafeheader.Slice)(v.ptr)
+	switch {
+	case n < 0:
+		panic("reflect.Value.Grow: negative len")
+	case p.Len+n < 0:
+		panic("reflect.Value.Grow: slice overflow")
+	case p.Len+n > p.Cap:
+		t := v.typ.Elem().(*rtype)
+		*p = growslice(t, *p, n)
 	}
-	m := s.Cap()
-	if i1 <= m {
-		return s.Slice(0, i1), i0, i1
-	}
-	if m == 0 {
-		m = extra
-	} else {
-		const threshold = 256
-		for m < i1 {
-			if i0 < threshold {
-				m += m
-			} else {
-				m += (m + 3*threshold) / 4
-			}
-		}
-	}
-	t := MakeSlice(s.Type(), i1, m)
-	Copy(t, s)
-	return t, i0, i1
+}
+
+// extendSlice extends a slice by n elements.
+//
+// Unlike Value.grow, which modifies the slice in place and
+// does not change the length of the slice in place,
+// extendSlice returns a new slice value with the length
+// incremented by the number of specified elements.
+func (v Value) extendSlice(n int) Value {
+	v.mustBeExported()
+	v.mustBe(Slice)
+
+	// Shallow copy the slice header to avoid mutating the source slice.
+	sh := *(*unsafeheader.Slice)(v.ptr)
+	s := &sh
+	v.ptr = unsafe.Pointer(s)
+	v.flag = flagIndir | flag(Slice) // equivalent flag to MakeSlice
+
+	v.grow(n) // fine to treat as assignable since we allocate a new slice header
+	s.Len += n
+	return v
 }
 
 // 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 {
 	s.mustBe(Slice)
-	s, i0, i1 := grow(s, len(x))
-	for i, j := i0, 0; i < i1; i, j = i+1, j+1 {
-		s.Index(i).Set(x[j])
+	n := s.Len()
+	s = s.extendSlice(len(x))
+	for i, v := range x {
+		s.Index(n + i).Set(v)
 	}
 	return s
 }
@@ -2826,8 +2845,10 @@ func AppendSlice(s, t Value) Value {
 	s.mustBe(Slice)
 	t.mustBe(Slice)
 	typesMustMatch("reflect.AppendSlice", s.Type().Elem(), t.Type().Elem())
-	s, i0, i1 := grow(s, t.Len())
-	Copy(s.Slice(i0, i1), t)
+	ns := s.Len()
+	nt := t.Len()
+	s = s.extendSlice(nt)
+	Copy(s.Slice(ns, ns+nt), t)
 	return s
 }
 
@@ -3764,6 +3785,9 @@ func typehash(t *rtype, p unsafe.Pointer, h uintptr) uintptr
 
 func verifyNotInHeapPtr(p uintptr) bool
 
+//go:noescape
+func growslice(t *rtype, old unsafeheader.Slice, num int) unsafeheader.Slice
+
 // Dummy annotation marking that the value x escapes,
 // for use in cases where the reflect code is so clever that
 // the compiler cannot follow.

src/runtime/slice.go

@@ -126,16 +126,18 @@ func mulUintptr(a, b uintptr) (uintptr, bool) {
 // growslice allocates new backing store for a slice.
 //
 // arguments:
-//   oldPtr = pointer to the slice's backing array
-//   newLen = new length (= oldLen + num)
-//   oldCap = original slice's capacity.
-//      num = number of elements being added
-//       et = element type
+//
+//	oldPtr = pointer to the slice's backing array
+//	newLen = new length (= oldLen + num)
+//	oldCap = original slice's capacity.
+//	   num = number of elements being added
+//	    et = element type
 //
 // return values:
-//   newPtr = pointer to the new backing store
-//   newLen = same value as the argument
-//   newCap = capacity of the new backing store
+//
+//	newPtr = pointer to the new backing store
+//	newLen = same value as the argument
+//	newCap = capacity of the new backing store
 //
 // Requires that uint(newLen) > uint(oldCap).
 // Assumes the original slice length is newLen - num
@@ -264,6 +266,8 @@ func growslice(oldPtr unsafe.Pointer, newLen, oldCap, num int, et *_type) slice
 		p = mallocgc(capmem, nil, false)
 		// The append() that calls growslice is going to overwrite from oldLen to newLen.
 		// Only clear the part that will not be overwritten.
+		// The reflect_growslice() that calls growslice will manually clear
+		// the region not cleared here.
 		memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem)
 	} else {
 		// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
@@ -279,6 +283,25 @@ func growslice(oldPtr unsafe.Pointer, newLen, oldCap, num int, et *_type) slice
 	return slice{p, newLen, newcap}
 }
 
+//go:linkname reflect_growslice reflect.growslice
+func reflect_growslice(et *_type, old slice, num int) slice {
+	// Semantically equivalent to slices.Grow, except that the caller
+	// is responsible for ensuring that old.len+num > old.cap.
+	num -= old.cap - old.len // preserve memory of old[old.len:old.cap]
+	new := growslice(old.array, old.cap+num, old.cap, num, et)
+	// growslice does not zero out new[old.cap:new.len] since it assumes that
+	// the memory will be overwritten by an append() that called growslice.
+	// Since the caller of reflect_growslice is not append(),
+	// zero out this region before returning the slice to the reflect package.
+	if et.ptrdata == 0 {
+		oldcapmem := uintptr(old.cap) * et.size
+		newlenmem := uintptr(new.len) * et.size
+		memclrNoHeapPointers(add(new.array, oldcapmem), newlenmem-oldcapmem)
+	}
+	new.len = old.len // preserve the old length
+	return new
+}
+
 func isPowerOfTwo(x uintptr) bool {
 	return x&(x-1) == 0
 }