diff --git a/src/cmd/compile/internal/gc/builtin.go b/src/cmd/compile/internal/gc/builtin.go
index 6bdf78c8ab..f09dd5690f 100644
--- a/src/cmd/compile/internal/gc/builtin.go
+++ b/src/cmd/compile/internal/gc/builtin.go
@@ -130,7 +130,8 @@ const runtimeimport = "" +
 	"func @\"\".selectgo (@\"\".sel·1 *byte)\n" +
 	"func @\"\".block ()\n" +
 	"func @\"\".makeslice (@\"\".typ·2 *byte, @\"\".nel·3 int64, @\"\".cap·4 int64) (@\"\".ary·1 []any)\n" +
-	"func @\"\".growslice (@\"\".typ·2 *byte, @\"\".old·3 []any, @\"\".n·4 int) (@\"\".ary·1 []any)\n" +
+	"func @\"\".growslice (@\"\".typ·2 *byte, @\"\".old·3 []any, @\"\".cap·4 int) (@\"\".ary·1 []any)\n" +
+	"func @\"\".growslice_n (@\"\".typ·2 *byte, @\"\".old·3 []any, @\"\".n·4 int) (@\"\".ary·1 []any)\n" +
 	"func @\"\".memmove (@\"\".to·1 *any, @\"\".frm·2 *any, @\"\".length·3 uintptr)\n" +
 	"func @\"\".memclr (@\"\".ptr·1 *byte, @\"\".length·2 uintptr)\n" +
 	"func @\"\".memequal (@\"\".x·2 *any, @\"\".y·3 *any, @\"\".size·4 uintptr) (? bool)\n" +
diff --git a/src/cmd/compile/internal/gc/builtin/runtime.go b/src/cmd/compile/internal/gc/builtin/runtime.go
index 179a4ddd9a..6210f10cdf 100644
--- a/src/cmd/compile/internal/gc/builtin/runtime.go
+++ b/src/cmd/compile/internal/gc/builtin/runtime.go
@@ -162,7 +162,8 @@ func selectgo(sel *byte)
 func block()
 
 func makeslice(typ *byte, nel int64, cap int64) (ary []any)
-func growslice(typ *byte, old []any, n int) (ary []any)
+func growslice(typ *byte, old []any, cap int) (ary []any)
+func growslice_n(typ *byte, old []any, n int) (ary []any)
 func memmove(to *any, frm *any, length uintptr)
 func memclr(ptr *byte, length uintptr)
 
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index bbbc990cb1..f5ae9fbe21 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -2884,7 +2884,7 @@ func addstr(n *Node, init **NodeList) *Node {
 //   init {
 //     s := l1
 //     if n := len(l1) + len(l2) - cap(s); n > 0 {
-//       s = growslice(s, n)
+//       s = growslice_n(s, n)
 //     }
 //     s = s[:len(l1)+len(l2)]
 //     memmove(&s[len(l1)], &l2[0], len(l2)*sizeof(T))
@@ -2918,11 +2918,11 @@ func appendslice(n *Node, init **NodeList) *Node {
 
 	nif.Left = Nod(OGT, nt, Nodintconst(0))
 
-	// instantiate growslice(Type*, []any, int) []any
-	fn := syslook("growslice", 1) //   growslice(<type>, old []T, n int64) (ret []T)
+	// instantiate growslice_n(Type*, []any, int) []any
+	fn := syslook("growslice_n", 1) //   growslice_n(<type>, old []T, n int64) (ret []T)
 	substArgTypes(fn, s.Type.Type, s.Type.Type)
 
-	// s = growslice(T, s, n)
+	// s = growslice_n(T, s, n)
 	nif.Nbody = list1(Nod(OAS, s, mkcall1(fn, s.Type, &nif.Ninit, typename(s.Type), s, nt)))
 
 	l = list(l, nif)
@@ -2997,7 +2997,7 @@ func appendslice(n *Node, init **NodeList) *Node {
 //     s := src
 //     const argc = len(args) - 1
 //     if cap(s) - len(s) < argc {
-//	    s = growslice(s, argc)
+//	    s = growslice(s, len(s)+argc)
 //     }
 //     n := len(s)
 //     s = s[:n+argc]
@@ -3050,10 +3050,10 @@ func walkappend(n *Node, init **NodeList, dst *Node) *Node {
 	nx := Nod(OIF, nil, nil)       // if cap(s) - len(s) < argc
 	nx.Left = Nod(OLT, Nod(OSUB, Nod(OCAP, ns, nil), Nod(OLEN, ns, nil)), na)
 
-	fn := syslook("growslice", 1) //   growslice(<type>, old []T, n int) (ret []T)
+	fn := syslook("growslice", 1) //   growslice(<type>, old []T, mincap int) (ret []T)
 	substArgTypes(fn, ns.Type.Type, ns.Type.Type)
 
-	nx.Nbody = list1(Nod(OAS, ns, mkcall1(fn, ns.Type, &nx.Ninit, typename(ns.Type), ns, na)))
+	nx.Nbody = list1(Nod(OAS, ns, mkcall1(fn, ns.Type, &nx.Ninit, typename(ns.Type), ns, Nod(OADD, Nod(OLEN, ns, nil), na))))
 
 	l = list(l, nx)
 
diff --git a/src/runtime/runtime_test.go b/src/runtime/runtime_test.go
index f65562ab91..75fc9bcb84 100644
--- a/src/runtime/runtime_test.go
+++ b/src/runtime/runtime_test.go
@@ -261,3 +261,43 @@ func TestBadOpen(t *testing.T) {
 		t.Errorf("close()=%d, want -1", c)
 	}
 }
+
+func TestAppendGrowth(t *testing.T) {
+	var x []int64
+	check := func(want int) {
+		if cap(x) != want {
+			t.Errorf("len=%d, cap=%d, want cap=%d", len(x), cap(x), want)
+		}
+	}
+
+	check(0)
+	want := 1
+	for i := 1; i <= 100; i++ {
+		x = append(x, 1)
+		check(want)
+		if i&(i-1) == 0 {
+			want = 2 * i
+		}
+	}
+}
+
+var One = []int64{1}
+
+func TestAppendSliceGrowth(t *testing.T) {
+	var x []int64
+	check := func(want int) {
+		if cap(x) != want {
+			t.Errorf("len=%d, cap=%d, want cap=%d", len(x), cap(x), want)
+		}
+	}
+
+	check(0)
+	want := 1
+	for i := 1; i <= 100; i++ {
+		x = append(x, One...)
+		check(want)
+		if i&(i-1) == 0 {
+			want = 2 * i
+		}
+	}
+}
diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index 15820a5181..5cda11d9b0 100644
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
@@ -33,12 +33,22 @@ func makeslice(t *slicetype, len64, cap64 int64) slice {
 	return slice{p, len, cap}
 }
 
-func growslice(t *slicetype, old slice, n int) slice {
+// growslice_n is a variant of growslice that takes the number of new elements
+// instead of the new minimum capacity.
+// TODO(rsc): This is used by append(slice, slice...).
+// The compiler should change that code to use growslice directly (issue #11419).
+func growslice_n(t *slicetype, old slice, n int) slice {
 	if n < 1 {
 		panic(errorString("growslice: invalid n"))
 	}
+	return growslice(t, old, old.cap+n)
+}
 
-	cap := old.cap + n
+// growslice handles slice growth during append.
+// It is passed the slice type, the old slice, and the desired new minimum capacity,
+// and it returns a new slice with at least that capacity, with the old data
+// copied into it.
+func growslice(t *slicetype, old slice, cap int) slice {
 	if cap < old.cap || t.elem.size > 0 && uintptr(cap) > _MaxMem/uintptr(t.elem.size) {
 		panic(errorString("growslice: cap out of range"))
 	}