runtime: use old capacity to decide on append growth regime

We grow the backing store on append by 2x for small sizes and 1.25x
for large sizes. The threshold we use for picking the growth factor
used to depend on the old length, not the old capacity. That's kind of
unfortunate, because then doing append(s, 0, 0) and append(append(s,
0), 0) do different things. (If s has one more spot available, then
the former expression chooses its growth based on len(s) and the
latter on len(s)+1.)  If we instead use the old capacity, we get more
consistent behavior. (Both expressions use len(s)+1 == cap(s) to
decide.)

Fixes #41239

Change-Id: I40686471d256edd72ec92aef973a89b52e235d4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/257338
Trust: Keith Randall <khr@golang.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/runtime/slice.go

@@ -146,7 +146,7 @@ func growslice(et *_type, old slice, cap int) slice {
 	if cap > doublecap {
 		newcap = cap
 	} else {
-		if old.len < 1024 {
+		if old.cap < 1024 {
 			newcap = doublecap
 		} else {
 			// Check 0 < newcap to detect overflow

test/fixedbugs/issue41239.go

@@ -0,0 +1,19 @@
+// run
+
+// Copyright 2020 The Go Authors. All rights reserved.  Use of this
+// source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+package main
+
+import "fmt"
+
+func main() {
+	const N = 1024
+	var a [N]int
+	x := cap(append(a[:N-1:N], 9, 9))
+	y := cap(append(a[:N:N], 9))
+	if x != y {
+		panic(fmt.Sprintf("different capacity on append: %d vs %d", x, y))
+	}
+}