From 2333c6299f340a5f76a73a4fec6db23ffa388e97 Mon Sep 17 00:00:00 2001 From: Keith Randall Date: Thu, 24 Sep 2020 19:26:33 -0700 Subject: [PATCH] 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 Trust: Josh Bleecher Snyder Run-TryBot: Keith Randall TryBot-Result: Go Bot Reviewed-by: Josh Bleecher Snyder --- src/runtime/slice.go | 2 +- test/fixedbugs/issue41239.go | 19 +++++++++++++++++++ 2 files changed, 20 insertions(+), 1 deletion(-) create mode 100644 test/fixedbugs/issue41239.go diff --git a/src/runtime/slice.go b/src/runtime/slice.go index 82a45c78a9..c0647d95a0 100644 --- a/src/runtime/slice.go +++ b/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 diff --git a/test/fixedbugs/issue41239.go b/test/fixedbugs/issue41239.go new file mode 100644 index 0000000000..3e9ef5eb66 --- /dev/null +++ b/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)) + } +} -- 2.50.0