Commit 6fff980c authored by Keith Randall's avatar Keith Randall Committed by Keith Randall

cmd/compile: initialize sparse slice literals dynamically

When a slice composite literal is sparse, initialize it dynamically
instead of statically.

s := []int{5:5, 20:20}

To initialize the backing store for s, use 2 constant writes instead
of copying from a static array with 21 entries.

This CL also fixes pathologies in the compiler when the slice is
*very* sparse.

Fixes #23780

Change-Id: Iae95c6e6f6a0e2994675cbc750d7a4dd6436b13b
Reviewed-on: https://go-review.googlesource.com/c/151319
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: 's avatarRobert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
parent 1602e497
......@@ -612,6 +612,15 @@ func getdyn(n *Node, top bool) initGenType {
if !top {
return initDynamic
}
if n.Right.Int64()/4 > int64(n.List.Len()) {
// <25% of entries have explicit values.
// Very rough estimation, it takes 4 bytes of instructions
// to initialize 1 byte of result. So don't use a static
// initializer if the dynamic initialization code would be
// smaller than the static value.
// See issue 23780.
return initDynamic
}
case OARRAYLIT, OSTRUCTLIT:
}
......@@ -902,7 +911,7 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
continue
}
if isLiteral(value) {
if vstat != nil && isLiteral(value) { // already set by copy from static value
continue
}
......
......@@ -622,7 +622,7 @@ const (
OMAPLIT // Type{List} (composite literal, Type is map)
OSTRUCTLIT // Type{List} (composite literal, Type is struct)
OARRAYLIT // Type{List} (composite literal, Type is array)
OSLICELIT // Type{List} (composite literal, Type is slice)
OSLICELIT // Type{List} (composite literal, Type is slice) Right.Int64() = slice length.
OPTRLIT // &Left (left is composite literal)
OCONV // Type(Left) (type conversion)
OCONVIFACE // Type(Left) (type conversion, to interface)
......
// compile
// Copyright 2018 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 p
func f() {
_ = []byte{1 << 30: 1}
}
func g() {
sink = []byte{1 << 30: 1}
}
var sink []byte
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