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![Matthew Dempsky](/assets/img/avatar_default.png)
When compiling expression switches, we try to optimize runs of constants into binary searches. The ordering used isn't visible to the application, so it's unimportant as long as we're consistent between sorting and searching. For strings, it's much cheaper to compare string lengths than strings themselves, so instead of ordering strings by "si <= sj", we currently order them by "len(si) < len(sj) || len(si) == len(sj) && si <= sj" (i.e., the lexicographical ordering on the 2-tuple (len(s), s)). However, it's also somewhat cheaper to compare strings for equality (i.e., ==) than for ordering (i.e., <=). And if there were two or three string constants of the same length in a switch statement, we might unnecessarily emit ordering comparisons. For example, given: switch s { case "", "1", "2", "3": // ordered by length then content goto L } we currently compile this as: if len(s) < 1 || len(s) == 1 && s <= "1" { if s == "" { goto L } else if s == "1" { goto L } } else { if s == "2" { goto L } else if s == "3" { goto L } } This CL switches to using a 2-level binary search---first on len(s), then on s itself---so that string ordering comparisons are only needed when there are 4 or more strings of the same length. (4 being the cut-off for when using binary search is actually worthwhile.) So the above switch instead now compiles to: if len(s) == 0 { if s == "" { goto L } } else if len(s) == 1 { if s == "1" { goto L } else if s == "2" { goto L } else if s == "3" { goto L } } which is better optimized by walk and SSA. (Notably, because there are only two distinct lengths and no more than three strings of any particular length, this example ends up falling back to simply using linear search.) Test case by khr@ from CL 195138. Fixes #33934. Change-Id: I8eeebcaf7e26343223be5f443d6a97a0daf84f07 Reviewed-on: https://go-review.googlesource.com/c/go/+/195340 Run-TryBot: Matthew Dempsky <mdempsky@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Reviewed-by: Keith Randall <khr@golang.org>
23 lines
409 B
Go
23 lines
409 B
Go
// asmcheck
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// Copyright 2019 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// These tests check code generation of switch statements.
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package codegen
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// see issue 33934
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func f(x string) int {
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// amd64:-`cmpstring`
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switch x {
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case "":
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return -1
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case "1", "2", "3":
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return -2
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default:
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return -3
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}
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}
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