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author | Martin Möhrmann <martisch@uos.de> | 2016-04-10 17:32:35 +0200 |
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committer | Ian Lance Taylor <iant@golang.org> | 2016-04-10 23:39:46 +0000 |
commit | ad7448fe982d83de15deec9c55c56d0cd9261c6c (patch) | |
tree | e4d44cc145b562c0b1e184a4a5d2bec78b8cdfd3 /src/runtime/slice.go | |
parent | 5b3e5766bcc5e1090d0512a5916886ffc24ab246 (diff) | |
download | go-ad7448fe982d83de15deec9c55c56d0cd9261c6c.tar.gz go-ad7448fe982d83de15deec9c55c56d0cd9261c6c.zip |
runtime: speed up makeslice by avoiding divisions
Only compute the number of maximum allowed elements per slice once.
name old time/op new time/op delta
MakeSlice-2 55.5ns ± 1% 45.6ns ± 2% -17.88% (p=0.000 n=99+100)
Change-Id: I951feffda5d11910a75e55d7e978d306d14da2c5
Reviewed-on: https://go-review.googlesource.com/21801
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Diffstat (limited to 'src/runtime/slice.go')
-rw-r--r-- | src/runtime/slice.go | 14 |
1 files changed, 11 insertions, 3 deletions
diff --git a/src/runtime/slice.go b/src/runtime/slice.go index 4ab221056c..f36ec0b466 100644 --- a/src/runtime/slice.go +++ b/src/runtime/slice.go @@ -16,19 +16,27 @@ type slice struct { // TODO: take uintptrs instead of int64s? func makeslice(t *slicetype, len64, cap64 int64) slice { - // NOTE: The len > MaxMem/elemsize check here is not strictly necessary, + // NOTE: The len > maxElements check here is not strictly necessary, // but it produces a 'len out of range' error instead of a 'cap out of range' error // when someone does make([]T, bignumber). 'cap out of range' is true too, // but since the cap is only being supplied implicitly, saying len is clearer. // See issue 4085. + + maxElements := ^uintptr(0) + if t.elem.size > 0 { + maxElements = _MaxMem / t.elem.size + } + len := int(len64) - if len64 < 0 || int64(len) != len64 || t.elem.size > 0 && uintptr(len) > _MaxMem/t.elem.size { + if len64 < 0 || int64(len) != len64 || uintptr(len) > maxElements { panic(errorString("makeslice: len out of range")) } + cap := int(cap64) - if cap < len || int64(cap) != cap64 || t.elem.size > 0 && uintptr(cap) > _MaxMem/t.elem.size { + if cap < len || int64(cap) != cap64 || uintptr(cap) > maxElements { panic(errorString("makeslice: cap out of range")) } + p := newarray(t.elem, uintptr(cap)) return slice{p, len, cap} } |