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runtime: speed up growslice by avoiding divisions 2

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This is a follow-up of https://go-review.googlesource.com/#/c/20653/

Special case computation for slices with elements of byte size or
pointer size.

name                      old time/op  new time/op  delta
GrowSliceBytes-4          86.2ns ± 3%  75.4ns ± 2%  -12.50%  (p=0.000 n=20+20)
GrowSliceInts-4            161ns ± 3%   136ns ± 3%  -15.59%  (p=0.000 n=19+19)
GrowSlicePtr-4             239ns ± 2%   233ns ± 2%   -2.52%  (p=0.000 n=20+20)
GrowSliceStruct24Bytes-4   258ns ± 3%   256ns ± 3%     ~     (p=0.134 n=20+20)

Change-Id: Ice5fa648058fe9d7fa89dee97ca359966f671128
Reviewed-on: https://go-review.googlesource.com/21101
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Marvin Stenger 2016-03-24 17:42:01 +01:00 • committed by Ian Lance Taylor
parent 1664ff96f7
commit 6b0688f742
2 changed files with 42 additions and 14 deletions
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32
src/runtime/slice.go
View file

@ -56,11 +56,6 @@ func growslice(t *slicetype, old slice, cap int) slice {
return slice{unsafe.Pointer(&zerobase), old.len, cap}
}
maxcap := _MaxMem / et.size
if cap < old.cap || uintptr(cap) > maxcap {
panic(errorString("growslice: cap out of range"))
}
newcap := old.cap
doublecap := newcap + newcap
if cap > doublecap {
@ -73,17 +68,30 @@ func growslice(t *slicetype, old slice, cap int) slice {
newcap += newcap / 4
}
}
if uintptr(newcap) > maxcap {
panic(errorString("growslice: cap out of range"))
}
}
lenmem := uintptr(old.len) * et.size
capmem := roundupsize(uintptr(newcap) * et.size)
if et.size == 1 {
var lenmem, capmem, maxcap uintptr
const ptrSize = unsafe.Sizeof((*byte)(nil))
switch et.size {
case 1:
lenmem = uintptr(old.len)
capmem = roundupsize(uintptr(newcap))
newcap = int(capmem)
} else {
maxcap = _MaxMem
case ptrSize:
lenmem = uintptr(old.len) * ptrSize
capmem = roundupsize(uintptr(newcap) * ptrSize)
newcap = int(capmem / ptrSize)
maxcap = _MaxMem / ptrSize
default:
lenmem = uintptr(old.len) * et.size
capmem = roundupsize(uintptr(newcap) * et.size)
newcap = int(capmem / et.size)
maxcap = _MaxMem / et.size
}
if cap < old.cap || uintptr(newcap) > maxcap {
panic(errorString("growslice: cap out of range"))
}
var p unsafe.Pointer