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author | Josh Bleecher Snyder <josharian@gmail.com> | 2016-04-19 09:18:59 -0700 |
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committer | Josh Bleecher Snyder <josharian@gmail.com> | 2016-04-19 21:38:52 +0000 |
commit | a4dd6ea1524901fab5deac60772345babd058ae7 (patch) | |
tree | 413af3884aff44e753fe46c98cb4f66f95763a7c /src/runtime/slice.go | |
parent | 55ab07c224a358cabe795fb1e52a627194d7daee (diff) | |
download | go-a4dd6ea1524901fab5deac60772345babd058ae7.tar.gz go-a4dd6ea1524901fab5deac60772345babd058ae7.zip |
runtime: add maxSliceCap
This avoids expensive division calculations
for many common slice element sizes.
name old time/op new time/op delta
MakeSlice-8 51.9ns ± 3% 35.1ns ± 2% -32.41% (p=0.000 n=10+10)
GrowSliceBytes-8 44.1ns ± 2% 44.1ns ± 1% ~ (p=0.984 n=10+10)
GrowSliceInts-8 60.9ns ± 3% 60.9ns ± 3% ~ (p=0.698 n=10+10)
GrowSlicePtr-8 131ns ± 1% 120ns ± 2% -8.41% (p=0.000 n=8+10)
GrowSliceStruct24Bytes-8 111ns ± 2% 103ns ± 3% -7.23% (p=0.000 n=8+8)
Change-Id: I2630eb3d73c814db030cad16e620ea7fecbbd312
Reviewed-on: https://go-review.googlesource.com/22223
Reviewed-by: Keith Randall <khr@golang.org>
Diffstat (limited to 'src/runtime/slice.go')
-rw-r--r-- | src/runtime/slice.go | 35 |
1 files changed, 25 insertions, 10 deletions
diff --git a/src/runtime/slice.go b/src/runtime/slice.go index f36ec0b466..f9414d7658 100644 --- a/src/runtime/slice.go +++ b/src/runtime/slice.go @@ -14,6 +14,28 @@ type slice struct { cap int } +// maxElems is a lookup table containing the maximum capacity for a slice. +// The index is the size of the slice element. +var maxElems = [...]uintptr{ + ^uintptr(0), + _MaxMem / 1, _MaxMem / 2, _MaxMem / 3, _MaxMem / 4, + _MaxMem / 5, _MaxMem / 6, _MaxMem / 7, _MaxMem / 8, + _MaxMem / 9, _MaxMem / 10, _MaxMem / 11, _MaxMem / 12, + _MaxMem / 13, _MaxMem / 14, _MaxMem / 15, _MaxMem / 16, + _MaxMem / 17, _MaxMem / 18, _MaxMem / 19, _MaxMem / 20, + _MaxMem / 21, _MaxMem / 22, _MaxMem / 23, _MaxMem / 24, + _MaxMem / 25, _MaxMem / 26, _MaxMem / 27, _MaxMem / 28, + _MaxMem / 29, _MaxMem / 30, _MaxMem / 31, _MaxMem / 32, +} + +// maxSliceCap returns the maximum capacity for a slice. +func maxSliceCap(elemsize uintptr) uintptr { + if elemsize < uintptr(len(maxElems)) { + return maxElems[elemsize] + } + return _MaxMem / elemsize +} + // TODO: take uintptrs instead of int64s? func makeslice(t *slicetype, len64, cap64 int64) slice { // NOTE: The len > maxElements check here is not strictly necessary, @@ -22,11 +44,7 @@ func makeslice(t *slicetype, len64, cap64 int64) slice { // 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 - } - + maxElements := maxSliceCap(t.elem.size) len := int(len64) if len64 < 0 || int64(len) != len64 || uintptr(len) > maxElements { panic(errorString("makeslice: len out of range")) @@ -84,27 +102,24 @@ func growslice(t *slicetype, old slice, cap int) slice { } } - var lenmem, capmem, maxcap uintptr + var lenmem, capmem uintptr const ptrSize = unsafe.Sizeof((*byte)(nil)) switch et.size { case 1: lenmem = uintptr(old.len) capmem = roundupsize(uintptr(newcap)) newcap = int(capmem) - 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 { + if cap < old.cap || uintptr(newcap) > maxSliceCap(et.size) { panic(errorString("growslice: cap out of range")) } |