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runtime: avoid division in growslice

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Add a special case for power-of-2 sized elements.
We can replace div/mul with left/right shift and avoid expensive operation.
growslice is hotter for short slices of small elements, such as int16, so
add an int16 version for GrowSlice benchmark.

name                   old time/op  new time/op  delta
GrowSlice/Byte-6       61.3ns ± 3%  60.5ns ± 4%  -1.33%  (p=0.002 n=30+30)
GrowSlice/Int16-6      94.0ns ± 4%  84.7ns ± 2%  -9.82%  (p=0.000 n=30+30)
GrowSlice/Int-6         100ns ± 1%    99ns ± 1%  -0.25%  (p=0.032 n=29+28)
GrowSlice/Ptr-6         197ns ± 2%   195ns ± 2%  -0.94%  (p=0.001 n=30+29)
GrowSlice/Struct/24-6   168ns ± 1%   166ns ± 2%  -1.09%  (p=0.000 n=25+30)
GrowSlice/Struct/32-6   187ns ± 2%   180ns ± 1%  -3.59%  (p=0.000 n=30+30)
GrowSlice/Struct/40-6   241ns ± 2%   238ns ± 2%  -1.41%  (p=0.000 n=30+30)

Change-Id: I31e8388d73fd9356e2dcc091d8d92eef3e3ccdbc
Reviewed-on: https://go-review.googlesource.com/102279
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
This commit is contained in:
Ilya Tocar 2018-03-23 15:45:03 -05:00
parent 70d83eda99
commit e53cc7ae93
2 changed files with 31 additions and 3 deletions
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28
src/runtime/slice.go
View file

@ -5,6 +5,7 @@
package runtime
import (
"runtime/internal/sys"
"unsafe"
)
@ -128,19 +129,36 @@ func growslice(et *_type, old slice, cap int) slice {
var overflow bool
var lenmem, newlenmem, capmem uintptr
const ptrSize = unsafe.Sizeof((*byte)(nil))
switch et.size {
case 1:
// Specialize for common values of et.size.
// For 1 we don't need any division/multiplication.
// For ptrSize, compiler will optimize division/multiplication into a shift by a constant.
// For powers of 2, use a variable shift.
switch {
case et.size == 1:
lenmem = uintptr(old.len)
newlenmem = uintptr(cap)
capmem = roundupsize(uintptr(newcap))
overflow = uintptr(newcap) > maxAlloc
newcap = int(capmem)
case ptrSize:
case et.size == ptrSize:
lenmem = uintptr(old.len) * ptrSize
newlenmem = uintptr(cap) * ptrSize
capmem = roundupsize(uintptr(newcap) * ptrSize)
overflow = uintptr(newcap) > maxAlloc/ptrSize
newcap = int(capmem / ptrSize)
case isPowerOfTwo(et.size):
var shift uintptr
if ptrSize == 8 {
// Mask shift for better code generation.
shift = uintptr(sys.Ctz64(uint64(et.size))) & 63
} else {
shift = uintptr(sys.Ctz32(uint32(et.size))) & 31
}
lenmem = uintptr(old.len) << shift
newlenmem = uintptr(cap) << shift
capmem = roundupsize(uintptr(newcap) << shift)
overflow = uintptr(newcap) > (maxAlloc >> shift)
newcap = int(capmem >> shift)
default:
lenmem = uintptr(old.len) * et.size
newlenmem = uintptr(cap) * et.size
@ -189,6 +207,10 @@ func growslice(et *_type, old slice, cap int) slice {
return slice{p, old.len, newcap}
}
func isPowerOfTwo(x uintptr) bool {
return x&(x-1) == 0
}
func slicecopy(to, fm slice, width uintptr) int {
if fm.len == 0 || to.len == 0 {
return 0