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make Select64 inlinable (#8267)

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Alex Sharov 2023-09-22 14:16:57 +07:00 committed by GitHub
parent 80d40ef898
commit 101815ffac
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1 changed files with 17 additions and 4 deletions
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21
erigon-lib/common/bitutil/select.go
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@ -20,7 +20,7 @@ import (
)
// Required by select64
var kSelectInByte []byte = []byte{
var kSelectInByte = [2048]byte{
8, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
7, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
@ -58,6 +58,10 @@ const (
kOnesStep4 uint64 = 0x1111111111111111
kOnesStep8 uint64 = 0x0101010101010101
kLAMBDAsStep8 uint64 = 0x80 * kOnesStep8
kOnesStep4x3 = 0x3 * kOnesStep4
kOnesStep4xA = 0xA * kOnesStep4
kOnesStep8xF = 0xF * kOnesStep8
)
/** Returns the index of the k-th 1-bit in the 64-bit word x.
@ -77,16 +81,25 @@ const (
* [4] Facebook Folly library: https://github.com/facebook/folly
*
*/
func Select64(x uint64, k int) int {
func Select64(x uint64, k int) (place int) {
/* Original implementation - a bit obfuscated to satisfy Golang's inlining costs
s := x
s = s - ((s & (0xA * kOnesStep4)) >> 1)
s = (s & (0x3 * kOnesStep4)) + ((s >> 2) & (0x3 * kOnesStep4))
s = (s + (s >> 4)) & (0xF * kOnesStep8)
byteSums := s * kOnesStep8
*/
s := x - ((x & kOnesStep4xA) >> 1)
s = (s & kOnesStep4x3) + ((s >> 2) & kOnesStep4x3)
byteSums := ((s + (s >> 4)) & kOnesStep8xF) * kOnesStep8
/* Original implementaiton:
kStep8 := uint64(k) * kOnesStep8
geqKStep8 := ((kStep8 | kLAMBDAsStep8) - byteSums) & kLAMBDAsStep8
place := bits.OnesCount64(geqKStep8) * 8
place = bits.OnesCount64(geqKStep8) * 8
byteRank := uint64(k) - (((byteSums << 8) >> place) & uint64(0xFF))
*/
place = bits.OnesCount64((((uint64(k)*kOnesStep8)|kLAMBDAsStep8)-byteSums)&kLAMBDAsStep8) * 8
byteRank := uint64(k) - (((byteSums << 8) >> place) & uint64(0xFF))
return place + int(kSelectInByte[((x>>place)&0xFF)|(byteRank<<8)])
}