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Use OnesCount8 from math/bits to implement countBits

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This allows to drop the bitCounts table. Also, bits.OnesCount8 can be
intrinsified to a single instruction on several GOARCHes.
This commit is contained in:
Tobias Klauser 2020-03-26 16:52:58 +01:00
parent 894916b665
commit 811f9d8abf
2 changed files with 5 additions and 52 deletions
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43
pkg/registry/core/service/allocator/utils.go
View File

@ -16,49 +16,16 @@ limitations under the License.
package allocator package allocator
import "math/big" import (
"math/big"
"math/bits"
)
// countBits returns the number of set bits in n // countBits returns the number of set bits in n
func countBits(n *big.Int) int { func countBits(n *big.Int) int {
var count int = 0 var count int = 0
for _, b := range n.Bytes() { for _, b := range n.Bytes() {
count += int(bitCounts[b]) count += bits.OnesCount8(uint8(b))
} }
return count return count
} }
// bitCounts is all of the bits counted for each number between 0-255
var bitCounts = []int8{
0, 1, 1, 2, 1, 2, 2, 3,
1, 2, 2, 3, 2, 3, 3, 4,
1, 2, 2, 3, 2, 3, 3, 4,
2, 3, 3, 4, 3, 4, 4, 5,
1, 2, 2, 3, 2, 3, 3, 4,
2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5,
3, 4, 4, 5, 4, 5, 5, 6,
1, 2, 2, 3, 2, 3, 3, 4,
2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5,
3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5,
3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6,
4, 5, 5, 6, 5, 6, 6, 7,
1, 2, 2, 3, 2, 3, 3, 4,
2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5,
3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5,
3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6,
4, 5, 5, 6, 5, 6, 6, 7,
2, 3, 3, 4, 3, 4, 4, 5,
3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6,
4, 5, 5, 6, 5, 6, 6, 7,
3, 4, 4, 5, 4, 5, 5, 6,
4, 5, 5, 6, 5, 6, 6, 7,
4, 5, 5, 6, 5, 6, 6, 7,
5, 6, 6, 7, 6, 7, 7, 8,
}

14
pkg/registry/core/service/allocator/utils_test.go
View File

@ -21,20 +21,6 @@ import (
"testing" "testing"
) )
func TestBitCount(t *testing.T) {
for i, c := range bitCounts {
actual := 0
for j := 0; j < 8; j++ {
if ((1 << uint(j)) & i) != 0 {
actual++
}
}
if actual != int(c) {
t.Errorf("%d should have %d bits but recorded as %d", i, actual, c)
}
}
}
func TestCountBits(t *testing.T) { func TestCountBits(t *testing.T) {
tests := []struct { tests := []struct {
n *big.Int n *big.Int