math/bits: faster TrailingZeroes8

For sizes > 8, the existing code is faster.

benchmark                     old ns/op     new ns/op     delta
BenchmarkTrailingZeros8-8     1.95          1.29          -33.85%

Measured on 2.3 GHz Intel Core i7 running macOS 10.12.3.

Change-Id: I6f3a33ec633a2c544ec29693c141f2f99335c745
Reviewed-on: https://go-review.googlesource.com/37581
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/math/bits/bits.go b/src/math/bits/bits.go
index 24aa7c5f3fb8ec10cbc1d23a728db7de7d9204cc..1aaa9eea9d4c2f4b51ce79dd0538523ea1dc3f45 100644 (file)
--- a/src/math/bits/bits.go
+++ b/src/math/bits/bits.go
@@ -34,7 +34,7 @@ func LeadingZeros64(x uint64) int { return 64 - blen(uint64(x)) }
 func TrailingZeros(x uint) int { return ntz(x) }
 
 // TrailingZeros8 returns the number of trailing zero bits in x; the result is 8 for x == 0.
-func TrailingZeros8(x uint8) int { return ntz8(x) }
+func TrailingZeros8(x uint8) int { return int(ntz8tab[x]) }
 
 // TrailingZeros16 returns the number of trailing zero bits in x; the result is 16 for x == 0.
 func TrailingZeros16(x uint16) int { return ntz16(x) }

diff --git a/src/math/bits/bits_impl.go b/src/math/bits/bits_impl.go
index e7c1a8a5dc83a590f12845cd361cefd5619653df..cf5a12af2b84f081eed89f36b38f15a2fa05aeb2 100644 (file)
--- a/src/math/bits/bits_impl.go
+++ b/src/math/bits/bits_impl.go
@@ -23,14 +23,6 @@ var deBruijn32tab = [32]byte{
        31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9,
 }
 
-func ntz8(x uint8) (n int) {
-       if x == 0 {
-               return 8
-       }
-       // see comment in ntz64
-       return int(deBruijn32tab[uint32(x&-x)*deBruijn32>>(32-5)])
-}
-
 func ntz16(x uint16) (n int) {
        if x == 0 {
                return 16

diff --git a/src/math/bits/bits_tables.go b/src/math/bits/bits_tables.go
index 2c4e31b7966e7b3e687975f82ece30a8de662e9c..f79f83a01e1bcdbd2ed7ef4fa3b9f772e05130a5 100644 (file)
--- a/src/math/bits/bits_tables.go
+++ b/src/math/bits/bits_tables.go
@@ -6,6 +6,25 @@
 
 package bits
 
+var ntz8tab = [256]uint8{
+       0x08, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x05, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x06, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x05, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x07, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x05, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x06, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x05, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+       0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00,
+}
+
 var pop8tab = [256]uint8{
        0x00, 0x01, 0x01, 0x02, 0x01, 0x02, 0x02, 0x03, 0x01, 0x02, 0x02, 0x03, 0x02, 0x03, 0x03, 0x04,
        0x01, 0x02, 0x02, 0x03, 0x02, 0x03, 0x03, 0x04, 0x02, 0x03, 0x03, 0x04, 0x03, 0x04, 0x04, 0x05,

diff --git a/src/math/bits/make_tables.go b/src/math/bits/make_tables.go
index 7127bdf4130cee3b4c58ea3c8b3756a50537f169..c66afb5a96e41f21872e40571030c673b1ed2e63 100644 (file)
--- a/src/math/bits/make_tables.go
+++ b/src/math/bits/make_tables.go
@@ -30,6 +30,7 @@ package bits
 func main() {
        buf := bytes.NewBuffer(header)
 
+       gen(buf, "ntz8tab", ntz8)
        gen(buf, "pop8tab", pop8)
        gen(buf, "rev8tab", rev8)
        // add more tables as needed
@@ -58,6 +59,14 @@ func gen(w io.Writer, name string, f func(uint8) uint8) {
        fmt.Fprint(w, "\n}\n\n")
 }
 
+func ntz8(x uint8) (n uint8) {
+       for x&1 == 0 && n < 8 {
+               x >>= 1
+               n++
+       }
+       return
+}
+
 func pop8(x uint8) (n uint8) {
        for x != 0 {
                x &= x - 1