math/bits: much faster ReverseBytes, added respective benchmarks

Measured on 2.3 GHz Intel Core i7, running maxOS 10.12.3.

benchmark                     old ns/op     new ns/op     delta
BenchmarkReverseBytes-8       11.4          3.51          -69.21%
BenchmarkReverseBytes16-8     6.87          0.64          -90.68%
BenchmarkReverseBytes32-8     7.79          0.65          -91.66%
BenchmarkReverseBytes64-8     11.6          0.64          -94.48%

name              old time/op  new time/op  delta
ReverseBytes-8    11.4ns ± 0%   3.5ns ± 0%   ~     (p=1.000 n=1+1)
ReverseBytes16-8  6.87ns ± 0%  0.64ns ± 0%   ~     (p=1.000 n=1+1)
ReverseBytes32-8  7.79ns ± 0%  0.65ns ± 0%   ~     (p=1.000 n=1+1)
ReverseBytes64-8  11.6ns ± 0%   0.6ns ± 0%   ~     (p=1.000 n=1+1)

Change-Id: I67b529652b3b613c61687e9e185e8d4ee40c51a2
Reviewed-on: https://go-review.googlesource.com/37211
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

3 files changed, 52 insertions, 16 deletions

diff --git a/src/math/bits/bits.go b/src/math/bits/bits.go
index ab3f3eaa1e..9bbc2c5883 100644
--- a/src/math/bits/bits.go
+++ b/src/math/bits/bits.go
@@ -113,7 +113,7 @@ func Reverse(x uint) uint {
 
 // Reverse8 returns the value of x with its bits in reversed order.
 func Reverse8(x uint8) uint8 {
-	const m = 0xff
+	const m = 1<<8 - 1
 	x = x&(m0&m)>>1 | x&^(m0&m)<<1
 	x = x&(m1&m)>>2 | x&^(m1&m)<<2
 	x = x&(m2&m)>>4 | x&^(m2&m)<<4
@@ -122,7 +122,7 @@ func Reverse8(x uint8) uint8 {
 
 // Reverse16 returns the value of x with its bits in reversed order.
 func Reverse16(x uint16) uint16 {
-	const m = 0xffff
+	const m = 1<<16 - 1
 	x = x&(m0&m)>>1 | x&^(m0&m)<<1
 	x = x&(m1&m)>>2 | x&^(m1&m)<<2
 	x = x&(m2&m)>>4 | x&^(m2&m)<<4
@@ -132,7 +132,7 @@ func Reverse16(x uint16) uint16 {
 
 // Reverse32 returns the value of x with its bits in reversed order.
 func Reverse32(x uint32) uint32 {
-	const m = 0xffffffff
+	const m = 1<<32 - 1
 	x = x&(m0&m)>>1 | x&^(m0&m)<<1
 	x = x&(m1&m)>>2 | x&^(m1&m)<<2
 	x = x&(m2&m)>>4 | x&^(m2&m)<<4
@@ -143,7 +143,7 @@ func Reverse32(x uint32) uint32 {
 
 // Reverse64 returns the value of x with its bits in reversed order.
 func Reverse64(x uint64) uint64 {
-	const m = 0xffffffffffffffff
+	const m = 1<<64 - 1
 	x = x&(m0&m)>>1 | x&^(m0&m)<<1
 	x = x&(m1&m)>>2 | x&^(m1&m)<<2
 	x = x&(m2&m)>>4 | x&^(m2&m)<<4
@@ -156,16 +156,36 @@ func Reverse64(x uint64) uint64 {
 // --- ReverseBytes ---
 
 // ReverseBytes returns the value of x with its bytes in reversed order.
-func ReverseBytes(x uint) uint { return uint(swap(uint64(x), UintSize)) }
+func ReverseBytes(x uint) uint {
+	if UintSize == 32 {
+		return uint(ReverseBytes32(uint32(x)))
+	}
+	return uint(ReverseBytes64(uint64(x)))
+}
 
 // ReverseBytes16 returns the value of x with its bytes in reversed order.
-func ReverseBytes16(x uint16) uint16 { return uint16(swap(uint64(x), 16)) }
+func ReverseBytes16(x uint16) uint16 {
+	const m = 1<<16 - 1
+	x = x&(m3&m)>>8 | x&^(m3&m)<<8
+	return x
+}
 
 // ReverseBytes32 returns the value of x with its bytes in reversed order.
-func ReverseBytes32(x uint32) uint32 { return uint32(swap(uint64(x), 32)) }
+func ReverseBytes32(x uint32) uint32 {
+	const m = 1<<32 - 1
+	x = x&(m3&m)>>8 | x&^(m3&m)<<8
+	x = x&(m4&m)>>16 | x&^(m4&m)<<16
+	return x
+}
 
 // ReverseBytes64 returns the value of x with its bytes in reversed order.
-func ReverseBytes64(x uint64) uint64 { return uint64(swap(uint64(x), 64)) }
+func ReverseBytes64(x uint64) uint64 {
+	const m = 1<<64 - 1
+	x = x&(m3&m)>>8 | x&^(m3&m)<<8
+	x = x&(m4&m)>>16 | x&^(m4&m)<<16
+	x = x&(m5&m)>>32 | x&^(m5&m)<<32
+	return x
+}
 
 // --- Len ---
 
diff --git a/src/math/bits/bits_impl.go b/src/math/bits/bits_impl.go
index da38429fee..3a425e3b83 100644
--- a/src/math/bits/bits_impl.go
+++ b/src/math/bits/bits_impl.go
@@ -93,14 +93,6 @@ func rot(x uint64, size, k uint) uint64 {
 	return x<<k | x>>(size-k)&(1<<k-1)
 }
 
-func swap(x uint64, size uint) (r uint64) {
-	for i := size / 8; i > 0; i-- {
-		r = r<<8 | x&0xff
-		x >>= 8
-	}
-	return
-}
-
 func blen(x uint64) (i int) {
 	for ; x >= 1<<(16-1); x >>= 16 {
 		i += 16
diff --git a/src/math/bits/bits_test.go b/src/math/bits/bits_test.go
index bba35612fe..e1c7201daa 100644
--- a/src/math/bits/bits_test.go
+++ b/src/math/bits/bits_test.go
@@ -453,6 +453,30 @@ func testReverseBytes(t *testing.T, x64, want64 uint64) {
 	}
 }
 
+func BenchmarkReverseBytes(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		ReverseBytes(deBruijn64 & (1<<UintSize - 1))
+	}
+}
+
+func BenchmarkReverseBytes16(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		ReverseBytes16(deBruijn64 & (1<<16 - 1))
+	}
+}
+
+func BenchmarkReverseBytes32(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		ReverseBytes32(deBruijn64 & (1<<32 - 1))
+	}
+}
+
+func BenchmarkReverseBytes64(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		ReverseBytes64(deBruijn64 & (1<<64 - 1))
+	}
+}
+
 func TestLen(t *testing.T) {
 	for i := 0; i < 256; i++ {
 		len := 8 - tab[i].nlz