math/bits: faster Reverse8/16 functions using table lookups

Measured on 2.3 GHz Intel Core i7, running macOS 10.12.3:

benchmark                old ns/op     new ns/op     delta
BenchmarkReverse8-8      1.70          0.99          -41.76%
BenchmarkReverse16-8     2.24          1.32          -41.07%

Fixes #19279.

Change-Id: I398cf8a3513b7fa63c130efc7846a7c5353999d4
Reviewed-on: https://go-review.googlesource.com/37459
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/bits/bits.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:generate go run make_tables.go
+
 // Package bits implements bit counting and manipulation
 // functions for the predeclared unsigned integer types.
 package bits
@@ -232,19 +234,12 @@ func Reverse(x uint) uint {
 
 // Reverse8 returns the value of x with its bits in reversed order.
 func Reverse8(x uint8) uint8 {
-	const m = 1<<8 - 1
-	x = x>>1&(m0&m) | x&(m0&m)<<1
-	x = x>>2&(m1&m) | x&(m1&m)<<2
-	return x>>4 | x<<4
+	return rev8tab[x]
 }
 
 // Reverse16 returns the value of x with its bits in reversed order.
 func Reverse16(x uint16) uint16 {
-	const m = 1<<16 - 1
-	x = x>>1&(m0&m) | x&(m0&m)<<1
-	x = x>>2&(m1&m) | x&(m1&m)<<2
-	x = x>>4&(m2&m) | x&(m2&m)<<4
-	return x>>8 | x<<8
+	return uint16(rev8tab[x>>8]) | uint16(rev8tab[x&0xff])<<8
 }
 
 // Reverse32 returns the value of x with its bits in reversed order.