strconv: use bounded bits.TrailingZeros instead of shifts table

The strconv shifts table is 320 bytes (amd64) and is present in
many binaries since integer formatting is very common.

Instead of using a precalculated table with shift amounts
use a bounded bits.TrailingZeros to determine the shift amount
to format numbers in a base that is a power of 2.

amd64:
name        old time/op  new time/op  delta
AppendUint   379ns ± 1%   286ns ± 2%  -24.62%  (p=0.000 n=20+19)

Change-Id: Ib94d9b033321b41e975868943c7fcd9428c5111e
Reviewed-on: https://go-review.googlesource.com/110478
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/build/deps_test.go

@@ -64,7 +64,7 @@ var pkgDeps = map[string][]string{
 	"math/bits":     {},
 	"math/cmplx":    {"math"},
 	"math/rand":     {"L0", "math"},
-	"strconv":       {"L0", "unicode/utf8", "math"},
+	"strconv":       {"L0", "unicode/utf8", "math", "math/bits"},
 	"unicode/utf16": {},
 	"unicode/utf8":  {},
 

src/strconv/itoa.go

@@ -4,6 +4,8 @@
 
 package strconv
 
+import "math/bits"
+
 const fastSmalls = true // enable fast path for small integers
 
 // FormatUint returns the string representation of i in the given base,
@@ -79,14 +81,6 @@ const host32bit = ^uint(0)>>32 == 0
 
 const digits = "0123456789abcdefghijklmnopqrstuvwxyz"
 
-var shifts = [len(digits) + 1]uint{
-	1 << 1: 1,
-	1 << 2: 2,
-	1 << 3: 3,
-	1 << 4: 4,
-	1 << 5: 5,
-}
-
 // formatBits computes the string representation of u in the given base.
 // If neg is set, u is treated as negative int64 value. If append_ is
 // set, the string is appended to dst and the resulting byte slice is
@@ -158,14 +152,17 @@ func formatBits(dst []byte, u uint64, base int, neg, append_ bool) (d []byte, s
 			a[i] = smallsString[is]
 		}
 
-	} else if s := shifts[base]; s > 0 {
-		// base is power of 2: use shifts and masks instead of / and %
+	} else if isPowerOfTwo(base) {
+		// It is known that base is a power of two and
+		// 2 <= base <= len(digits).
+		// Use shifts and masks instead of / and %.
+		shift := uint(bits.TrailingZeros(uint(base))) & 31
 		b := uint64(base)
-		m := uint(base) - 1 // == 1<<s - 1
+		m := uint(base) - 1 // == 1<<shift - 1
 		for u >= b {
 			i--
 			a[i] = digits[uint(u)&m]
-			u >>= s
+			u >>= shift
 		}
 		// u < base
 		i--
@@ -200,3 +197,7 @@ func formatBits(dst []byte, u uint64, base int, neg, append_ bool) (d []byte, s
 	s = string(a[i:])
 	return
 }
+
+func isPowerOfTwo(x int) bool {
+	return x&(x-1) == 0
+}