cmd/internal/obj/arm64, image/gif, runtime, sort: use math/bits to calculate log2

In several places the integer log2 is calculated using loops or similar
mechanisms. math/bits.Len* provide a simpler and more efficient
mechanisms for this.

Annoyingly, every usage has slightly different ideas of what "log2"
means and how non-positive inputs should be handled. I verified the
replacements in each case by comparing the result for inputs from 0
to 1<<16.

Change-Id: Ie962a74674802da363e0038d34c06979ccb41cf3
Reviewed-on: https://go-review.googlesource.com/c/go/+/721880
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/cmd/internal/obj/arm64/asm7.go

@@ -1674,32 +1674,7 @@ func log2(x uint64) uint32 {
 	if x == 0 {
 		panic("log2 of 0")
 	}
-	n := uint32(0)
-	if x >= 1<<32 {
-		x >>= 32
-		n += 32
-	}
-	if x >= 1<<16 {
-		x >>= 16
-		n += 16
-	}
-	if x >= 1<<8 {
-		x >>= 8
-		n += 8
-	}
-	if x >= 1<<4 {
-		x >>= 4
-		n += 4
-	}
-	if x >= 1<<2 {
-		x >>= 2
-		n += 2
-	}
-	if x >= 1<<1 {
-		x >>= 1
-		n += 1
-	}
-	return n
+	return uint32(bits.Len64(x) - 1)
 }
 
 func autoclass(l int64) int {

src/image/gif/writer.go

@@ -15,6 +15,7 @@ import (
 	"image/draw"
 	"internal/byteorder"
 	"io"
+	"math/bits"
 )
 
 // Graphic control extension fields.
@@ -23,15 +24,11 @@ const (
 	gcBlockSize = 0x04
 )
 
-var log2Lookup = [8]int{2, 4, 8, 16, 32, 64, 128, 256}
-
 func log2(x int) int {
-	for i, v := range log2Lookup {
-		if x <= v {
-			return i
+	if x < 2 {
+		return 0
 	}
-	}
-	return -1
+	return bits.Len(uint(x-1)) - 1
 }
 
 // writer is a buffered writer.
@@ -192,7 +189,7 @@ func (e *encoder) writeHeader() {
 }
 
 func encodeColorTable(dst []byte, p color.Palette, size int) (int, error) {
-	if uint(size) >= uint(len(log2Lookup)) {
+	if uint(size) >= 8 {
 		return 0, errors.New("gif: cannot encode color table with more than 256 entries")
 	}
 	for i, c := range p {
@@ -212,7 +209,7 @@ func encodeColorTable(dst []byte, p color.Palette, size int) (int, error) {
 		dst[3*i+1] = g
 		dst[3*i+2] = b
 	}
-	n := log2Lookup[size]
+	n := 1 << (size + 1)
 	if n > len(p) {
 		// Pad with black.
 		clear(dst[3*len(p) : 3*n])

src/runtime/stack.go

@@ -12,6 +12,7 @@ import (
 	"internal/runtime/atomic"
 	"internal/runtime/gc"
 	"internal/runtime/sys"
+	"math/bits"
 	"unsafe"
 )
 
@@ -181,12 +182,10 @@ func stackinit() {
 
 // stacklog2 returns ⌊log_2(n)⌋.
 func stacklog2(n uintptr) int {
-	log2 := 0
-	for n > 1 {
-		n >>= 1
-		log2++
+	if n == 0 {
+		return 0
 	}
-	return log2
+	return bits.Len64(uint64(n))
 }
 
 // Allocates a stack from the free pool. Must be called with

src/sort/search_test.go

@@ -5,6 +5,7 @@
 package sort_test
 
 import (
+	"math/bits"
 	"runtime"
 	. "sort"
 	stringspkg "strings"
@@ -135,13 +136,10 @@ func TestFind(t *testing.T) {
 // log2 computes the binary logarithm of x, rounded up to the next integer.
 // (log2(0) == 0, log2(1) == 0, log2(2) == 1, log2(3) == 2, etc.)
 func log2(x int) int {
-	n := 0
-	for p := 1; p < x; p += p {
-		// p == 2**n
-		n++
+	if x < 1 {
+		return 0
 	}
-	// p/2 < x <= p == 2**n
-	return n
+	return bits.Len(uint(x - 1))
 }
 
 func TestSearchEfficiency(t *testing.T) {