[dev.regabi] go/constant: optimize BitLen

Avoids an unnecessary heap allocation when computing the bit length of
int64 values.

Change-Id: I69dfc510e461daf3e83b0b7b6c0707f6526a32d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/272646
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/constant/value.go

@@ -17,6 +17,7 @@ import (
 	"go/token"
 	"math"
 	"math/big"
+	"math/bits"
 	"strconv"
 	"strings"
 	"sync"
@@ -610,7 +611,11 @@ func Make(x interface{}) Value {
 func BitLen(x Value) int {
 	switch x := x.(type) {
 	case int64Val:
-		return i64toi(x).val.BitLen()
+		u := uint64(x)
+		if x < 0 {
+			u = uint64(-x)
+		}
+		return 64 - bits.LeadingZeros64(u)
 	case intVal:
 		return x.val.BitLen()
 	case unknownVal:

src/go/constant/value_test.go

@@ -655,3 +655,24 @@ func BenchmarkStringAdd(b *testing.B) {
 		})
 	}
 }
+
+var bitLenTests = []struct {
+	val  int64
+	want int
+}{
+	{0, 0},
+	{1, 1},
+	{-16, 5},
+	{1 << 61, 62},
+	{1 << 62, 63},
+	{-1 << 62, 63},
+	{-1 << 63, 64},
+}
+
+func TestBitLen(t *testing.T) {
+	for _, test := range bitLenTests {
+		if got := BitLen(MakeInt64(test.val)); got != test.want {
+			t.Errorf("%v: got %v, want %v", test.val, got, test.want)
+		}
+	}
+}