diff --git a/src/strconv/itoa.go b/src/strconv/itoa.go
index 8afe7af251..4aaf57830c 100644
--- a/src/strconv/itoa.go
+++ b/src/strconv/itoa.go
@@ -152,10 +152,14 @@ func formatBits(dst []byte, u uint64, base int, neg, append_ bool) (d []byte, s
 		}
 
 	} 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
+		// Base is a power of 2 and 2 <= base <= len(digits) where len(digits) is 36.
+		// The largest power of 2 below or equal to 36 is 32, which is 1 << 5;
+		// i.e., the largest possible shift count is 5. By &-ind that value with
+		// the constant 7 we tell the compiler that the shift count is always
+		// less than 8 which is smaller than any register width. This allows
+		// the compiler to generate better code for the shift operation.
+		shift := uint(bits.TrailingZeros(uint(base))) & 7
 		b := uint64(base)
 		m := uint(base) - 1 // == 1<<shift - 1
 		for u >= b {