kernel/util: Add fls to BitUtils.

Implemented using __builtin_clz where available, otherwise using
an algorithm derived from "Bit Twiddling Hacks" which is similar
to the one ramfs uses. GCC and Clang seem to unroll the loop on
x86 at least (but it doesn't matter there as the builtin exists,
implemented using the "bsr" instruction.)

headers/private/kernel/util/BitUtils.h

@@ -40,6 +40,31 @@ count_set_bits(uint32 v)
 }
 
 
+static inline uint32
+fls(uint32 value)
+{
+	if (value == 0)
+		return 0;
+
+#if __has_builtin(__builtin_clz)
+	return ((sizeof(value) * 8) - __builtin_clz(value));
+#else
+	// https://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
+	static const uint32 masks[] = {
+		0xaaaaaaaa,
+		0xcccccccc,
+		0xf0f0f0f0,
+		0xff00ff00,
+		0xffff0000
+	};
+	uint32 result = (value & masks[0]) != 0;
+	for (int i = 4; i > 0; i--)
+		result |= ((value & masks[i]) != 0) << i;
+	return result + 1;
+#endif
+}
+
+
 static inline uint32
 log2(uint32 v)
 {