Add __constant_flsl(), because it's useful for finding out the next

power-of-two up from a given constant via
  ( 1 << fls ( constant - 1 ) )

fls(), flsl(), ffs() and ffsl() appear in strings.h according to
POSIX.

src/include/strings.h

@@ -0,0 +1,54 @@
+#ifndef _STRINGS_H
+#define _STRINGS_H
+
+#include <limits.h>
+
+static inline __attribute__ (( always_inline )) int
+__constant_flsl ( unsigned long x ) {
+	int r = 0;
+
+#if ULONG_MAX > 0xffffffff
+	if ( x & 0xffffffff00000000UL ) {
+		x >>= 32;
+		r += 32;
+	}
+#endif
+	if ( x & 0xffff0000UL ) {
+		x >>= 16;
+		r += 16;
+	}
+	if ( x & 0xff00 ) {
+		x >>= 8;
+		r += 8;
+	}
+	if ( x & 0xf0 ) {
+		x >>= 4;
+		r += 4;
+	}
+	if ( x & 0xc ) {
+		x >>= 2;
+		r += 2;
+	}
+	if ( x & 0x2 ) {
+		x >>= 1;
+		r += 1;
+	}
+	if ( x & 0x1 ) {
+		r += 1;
+	}
+	return r;
+}
+
+#define __constant_fls(x) __constant_flsl(x)
+
+/* We don't actually have these functions yet */
+extern int __fls ( int x );
+extern int __flsl ( long x );
+
+#define flsl( x ) \
+	( __builtin_constant_p ( x ) ? __constant_flsl ( x ) : __flsl ( x ) )
+
+#define fls( x ) \
+	( __builtin_constant_p ( x ) ? __constant_fls ( x ) : __fls ( x ) )
+
+#endif /* _STRINGS_H */