Revert "core: faster bitarithm_msb"

This reverts commit ed7e233876.

core/bitarithm.c

@@ -15,36 +15,47 @@
  *
  * @author      Kaspar Schleiser <kaspar@schleiser.de>
  * @author      Martin Lenders <mlenders@inf.fu-berlin.de>
- * @author      René Kijewski <rene.kijewski@fu-berlin.de>
  *
  * @}
  */
 
 #include <stdio.h>
 
-#include "bitarithm.h"
-
 unsigned bitarithm_msb(unsigned v)
 {
-    if ((signed) v >= 0) {
-        v &= -v;
-        return bitarithm_lsb(v);
-    }
-    else {
-        return sizeof (v) * 8 - 1;
+    register unsigned r; // result of log2(v) will go here
+
+#if ARCH_32_BIT
+    register unsigned shift;
+
+    r =     (v > 0xFFFF) << 4; v >>= r;
+    shift = (v > 0xFF  ) << 3; v >>= shift; r |= shift;
+    shift = (v > 0xF   ) << 2; v >>= shift; r |= shift;
+    shift = (v > 0x3   ) << 1; v >>= shift; r |= shift;
+                                            r |= (v >> 1);
+#else
+    r = 0;
+    while (v >>= 1) { // unroll for more speed...
+        r++;
     }
-}
 
-unsigned bitarithm_lsb(unsigned v)
+#endif
+
+    return r;
+}
+/*---------------------------------------------------------------------------*/
+unsigned bitarithm_lsb(register unsigned v)
 {
-    unsigned r = 0;
-    while ((v & 1) == 0) {
+    register unsigned r = 0;
+
+    while ((v & 0x01) == 0) {
         v >>= 1;
         r++;
-    }
+    };
+
     return r;
 }
-
+/*---------------------------------------------------------------------------*/
 unsigned bitarithm_bits_set(unsigned v)
 {
     unsigned c; // c accumulates the total bits set in v