Merge pull request #7693 from kaspar030/improve_bitarithm

core: optimize bitarithm_lsb()

core/bitarithm.c

@@ -21,6 +21,8 @@
 
 #include <stdio.h>
 
+#include "bitarithm.h"
+
 unsigned bitarithm_msb(unsigned v)
 {
     register unsigned r; // result of log2(v) will go here
@@ -43,19 +45,7 @@ unsigned bitarithm_msb(unsigned v)
 
     return r;
 }
-/*---------------------------------------------------------------------------*/
-unsigned bitarithm_lsb(register unsigned v)
-{
-    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
@@ -66,3 +56,9 @@ unsigned bitarithm_bits_set(unsigned v)
 
     return c;
 }
+
+const uint8_t MultiplyDeBruijnBitPosition[32] =
+{
+    0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
+    31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
+};

core/include/bitarithm.h

 /*
- * Copyright (C) 2014 Freie Universität Berlin
+ * Copyright (C) 2017 Kaspar Schleiser <kaspar@schleiser.de>
+ *               2014 Freie Universität Berlin
  *
  * This file is subject to the terms and conditions of the GNU Lesser
  * General Public License v2.1. See the file LICENSE in the top level
@@ -20,6 +21,10 @@
 #ifndef BITARITHM_H
 #define BITARITHM_H
 
+#include <stdint.h>
+
+#include "cpu_conf.h"
+
 #ifdef __cplusplus
  extern "C" {
 #endif
@@ -105,19 +110,44 @@ unsigned bitarithm_msb(unsigned v);
  *                  function will produce an infinite loop
  * @return          Bit Number
  *
- * Source: http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogObvious
  */
-unsigned bitarithm_lsb(register unsigned v);
+static inline unsigned bitarithm_lsb(unsigned v);
 
 /**
  * @brief   Returns the number of bits set in a value
  * @param[in]   v   Input value
  * @return          Number of set bits
  *
- * Source: http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogObvious
  */
 unsigned bitarithm_bits_set(unsigned v);
 
+/* implementations */
+
+static inline unsigned bitarithm_lsb(unsigned v)
+#if defined(BITARITHM_LSB_BUILTIN)
+{
+    return __builtin_ffs(v) - 1;
+}
+#elif defined(BITARITHM_LSB_LOOKUP)
+{
+/* Source: http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup */
+    extern const uint8_t MultiplyDeBruijnBitPosition[32];
+    return MultiplyDeBruijnBitPosition[((uint32_t)((v & -v) * 0x077CB531U)) >> 27];
+}
+#else
+{
+/* Source: http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogObvious */
+    unsigned r = 0;
+
+    while ((v & 0x01) == 0) {
+        v >>= 1;
+        r++;
+    };
+
+    return r;
+}
+#endif
+
 #ifdef __cplusplus
 }
 #endif

cpu/cortexm_common/include/cpu_conf_common.h

@@ -57,6 +57,17 @@ extern "C" {
 #endif
 /** @} */
 
+/**
+ * @brief   Select fastest bitarithm_lsb implementation
+ * @{
+ */
+#ifdef __ARM_FEATURE_CLZ
+#define BITARITHM_LSB_BUILTIN
+#else
+#define BITARITHM_LSB_LOOKUP
+#endif
+/** @} */
+
 #ifdef __cplusplus
 }
 #endif