Merge pull request #5063 from Eureka1024/master

[kernel]增加一种新的查找字节最低非0位的算法

Author: BernardXiong

src/Kconfig

@@ -139,6 +139,10 @@ config RT_USING_ASM_MEMCPY
     bool
     default n
 
+config RT_USING_TINY_FFS
+    bool "Enable kservice to use tiny ffs"
+    default n
+
 endmenu
 
 menuconfig RT_DEBUG

src/kservice.c

@@ -1360,6 +1360,31 @@ RTM_EXPORT(rt_free_align);
 #endif /* RT_USING_HEAP */
 
 #ifndef RT_USING_CPU_FFS
+#ifdef RT_USING_TINY_FFS
+const rt_uint8_t __lowest_bit_bitmap[] =
+{
+    /*  0 - 7  */  0,  1,  2, 27,  3, 24, 28, 32,
+    /*  8 - 15 */  4, 17, 25, 31, 29, 12, 32, 14,
+    /* 16 - 23 */  5,  8, 18, 32, 26, 23, 32, 16,
+    /* 24 - 31 */ 30, 11, 13,  7, 32, 22, 15, 10,
+    /* 32 - 36 */  6, 21,  9, 20, 19
+};
+
+/**
+ * This function finds the first bit set (beginning with the least significant bit)
+ * in value and return the index of that bit.
+ *
+ * Bits are numbered starting at 1 (the least significant bit).  A return value of
+ * zero from any of these functions means that the argument was zero.
+ *
+ * @return return the index of the first bit set. If value is 0, then this function
+ * shall return 0.
+ */
+int __rt_ffs(int value)
+{
+    return __lowest_bit_bitmap[(rt_uint32_t)(value & (value - 1) ^ value) % 37];
+}
+#else
 const rt_uint8_t __lowest_bit_bitmap[] =
 {
     /* 00 */ 0, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
@@ -1405,6 +1430,7 @@ int __rt_ffs(int value)
 
     return __lowest_bit_bitmap[(value & 0xff000000) >> 24] + 25;
 }
+#endif /* RT_USING_TINY_FFS */
 #endif /* RT_USING_CPU_FFS */
 
 #ifdef RT_DEBUG