RTOS/Kernel/SingleCore/cpu_clz.c

#include "cpu_clz.h"
#include "os_config.h"
#include "cpu.h"

#if !defined(OS_CFG_CPU_CLZ_ASM_PRESENT) || (!OS_CFG_CPU_CLZ_ASM_PRESENT)
// 如果配置中没有提供汇编版本的clz函数，则使用C语言实现

static  const  uint8_t  clz_table[256] = {/*   索引           */
    8u,7u,6u,6u,5u,5u,5u,5u,4u,4u,4u,4u,4u,4u,4u,4u,  /*   0x00 to 0x0F   */
    3u,3u,3u,3u,3u,3u,3u,3u,3u,3u,3u,3u,3u,3u,3u,3u,  /*   0x10 to 0x1F   */
    2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,  /*   0x20 to 0x2F   */
    2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,2u,  /*   0x30 to 0x3F   */
    1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,  /*   0x40 to 0x4F   */
    1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,  /*   0x50 to 0x5F   */
    1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,  /*   0x60 to 0x6F   */
    1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,1u,  /*   0x70 to 0x7F   */
    0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,  /*   0x80 to 0x8F   */
    0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,  /*   0x90 to 0x9F   */
    0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,  /*   0xA0 to 0xAF   */
    0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,  /*   0xB0 to 0xBF   */
    0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,  /*   0xC0 to 0xCF   */
    0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,  /*   0xD0 to 0xDF   */
    0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,  /*   0xE0 to 0xEF   */
    0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u,0u   /*   0xF0 to 0xFF   */
};

os_uint_t cpu_clz(os_uint_t value){
    os_uint_t result = 0;
    uint8_t idx;

    // 检查高 16 bits 
    if(value > 0x0000FFFFu){
        // 检查 bits[24:31]    
        if(value > 0x00FFFFFFu){
            idx = (value >> 24u) & 0xFFu;       // 获取 bits[24:31] 的值
            result = clz_table[idx];            // 查表获取 bits[24:31] 中前导零的数量
        }else{
            idx = (value >> 16u) & 0xFFu;       // 获取 bits[16:23] 的值
            result = clz_table[idx] + 8u;       // 查表获取 bits[16:23] 中前导零的数量，并加上8，表示 bits[24:31] 中的0
        }
    }else{
        // 检查 bits[0:15]
        if(value > 0x000000FFu){
            idx = (value >> 8u) & 0xFFu;        // 获取 bits[8:15] 的值
            result = clz_table[idx] + 16u;      // 查表获取 bits[8:15] 中前导零的数量，并加上16，表示 bits[16:31] 中的0
        }else{
            idx = value & 0xFFu;                 // 获取 bits[0:7] 的值
            result = clz_table[idx] + 24u;      // 查表获取 bits[0:7] 中前导零的数量，并加上24，表示 bits[8:31] 中的0
        }
    }
    return result;
}
#endif /* OS_CFG_CLZ_ASM_PRESENT */