关于数据类型强制转换的效率问题

char *字符串;

unsigned char *临时指针=(unsigned char *)字符串;
for(long i=0;i<10000;i++){
    if(临时指针[i])
        临时指针[i]=255;
}

char *字符串;

for(long i=0;i<10000;i++){
    if(((unsigned char *)字符串)[i])
        ((unsigned char *)字符串)[i]=255;
}