网络子系统9_ip校验和计算

//ip校验和计算使用与体系结构相关的内联汇编
//x86_64版本
//b 1字节
//w 2字节
//l 4字节
//q 8字节
//1010 1010 逻辑左移 -> 0101 010[0]
//1010 1010 算数左移 -> 0101 010[0]

//1010 1010 逻辑右移 -> [0]101 0101
//1010 1010 算数右移 -> [1]101 0101
1.1 static inline unsigned short ip_fast_csum(unsigned char *iph, unsigned int ihl)
{
	unsigned int sum;
	asm(
		"  movl (%1), %0\n"//将sum = *iph
		"  subl $4, %2\n"//ihl = ihl-4
		"  jbe 2f\n"//长度为0，跳转到2处
		"  addl 4(%1), %0\n"//sum += *(iph+4)
		"  adcl 8(%1), %0\n"//sum += *(iph+8)
		"  adcl 12(%1), %0\n"//sum += *(iph + 12)
		"1: adcl 16(%1), %0\n"//sum += *(iph + 16)
		"  lea 4(%1), %1\n"//iph = iph + 4
		"  decl %2\n"//ihl -= 1
		"  jne	1b\n"//如果ihl！=0，跳转到1
		"  adcl $0, %0\n"//sum = sum+0
		"  movl %0, %2\n"//ihl = sum
		"  shrl $16, %0\n"//sum逻辑右移16位
		"  addw %w2, %w0\n"//取ihl低16位加到sum低16位
		"  adcl $0, %0\n"//sum = sum+0
		"  notl %0\n"//sum取反
		"2:"
	: "=r" (sum), "=r" (iph), "=r" (ihl)//输出，都使用寄存器保存输出
	: "1" (iph), "2" (ihl)//第一个输入为iph，第二个输入为ihl
	: "memory");//表明内存可能发生变化
	return(sum);//返回sum
}