在 GCC 中调用程序集？

Question

#include <stdlib.h>

static inline uint
xchg(volatile unsigned int *addr, unsigned int newval)
{
   uint result;
   asm volatile("lock; xchgl %0, %1" : "+m" (*addr), "=a" (result) : "1" (newval) : "cc");

return result;    
}

谁能告诉我这段代码到底做了什么？我的意思是我有一个想法或这个命令的一部分。 “1”newval是输入，“=a”是把它之前的值清空更新。 “m”用于内存操作，但我对这个函数的功能感到困惑。 “+m”符号有什么作用？这个函数是否做类似 m=a; 的事情？ m = 新值；返回一个

Answer 1

= 和 + 是约束修饰符。

http://gcc.gnu.org/onlinedocs/gcc/Modifiers.html#Modifiers

`=' Means that this operand is write-only for this instruction: the previous value is discarded and replaced by output data.

`+' Means that this operand is both read and written by the instruction.

基本约束在这里

http://gcc.gnu.org/onlinedocs/gcc/Simple-Constraints.html#Simple-Constraints

m A memory operand is allowed, with any kind of address that the machine supports in general.

..1.. An operand that matches the specified operand number is allowed. If a digit is used together with letters within the same alternative, the digit should come last.

'a' 是 i386 特定的

http://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints

a The a (eax) register.