volatile与内存屏障与Acquire&Release语义三者的差别和关系(一)

wait:
.LFB0:
        .cfi_startproc
        movl    flag(%rip), %edx //从内存里读取flag
        cmpl    $1, %edx
        je      .L1
        movl    $1, %eax
        .p2align 4,,10
        .p2align 3
.L3:
        notl    %eax
        cmpl    %edx, %eax   //直接使用edx, 不再去内存里面获取flag的内存值
        jne     .L3

wait:
.LFB0:
        .cfi_startproc
        movl    flag(%rip), %eax
        cmpl    $1, %eax
        je      .L1
        movl    $1, %eax
        .p2align 4,,10
        .p2align 3
.L3:
        movl    flag(%rip), %edx //再次从内存里面把flag读到寄存器edx
        notl    %eax
        cmpl    %eax, %edx
        jne     .L3

#include <unistd.h>
#include <pthread.h>
#include <stdio.h>
volatile int flag=10; //这里是否由volatile, 结果不同
void* wait(void* param) {
  int count = 1;
  while ( flag != count ) {
    count = ~count;
  }
  printf("wait\n");
}
void* wake(void* param) {
  flag = 1;
  printf("wake\n");
}
int main () {
  pthread_t t[2];
  pthread_create(&t[0], NULL, wait, NULL);
  sleep(1);
  pthread_create(&t[1], NULL, wake, NULL);
  while(1);
}

int A,B;
void foo() {
  A = B+1;
  B = 5;
}

movl    B(%rip), %eax
addl    $1, %eax          //先做加法
movl    %eax, A(%rip)
movl    $5, B(%rip)      //再赋值为5

movl    B(%rip), %eax
movl    $5, B(%rip)  //先赋值为5
addl    $1, %eax      //再执行加法, 顺序调换了
movl    %eax, A(%rip)

volatile int A,B;
void foo() {
  A = B+1;
  B = 5;
}

$gcc cordering.c -S -O2 && cat cordering.s
movl    B(%rip), %eax
addl    $1, %eax           //先加1
movl    %eax, A(%rip)
movl    $5, B(%rip)       //赋值为5