#include 
     
      #include 
      
       int main(){    volatile int m;    struct timeval start;    gettimeofday(&start, NULL);    for (int i = 0; i < 1000000; i++) {        m++;    }    struct timeval end;    gettimeofday(&end, NULL);    printf("add cost %lldus\n", (end.tv_sec - start.tv_sec) * 1000000 + (end.tv_usec - start.tv_usec));    int n;    gettimeofday(&start, NULL);    for (int i = 0; i < 1000000; i++) {        __sync_fetch_and_add(&n, 1);    }    gettimeofday(&end, NULL);    printf("atomic cost %lldus\n", (end.tv_sec - start.tv_sec) * 1000000 + (end.tv_usec - start.tv_usec));    return 0;}
      
     

之所以用volatile修饰m是拒绝编译器对m++做优化。

使用O2编译并查看性能：

$gcc -O2 -std=c99 -o perf atomic_perf.c$./perfadd cost 2638usatomic cost 8510us

　　可见如果你的变量压根不会被多线程访问，并且对性能极度苛刻的话，还是不要用原子变量了吧。因为在有些平台上“A full memory barrier is created when this function is invoked”。

 

可以通过下面的方法看到m++和原子操作的汇编之间的区别：

$gcc -O2 -std=c99 -g -c atomic_perf.c$objdump -Sl atomic_perf.oatomic_perf.o:     file format elf64-x86-64Disassembly of section .text:0000000000000000 
     
      :main():/home/admin/jinxin/test/atomic_perf.c:5#include 
      
       #include 
       
        int main(){   0: 55                    push   %rbp/home/admin/jinxin/test/atomic_perf.c:9    volatile int m;    struct timeval start;    gettimeofday(&start, NULL);   1: 31 f6                 xor    %esi,%esi/home/admin/jinxin/test/atomic_perf.c:5   3: 53                    push   %rbx   4: 48 83 ec 38           sub    $0x38,%rsp/home/admin/jinxin/test/atomic_perf.c:9   8: 48 8d 6c 24 10        lea    0x10(%rsp),%rbp   d: 48 89 ef              mov    %rbp,%rdi  10: e8 00 00 00 00        callq  15 
        
           15: 31 d2                 xor    %edx,%edx/home/admin/jinxin/test/atomic_perf.c:11    for (int i = 0; i < 1000000; i++) {        m++;  17: 8b 44 24 2c           mov    0x2c(%rsp),%eax/home/admin/jinxin/test/atomic_perf.c:10  1b: 83 c2 01              add    $0x1,%edx/home/admin/jinxin/test/atomic_perf.c:11  1e: 83 c0 01              add    $0x1,%eax/home/admin/jinxin/test/atomic_perf.c:10  21: 81 fa 40 42 0f 00     cmp    $0xf4240,%edx/home/admin/jinxin/test/atomic_perf.c:11  27: 89 44 24 2c           mov    %eax,0x2c(%rsp)/home/admin/jinxin/test/atomic_perf.c:10  2b: 75 ea                 jne    17 
         
          /home/admin/jinxin/test/atomic_perf.c:14    }    struct timeval end;    gettimeofday(&end, NULL);  2d: 31 f6                 xor    %esi,%esi  2f: 48 89 e7              mov    %rsp,%rdi  32: e8 00 00 00 00        callq  37 
          
           /home/admin/jinxin/test/atomic_perf.c:16 printf("add cost %lldus\n", (end.tv_sec - start.tv_sec) * 1000000 + (end.tv_usec - start.tv_usec)); 37: 48 8b 04 24 mov (%rsp),%rax 3b: 48 2b 44 24 10 sub 0x10(%rsp),%rax 40: bf 00 00 00 00 mov $0x0,%edi 45: 48 8b 74 24 08 mov 0x8(%rsp),%rsi 4a: 48 2b 74 24 18 sub 0x18(%rsp),%rsi 4f: 48 69 c0 40 42 0f 00 imul $0xf4240,%rax,%rax 56: 48 01 c6 add %rax,%rsi 59: 31 c0 xor %eax,%eax 5b: e8 00 00 00 00 callq 60 
           
            /home/admin/jinxin/test/atomic_perf.c:19 int n; gettimeofday(&start, NULL); 60: 31 f6 xor %esi,%esi 62: 48 89 ef mov %rbp,%rdi 65: e8 00 00 00 00 callq 6a 
            
              6a: 48 8d 54 24 28 lea 0x28(%rsp),%rdx 6f: 31 c0 xor %eax,%eax/home/admin/jinxin/test/atomic_perf.c:21 for (int i = 0; i < 1000000; i++) { __sync_fetch_and_add(&n, 1); 71: f0 83 02 01 lock addl $0x1,(%rdx)/home/admin/jinxin/test/atomic_perf.c:20 75: 83 c0 01 add $0x1,%eax 78: 3d 40 42 0f 00 cmp $0xf4240,%eax 7d: 75 f2 jne 71 
             
              /home/admin/jinxin/test/atomic_perf.c:23 } gettimeofday(&end, NULL); 7f: 48 89 e7 mov %rsp,%rdi 82: 31 f6 xor %esi,%esi 84: e8 00 00 00 00 callq 89 
              
               /home/admin/jinxin/test/atomic_perf.c:24 printf("atomic cost %lldus\n", (end.tv_sec - start.tv_sec) * 1000000 + (end.tv_usec - start.tv_usec)); 89: 48 8b 04 24 mov (%rsp),%rax 8d: 48 2b 44 24 10 sub 0x10(%rsp),%rax 92: bf 00 00 00 00 mov $0x0,%edi 97: 48 8b 74 24 08 mov 0x8(%rsp),%rsi 9c: 48 2b 74 24 18 sub 0x18(%rsp),%rsi a1: 48 69 c0 40 42 0f 00 imul $0xf4240,%rax,%rax a8: 48 01 c6 add %rax,%rsi ab: 31 c0 xor %eax,%eax ad: e8 00 00 00 00 callq b2 
               
                /home/admin/jinxin/test/atomic_perf.c:27 return 0;} b2: 48 83 c4 38 add $0x38,%rsp b6: 31 c0 xor %eax,%eax b8: 5b pop %rbx b9: 5d pop %rbp ba: c3 retq​​​