__builtin_expect详解

系统管理员 2022-03-19 08:40 228阅读 0赞

##     ##

转载自:[http://hi.baidu.com/uu\_dou/item/e9f6f41d570d817b7a5f25c7][http_hi.baidu.com_uu_dou_item_e9f6f41d570d817b7a5f25c7]

## 在GTK+2.0源码中有很多这样的宏：G\_LIKELY和G\_UNLIKELY。比如下面这段代码： ##

if (G\_LIKELY (acat == 1))     /\* allocate through magazine layer \*/  
    \{  
      ThreadMemory \*tmem = thread\_memory\_from\_self();  
      guint ix = SLAB\_INDEX (allocator, chunk\_size);  
      if (G\_UNLIKELY (thread\_memory\_magazine1\_is\_empty (tmem, ix)))  
        \{  
          thread\_memory\_swap\_magazines (tmem, ix);  
          if (G\_UNLIKELY (thread\_memory\_magazine1\_is\_empty (tmem, ix)))  
            thread\_memory\_magazine1\_reload (tmem, ix);  
        \}  
      mem = thread\_memory\_magazine1\_alloc (tmem, ix);  
    \}

在源码中，宏G\_LIKELY和G\_UNLIKELY 是这么定义的：

\#define G\_LIKELY(expr) (\_\_builtin\_expect (\_G\_BOOLEAN\_EXPR(expr), 1))  
\#define G\_UNLIKELY(expr) (\_\_builtin\_expect (\_G\_BOOLEAN\_EXPR(expr), 0))

宏\_G\_BOOLEAN\_EXPR的作用是把expr转换为0和1，即真假两种。要理解宏G\_LIKELY和G\_UNLIKELY ，很明显必须理解\_\_builtin\_expect。\_\_builtin\_expect是GCC(version>=2.9)引进的宏，其作用就是帮助编译器判断条件跳转的预期值，避免跳转造成时间乱费。拿上面的代码来说：

if (G\_LIKELY (acat == 1))     //表示大多数情况下if里面是真，程序大多数直接执行if里面的程序

而

if (G\_UNLIKELY (thread\_memory\_magazine1\_is\_empty (tmem, ix)))//表示大多数情况if里面为假，程序大多数直接执行else里面的程序

可能大家看到还是一头雾水，看下面一段就会明白其中的乐趣啦；

//test\_builtin\_expect.c   
\#define LIKELY(x) \_\_builtin\_expect(!!(x), 1)  
\#define UNLIKELY(x) \_\_builtin\_expect(!!(x), 0)  
int test\_likely(int x)  
\{  
if(LIKELY(x))  
\{  
   x = 5;  
\}  
else  
\{  
   x = 6;  
\}  
    
return x;  
\}  
int test\_unlikely(int x)  
\{  
if(UNLIKELY(x))  
\{  
   x = 5;  
\}  
else  
\{  
   x = 6;  
\}  
    
return x;  
\}\[lammy@localhost test\_builtin\_expect\]$ gcc -fprofile-arcs -O2 -c test\_builtin\_expect.c   
\[lammy@localhost test\_builtin\_expect\]$ objdump -d test\_builtin\_expect.otest\_builtin\_expect.o:     file format elf32-i386  
Disassembly of section .text:00000000 <test\_likely>:  
   0: 55                   push   %ebp  
   1: 89 e5                mov    %esp,%ebp  
   3: 8b 45 08             mov    0x8(%ebp),%eax  
   6: 83 05 38 00 00 00 01 addl   $0x1,0x38  
   d: 83 15 3c 00 00 00 00 adcl   $0x0,0x3c  
14: 85 c0                test   %eax,%eax  
16: 74 15                je     2d <test\_likely+0x2d>// **主要看这里**  
18: 83 05 40 00 00 00 01 addl   $0x1,0x40  
1f: b8 05 00 00 00       mov    $0x5,%eax  
24: 83 15 44 00 00 00 00 adcl   $0x0,0x44  
2b: 5d                   pop    %ebp  
2c: c3                   ret      
2d: 83 05 48 00 00 00 01 addl   $0x1,0x48  
34: b8 06 00 00 00       mov    $0x6,%eax  
39: 83 15 4c 00 00 00 00 adcl   $0x0,0x4c  
40: 5d                   pop    %ebp  
41: c3                   ret      
42: 8d b4 26 00 00 00 00 lea    0x0(%esi,%eiz,1),%esi  
49: 8d bc 27 00 00 00 00 lea    0x0(%edi,%eiz,1),%edi00000050 <test\_unlikely>:  
50: 55                   push   %ebp  
51: 89 e5                mov    %esp,%ebp  
53: 8b 55 08             mov    0x8(%ebp),%edx  
56: 83 05 20 00 00 00 01 addl   $0x1,0x20  
5d: 83 15 24 00 00 00 00 adcl   $0x0,0x24  
64: 85 d2                test   %edx,%edx  
66: 75 15                jne    7d <test\_unlikely+0x2d>// **主要看这里**  
68: 83 05 30 00 00 00 01 addl   $0x1,0x30  
6f: b8 06 00 00 00       mov    $0x6,%eax  
74: 83 15 34 00 00 00 00 adcl   $0x0,0x34  
7b: 5d                   pop    %ebp  
7c: c3                   ret      
7d: 83 05 28 00 00 00 01 addl   $0x1,0x28  
84: b8 05 00 00 00       mov    $0x5,%eax  
89: 83 15 2c 00 00 00 00 adcl   $0x0,0x2c  
90: 5d                   pop    %ebp  
91: c3                   ret      
92: 8d b4 26 00 00 00 00 lea    0x0(%esi,%eiz,1),%esi  
99: 8d bc 27 00 00 00 00 lea    0x0(%edi,%eiz,1),%edi000000a0 <\_GLOBAL\_\_I\_65535\_0\_test\_likely>:  
a0: 55                   push   %ebp  
a1: 89 e5                mov    %esp,%ebp  
a3: 83 ec 08             sub    $0x8,%esp  
a6: c7 04 24 00 00 00 00 movl   $0x0,(%esp)  
ad: e8 fc ff ff ff       call   ae <\_GLOBAL\_\_I\_65535\_0\_test\_likely+0xe>  
b2: c9                   leave   
b3: c3                   ret      
\[lammy@localhost test\_builtin\_expect\]$

两个函数编译生成的汇编语句所使用到的跳转指令不一样，仔细分析下会发现\_\_builtin\_expect实际上是为了满足在大多数情况不执行跳转指令，所以\_\_builtin\_expect仅仅是告诉编译器优化，并没有改变其对真值的判断。

这种用法在linux内核中也经常用到，国外也有一篇相关的文章，大家不妨看看：[http://kernelnewbies.org/FAQ/LikelyUnlikely][http_kernelnewbies.org_FAQ_LikelyUnlikely]

不知大家注意到没有，我在生产汇编时用的是gcc -fprofile-arcs -O2 -c test\_builtin\_expect.c，而不是gcc -O2 -c test\_builtin\_expect.c，具体可以参考[http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html][http_gcc.gnu.org_onlinedocs_gcc_Other-Builtins.html]。

[http_hi.baidu.com_uu_dou_item_e9f6f41d570d817b7a5f25c7]: http://hi.baidu.com/uu_dou/item/e9f6f41d570d817b7a5f25c7
[http_kernelnewbies.org_FAQ_LikelyUnlikely]: http://kernelnewbies.org/FAQ/LikelyUnlikely
[http_gcc.gnu.org_onlinedocs_gcc_Other-Builtins.html]: http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html