__builtin_expect详解

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.o

test_builtin_expect.o:     file format elf32-i386

Disassembly of section .text:

00000000 
  
    : 
   
   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 
          
            // 
           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),%edi 
 
 
 
 
 
 
 
 
 
 
 
 























 
           
 
 
 
 
 
 
 













 
  

00000050 
  
    : 
   
 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 
          
            // 
           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),%edi 
 
 
 
 
 
 
 
 
 
 
 





















 
           
 
 
 
 
 
 
 













 
  

000000a0 <_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

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。