__builtin_expect详解

在GTK&＃43;2.0源码中有很多这样的宏&＃xff1a;G_LIKELY和G_UNLIKELY。比如下面这段代码&＃xff1a;

if (G_LIKELY (acat &＃61;&＃61; 1))     /* allocate through magazine layer */
    {
      ThreadMemory *tmem &＃61; thread_memory_from_self();
      guint ix &＃61; 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 &＃61; thread_memory_magazine1_alloc (tmem, ix);
    }

在源码中&＃xff0c;宏G_LIKELY和G_UNLIKELY 是这么定义的&＃xff1a;

#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&＃xff0c;即真假两种。要理解宏G_LIKELY和G_UNLIKELY &＃xff0c;很明显必须理解__builtin_expect。__builtin_expect是GCC(version>&＃61;2.9)引进的宏&＃xff0c;其作用就是帮助编译器判断条件跳转的预期值&＃xff0c;避免跳转造成时间乱费。拿上面的代码来说&＃xff1a;

if (G_LIKELY (acat &＃61;&＃61; 1))     //表示大多数情况下if里面是真&＃xff0c;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&＃xff0c;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&＃xff0c;看下面一段就会明白其中的乐趣啦&＃xff1b;

//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 &＃61; 5;
}
else
{
   x &＃61; 6;
}
  
return x;
}

int test_unlikely(int x)
{
if(UNLIKELY(x))
{
   x &＃61; 5;
}
else
{
   x &＃61; 6;
}
  
return x;
}

[lammy&＃64;localhost test_builtin_expect]$ gcc -fprofile-arcs -O2 -c test_builtin_expect.c
[lammy&＃64;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   $0x0,(%esp)
ad: e8 fc ff ff ff       call   ae <_GLOBAL__I_65535_0_test_likely&＃43;0xe>
b2: c9                   leave
b3: c3                   ret   
[lammy&＃64;localhost test_builtin_expect]$

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&＃xff0c;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&＃xff0c;所以__builtin_expect仅仅是告诉编译器优化&＃xff0c;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&＃xff0c;国外也有一篇相关的文章&＃xff0c;大家不妨看看&＃xff1a;http://kernelnewbies.org/FAQ/LikelyUnlikely

不知大家注意到没有&＃xff0c;我在生产汇编时用的是gcc -fprofile-arcs -O2 -c test_builtin_expect.c&＃xff0c;而不是gcc -O2 -c test_builtin_expect.c&＃xff0c;具体可以参考http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html。