__builtin_expect详解

来源：互联网发布：不锈钢管重量计算软件编辑：程序博客网时间：2024/06/08 18:36

转自：http://hi.baidu.com/lammy/blog/item/bc5e3d4e869073c3d1c86a89.html

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 <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),%edi

00000050 <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),%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。