宏likely和unlikely

在源码中，宏likely和unlikely 是这么定义的（位于include/linux/compiler.h）：

    #define likely(x) __builtin_expect(!!(x), 1)
    #define unlikely(x) __builtin_expect(!!(x), 0)

要理解宏likely和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，避免跳转造成时间浪费。拿下面的代码来说：

    if (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 (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

    两段代码编译生成的汇编语句所使用到的跳转指令不一样，__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。

示例：

//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     $0x0,(%esp)
  ad: e8 fc ff ff ff          call     ae <_GLOBAL__I_65535_0_test_likely+0xe>
  b2: c9                      leave  
  b3: c3                      ret      
[lammy@localhost test_builtin_expect]$