likely和unlikely宏

    首先，无论在何处，当我们遇到宏likely和宏unlikely时，都要明确一点：

　　if(likely(value)) 等价于 if(value)

　　if(unlikely(value)) 也等价于 if(value)

    也就是说 ，当value值为真时执行if分支，为假时执行else分支，从阅读和理解代码的角度来看，是一样的！！！

    以下为这两个宏的一般定义：

#define likely(x)       __builtin_expect(!!(x), 1)
#define unlikely(x)     __builtin_expect(!!(x), 0)
    而__builtin_expect() 是 GCC (version >= 2.96）提供给程序员使用的，目的是将“分支转移”的信息提供给编译器，这样编译器可以对代码进行优化，以减少指令跳转带来的性能下降。__builtin_expect()在GCC的官方文档中解释如下（可跳过）：

-- Built-in Function: long __builtin_expect (long EXP, long C)
     You may use `__builtin_expect' to provide the compiler with branch
     prediction information.  In general, you should prefer to use
     actual profile feedback for this (`-fprofile-arcs'), as
     programmers are notoriously bad at predicting how their programs
     actually perform.  However, there are applications in which this
     data is hard to collect.

     The return value is the value of EXP, which should be an integral
     expression.  The value of C must be a compile-time constant.  The
     semantics of the built-in are that it is expected that EXP == C.
     For example:

          if (__builtin_expect (x, 0))
            foo ();

     would indicate that we do not expect to call `foo', since we
     expect `x' to be zero.  Since you are limited to integral
     expressions for EXP, you should use constructions such as

          if (__builtin_expect (ptr != NULL, 1))
            error ();

     when testing pointer or floating-point values.

    也就是说，GCC的内建方法会判断 EXP == C 是否成立，成立则将if分支中的执行语句紧跟放在汇编跳转指令之后，否则将else分支中的执行语句紧跟汇编跳转指令之后。如下例子所示：

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//test.c
    #define likely(x) __builtin_expect(!!(x),1)
    #define unlikely(x) __builtin_expect(!!(x),0)

    int test_unlikely(int x)
    {
        if(unlikely(x == 2))
        {
            x++;
        }
        else
        {
            x--;
        }

        return x;
    }

    int test_likely(int x)
    {
        if(likely(x == 2))
        {
            x++;
        }
        else
        {
            x--;
        }

        return x;
    }

编译并导出目标文件的汇编表示：

gcc -fprofile-arcs -O2 -c test.c

objdump -d test.o

得到如下汇编：

test.o:     file format elf32-i386

Disassembly of section .text:

00000000 <test_likely>:
   0: 55                                 push   %ebp
   1: 83 05 00 00 00 00 01       addl   $0x1,0x0
   8: 89 e5                            mov    %esp,%ebp
   a: 83 15 04 00 00 00 00       adcl   $0x0,0x4
  11: 83 7d 08 02                   cmpl   $0x2,0x8(%ebp)                   //留意这里！！！判断 x == 2
  15: 75 15                            jne    2c                //跳转指令jne！！！x != 2 时才跳转！
  17: 83 05 08 00 00 00 01       addl   $0x1,0x8                          //if分支代码 x++
  1e: b8 03 00 00 00               mov    $0x3,%eax
  23: 83 15 0c 00 00 00 00       adcl   $0x0,0xc
  2a: 5d                                pop    %ebp
  2b: c3                                ret   
  2c: 8b 45 08                       mov    0x8(%ebp),%eax                   //跳转到这里，else分支代码 x--
  2f: 5d                                pop    %ebp
  30: 83 e8 01                       sub    $0x1,%eax
  33: 83 05 10 00 00 00 01      addl   $0x1,0x10
  3a: 83 15 14 00 00 00 00      adcl   $0x0,0x14
  41: c3                               ret   
  42: 8d b4 26 00 00 00 00      lea    0x0(%esi),%esi
  49: 8d bc 27 00 00 00 00      lea    0x0(%edi),%edi

00000050 <test_unlikely>:
  50: 55                               push   %ebp
  51: 89 e5                           mov    %esp,%ebp
  53: 8b 45 08                       mov    0x8(%ebp),%eax
  56: 83 05 18 00 00 00 01      addl   $0x1,0x18
  5d: 83 15 1c 00 00 00 00      adcl   $0x0,0x1c
  64: 83 f8 02                       cmp    $0x2,%eax                          //留意这里！！！判断 x == 2
  67: 74 13                           je     7c            //跳转指令je！！！x == 2 时就跳转！
  69: 83 e8 01                       sub    $0x1,%eax                          //else分支代码 x--
  6c: 83 05 28 00 00 00 01      addl   $0x1,0x28
  73: 83 15 2c 00 00 00 00      adcl   $0x0,0x2c
  7a: 5d                               pop    %ebp
  7b: c3                               ret   
  7c: 83 05 20 00 00 00 01      addl   $0x1,0x20                       //跳转到这里，if分支代码 x++
  83: b0 03                           mov    $0x3,%al
  85: 83 15 24 00 00 00 00      adcl   $0x0,0x24
  8c: 5d                               pop    %ebp
  8d: c3                               ret   
  8e: 66 90                           xchg   %ax,%ax

00000090 <_GLOBAL__I_0_test_unlikely>:
  90: 55                    push   %ebp
  91: 89 e5                 mov    %esp,%ebp
  93: 83 ec 08              sub    $0x8,%esp
  96: c7 04 24 00 00 00 00  movl   $0x0,(%esp)
  9d: e8 fc ff ff ff        call   9e <_GLOBAL__I_0_test_unlikely+0xe>
  a2: c9                    leave 
  a3: c3                    ret   

   注意：likely和unlikely所生成的跳转指令是不同的，分别是jne和je！！！

    如上述例子分析所示，宏likely和宏unlikely唯一的作用就是选择“将if分支还是else分支放在跳转指令之后，从而优化程序的执行效率”。因为likely(EXP)代表条件表达式EXP很可能成立，而unlikely(EXP)代表条件表达式EXP很可能不成立，当程序员清楚EXP表达式多数情况成立（不成立）时，就可使用likely（unlikely），使if分支（else分支）紧跟跳转指令其后，从而在大多数情况下不用执行跳转指令，避开跳转指令所带来的开销，从而达到优化的目的。