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標題: __builtin_expect詳解 [打印本頁]

作者: liuda 時間: 2015-1-23 19:25
標題: __builtin_expect詳解
在GTK+2.0源碼中有很多這樣的宏：G_LIKELY和G_UNLIKELY。比如下面這段代碼：
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);
   }在源碼中，宏G_LIKELY和G_UNLIKELY 是這么定義的：
#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轉換為0和1，即真假兩種。要理解宏G_LIKELY和G_UNLIKELY ，很明顯必須理解__builtin_expect。__builtin_expect是GCC(version>=2.9)引進的宏，其作用就是幫助編譯器判斷條件跳轉的預期值，避免跳轉造成時間亂費。拿上面的代碼來說：
if (G_LIKELY (acat == 1))    //表示大多數情況下if里面是真，程序大多數直接執行if里面的程序
而
if (G_UNLIKELY (thread_memory_magazine1_is_empty (tmem, ix)))//表示大多數情況if里面為假，程序大多數直接執行else里面的程序
可能大家看到還是一頭霧水，看下面一段就會明白其中的樂趣啦；
//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.otest_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>//主要看這里
  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),%edi00000050 <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>//主要看這里
  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),%edi000000a0 <_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    $0x0,(%esp)
  ad: e8 fc ff ff ff       call    ae <_GLOBAL__I_65535_0_test_likely+0xe>
  b2: c9                   leave
  b3: c3                   ret
[lammy@localhost test_builtin_expect]$兩個函數編譯生成的匯編語句所使用到的跳轉指令不一樣，仔細分析下會發現__builtin_expect實際上是為了滿足在大多數情況不執行跳轉指令，所以__builtin_expect僅僅是告訴編譯器優化，并沒有改變其對真值的判斷。
這種用法在linux內核中也經常用到，國外也有一篇相關的文章，大家不妨看看：http://kernelnewbies.org/FAQ/LikelyUnlikely
不知大家注意到沒有，我在生產匯編時用的是gcc -fprofile-arcs -O2 -c test_builtin_expect.c，而不是gcc -O2 -c test_builtin_expect.c，具體可以參考http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html。

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