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 是這麼定義的：
#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轉換爲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裏面是真，程序大多數直接執行if裏面的程序
而
if (G_UNLIKELY (thread_memory_magazine1_is_empty (tmem, ix)))//表示大多數情況if裏面爲假，程序大多數直接執行else裏面的程序
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.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>//主要看這裏
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>//主要看這裏
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 $0x0,(%esp)
ad: e8 fc ff ff ff call ae <_GLOBAL__I_65535_0_test_likely+0xe>
b2: c9 leave
b3: c3 ret
[lammy@localhost test_builtin_expect]$
兩個函數編譯生成的彙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實際上是爲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，並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，我在生產彙編時用的是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。
轉自：http://babybandf.blog.163.com/blog/static/619935320105942336930/
