Linux应用程序之likely and unlikely宏定义

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在很多代码里可以看到likely和unlikely宏定义，其定义为：

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

这两个宏定义使用了GCC内置函数，即便使用了-fno-buildin参数，这些函数仍然会被编译。

GCC includes built-in versions of many of the functions in the standard C library. The versions prefixed with __builtin_ will always be treated as having the same meaning as the C library function even if you specify the -fno-builtin option.

抽取这两个宏定义，并结合使用定义如下出错提示定义：

#define BUG_ALERT_FALSE(condition) do \{ \if (unlikely((condition) != 0)) \printf("BUG: failure at %s:%d/%s()!\n", \       __FILE__, __LINE__, __FUNCTION__); \} while(0)#define BUG_ALERT_TRUE(condition) do \{ \    if (likely((condition) != 0)) \        printf("BUG: failure at %s:%d/%s()!\n", \               __FILE__, __LINE__, __FUNCTION__); \} while(0)

有了提示函数BUG_ALERT_FALSE和BUG_ALERT_TRUE，这里简单验证下效果：

验证代码如下：

int main(void){int i = 7;BUG_ALERT_FALSE(i!=10);BUG_ALERT_TRUE(i==7);return 0;}

从这里我们可以看到这两个ALERT定义是如何使用的了吧。 ^_^

当然这样类似的内置函数，GCC提供了很多，其开头为__builtin_xxx，在项目中合理的使用这些内置函数，将大有裨益。

系列函数为：

编号gcc内置函数列表1int __builtin_types_compatible_p (type1, type2)2type __builtin_choose_expr (const_exp, exp1, exp2)3type __builtin_complex (real, imag)4int __builtin_constant_p (exp)5long __builtin_expect (long exp, long c)6void __builtin_trap (void)7void __builtin_unreachable (void)8void *__builtin_assume_aligned (const void *exp, size_t align, ...)9void __builtin___clear_cache (char *begin, char *end)10void __builtin_prefetch (const void *addr, ...)11double __builtin_huge_val (void)12float __builtin_huge_valf (void)13long double __builtin_huge_vall (void)14int __builtin_fpclassify (int, int, int, int, int, ...)15double __builtin_inf (void)16_Decimal32 __builtin_infd32 (void)17_Decimal64 __builtin_infd64 (void)18_Decimal128 __builtin_infd128 (void)19float __builtin_inff (void)20long double __builtin_infl (void)21int __builtin_isinf_sign (...)22double __builtin_nan (const char *str)23_Decimal32 __builtin_nand32 (const char *str)24_Decimal64 __builtin_nand64 (const char *str)25_Decimal128 __builtin_nand128 (const char *str)26float __builtin_nanf (const char *str)27long double __builtin_nanl (const char *str)28double __builtin_nans (const char *str)29float __builtin_nansf (const char *str)30long double __builtin_nansl (const char *str)31int __builtin_ffs (unsigned int x)32int __builtin_clz (unsigned int x)33int __builtin_ctz (unsigned int x)34int __builtin_clrsb (int x)35int __builtin_popcount (unsigned int x)36int __builtin_parity (unsigned int x)37int __builtin_ffsl (unsigned long)38int __builtin_clzl (unsigned long)39int __builtin_ctzl (unsigned long)40int __builtin_clrsbl (long)41int __builtin_popcountl (unsigned long)42int __builtin_parityl (unsigned long)43int __builtin_ffsll (unsigned long long)44int __builtin_clzll (unsigned long long)45int __builtin_ctzll (unsigned long long)46int __builtin_clrsbll (long long)47int __builtin_popcountll (unsigned long long)48int __builtin_parityll (unsigned long long)49double __builtin_powi (double, int)50float __builtin_powif (float, int)51long double __builtin_powil (long double, int)52int32_t __builtin_bswap32 (int32_t x)53int64_t __builtin_bswap64 (int64_t x)

参考资料：

【1】gcc内置函数