GCC __attribute__((mode(XX))

用gcc -E可以得到下面的预编译结果：
typedef unsigned int u_int8_t __attribute__ ((__mode__ (__QI__)));
typedef unsigned int u_int16_t __attribute__ ((__mode__ (__HI__)));
typedef unsigned int u_int32_t __attribute__ ((__mode__ (__SI__)));
typedef unsigned int u_int64_t __attribute__ ((__mode__ (__DI__)));


QI: An integer that is as wide as the smallest addressable unit, usually 8 bits.

HI: An integer, twice as wide as a QI mode integer, usually 16 bits.

SI: An integer, four times as wide as a QI mode integer, usually 32 bits.

DI: An integer, eight times as wide as a QI mode integer, usually 64 bits.

SF: A floating point value, as wide as a SI mode integer, usually 32 bits.

DF: A floating point value, as wide as a DI mode integer, usually 64 bits.

So DI is essentially sizeof(char) * 8.