1.1 类型表示

下面是两种类型的数据表示：

	float	long double
字节数	4	8
符号位	1	1
指数位数	8	11
尾数位数	23	52
最小值	1.1754943508222875E-38F	2.2250738585072014E-308L
最大值	3.4028234663852886E+38F	1.797693134862315708E+308L
分辨率	1.1920928955078125E-07F	2.2204460492503131E-16L

1.2 正常情况下的运算效率

下表给出两种类型下做运算的效率比较，选择High performance

	float	long double
加减运算	132 cycle	166 cycle
乘法运算	92 cycle	179 cycle
除法运算	240 cycle	1506 cycle

狂汗，这个long double做除法也太狠了点！

下表给出两种类型下做运算的效率比较，选择Strict IEEE：

	Float	long double
加减运算	310 cycle	381 cycle
乘法运算	274 cycle	585 cycle
除法运算	497 cycle	1063 cycle

这里有一个值显得很奇怪，在选择了Strict IEEE选项之后，long double进行除法运算反而变快了！

在vdsp的文档里这样解释这两种不同的选项：

The -fast-fp (fast floating point) switch directs the compiler to link with the high-speed floating-point emulation library. This library relaxes some of the IEEE floating-point standard’s rules for checking inputs against not-a-number (NaN) and denormalized numbers to improve performance. This switch is enabled by default.

The -ieee-fp (slower floating point) switch directs the compiler to link with the fully-compliant floating-point emulation library. This library obeys all of the IEEE floating-point standard’s rules, and incurs a performance penalty when compared with the default floating-point emulation library.

照说使用Strict IEEE选项之后应该所有的计算都变慢，但long double的浮点除法似乎是个例外！