Loop optimization

[Don]

strtr wrote:
> == Quote from Don (nospam at nospam.com)'s article
>> strtr wrote:
>>> == Quote from bearophile (bearophileHUGS at lycos.com)'s article
>>>> But the bigger problem in your code is that you are performing operations on
>>> NaNs (that's the default initalization of FP values in D), and operations on NaNs
>>> are usually quite slower.
>>>
>>> I didn't know that. Is it the same for inf?
>> Yes, nan and inf are usually the same speed. However, it's very CPU
>> dependent, and even *within* a CPU! On Pentium 4, for example, for x87,
>> nan is 200 times slower than a normal value (!), but on Pentium 4 SSE
>> there's no speed difference at all between nan and normal. I think
>> there's no speed difference on AMD, but I'm not sure.
>> There's almost no documentation on it at all.
> 
> Thanks!
> NaNs being slower I can understand but inf might well be a value you want to use.

Yes. What's happened is that none of the popular programming languages 
support special IEEE values, so they're given very low priority by chip 
designers. In the Pentium 4 case, they're implemented entirely in 
microcode. A 200X slowdown is really significant.

However, the bit pattern for NaN is 0xFFFF..., which is the same as a 
negative integer, so an uninitialized floating-point variable has a 
quite high probability of being a NaN. I'm certain there's a lot of C 
programs out there which are inadvertantly using NaNs.