Dumping long double

[Hal Murray]

> Dropping long double opened a can of worms about overflow/precision problems
> and how to redo the pivot logic that I didn't want to broach while were
> trying to avoid destabilizing changes. 

We ran for years without long doubles.  There were no destabilizing problems.

> Now would be a reasonable time to repen discussion of that cluster of
> issues. 

long double isn't cleanly supported by c.  I'm amazed that wasn't enough for 
you to reject using it.

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The potential problem is loss of precision.  The problem is converting an 
offset from l_fp to double.  A l_fp is sign plus 63 bits.  A double is only 
53 bits.  But most of the time, the high bits in a l_fp are 0 so they don't 
use any bits after conversion to float/double.

The potential interesting case is the first long jump at startup.  If all 63 
bits are interesting, we drop the bottom 10 bits when converting to a double. 
 That leaves over 20 bits of fraction which is good to better than a 
microsecond.


-- 
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