Dumping long double
hmurray at megapathdsl.net
Sun Nov 5 00:20:32 UTC 2017
> 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
long double isn't cleanly supported by c. I'm amazed that wasn't enough for
you to reject using it.
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
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