Re: [boost] [review][constrained_value] Review of Constrained Value Library begins today

Subject: Re: [boost] [review][constrained_value] Review of Constrained Value Library begins today
From: Gordon Woodhull (gordon_at_[hidden])
Date: 2008-12-11 20:14:58

On Dec 11, 2008, at 11:45 AM, Thorsten Ottosen wrote:
> We need to make either x or y one of the bounds of our interval. Let  
> us assume it is y.
>
> Furthermore, we know y has been specified such that it is  
> representable exactly in float or double. Since the 80-bit floating  
> point values are a  superset, we know y can also be represented  
> exactly in this format, and we know that truncation on y will have  
> no effect.
>
> Basically, y will have the same value no matter if it is stored in a  
> register or at some memory location.
>
> The question is know, can
>
>  x < y
>
> pass, and then x is assigned to the a bounded_float
> (where it will be stored in x'), only to
> discover that
>
> x' < y
>
> ? We know y will always have the same value, so only x may
> switch location.
>
> Case 1: x is in memory, x' is in register. Then x == x' by the  
> superset property.
>
> Case 2: x is in register, x' is register. Then x == x'.
>
> Case 3: x is in register, x' is in memory. Then we might have x' ==  
> y, but not x' > y, since that would violate the fact that y is  
> exactly representable in both formats.

Well said.  It seems that a truncated bound is key.  Surely it will be  
if it's stored as a runtime field, I am not so sure when it's one of  
those nice optimized-away functors.

So the next questions are
1. how to implement
0. how to test

I will inquire of a local float enthusiast, one of my profs at NYU.

Gordon