[M3devel] range analysis?

[Hendrik Boom]

On Sun, May 22, 2011 at 11:18:02PM +0000, Jay K wrote:
> 
> Well, I can build the whole tree and see how many times it helps.
> I think this is a pretty standard optimization technique.
> Though it'd work better with compiler-derived additional information.
> 
> I initially happened upon this idea developing test cases.
> 
>  > The code assumes the minimum of a type is less than or equal to its maximum.
> 
> I'll change it not have that problem -- to just fall back to usual pessimistic code for such types, I guess.

What *is* the semantics of a range whose minimum is greater than its 
maximum?  There plainly can't be any values in this range.  How is a 
variable of this tyoe initialized?  Not to some arbitrary value of the 
type, because there aren't any.  I can see this type being useful to 
admit convenient generalizations -- for example, an array with n 
elements can still exist if n happens to be zero, but it seems to me 
that any code involving a value of the range for subscripts for this 
array must be simple unexecutable.

Or is there some latitude available in the principle that a value of a 
variable must always be of the correct type?

-- hendrik