Numeric limits tracking

[John Colvin]

On Monday, 13 May 2013 at 00:00:54 UTC, Manu wrote:
> So, here's an issue that constantly drives me nuts, and an 
> elegant solution
> seems so do-able.
>
> void func(int x)
> {
>   x &= 0xFFFF;
>   short s = x; // Error! (but we know x is 0 .. 65535)
>
>   if(x < 256)
>   {
>     byte b = x; // Error! (we also know x is 0 .. 255)
>   }
> }
>
> It would be really nice if the compiler would carry around the 
> known
> possible range of values, and refer to that information when 
> performing
> down-casting assignments.
> It would also be very useful information for the back end, it 
> can choose
> more efficient types if it knows the range, or produce jump 
> tables rather
> than branch sequences.
>
> I think the compiler would need a min, max, and mask for any 
> integers, and
> update them as it works.
>
> There are many sources of this information.
> Comparisons effectively limit the range:
> if(x > 0)
> {
>   // x = 0 .. int.max
> }
> else
> {
>   // x = int.min .. -1
> }
>
> Masks, obviously:
> x &= 15;
>
> Also contracts are a really great source of seeding this 
> information on
> function entry.
>
> Has this been discussed? It seems simple, and it's invisible to 
> the
> language. It would just reduce some boilerplate (tedious 
> casts), and also
> offer some great optimisation opportunities.

I've been thinking about this for a while from an optimisation 
point of view. Is this not a common practice for an optimising 
compiler?