Re: [PATCH] Improve PR30911 and PR31023, Ada and VRP

[kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)]

> That said, it is not clear exactly what is allowed when there is a bounded
> error.  For example, the Ada reference manual seems to require that
> uninitialized values behave deterministically.  Suppose X is uninitialized
> (using it is then a bounded error).  In the following code
>   if X = 0 then
>     do_something_with_X;
>   end if;
> must X have the value 0 in do_something_with_X?  

> I think this was Bob Duff's point.

Yes.  The Ada RM "seems" to require this.  One could argue that it doesn't
definitely, but since Bob was one of the two authors of that document, a
lot of deference should be given to his position.

> In fact, even without VRP this might be a problem: do gcc's constant
>  propagation passes turn
>   ...X unitialized...
>   if X = 0 then use_X; end if;
> into
>   if true then use_X; end if; (without X being "true" in use_X)
> for example?

Not as far as I know.