Null references redux

[Walter Bright]

Rainer Deyke wrote:
> OT, but declaring the variable at the top of the function increases
> stack size.
> 
> Example with changed variable names:
> 
>   void bar(bool foo) {
>     if (foo) {
>       int a = 1;
>     } else {
>       int b = 2;
>     }
>     int c = 3;
>   }
> 
> In this example, there are clearly three different (and differently
> named) variables, but their lifetimes do not overlap.  Only one variable
> can exist at a time, therefore the compiler only needs to allocate space
> for one variable.  Now, if you move your declaration to the top:
> 
>   void bar(bool foo) {
>     int a = void;
>     if (foo) {
>       a = 1;
>     } else {
>       a = 2; // Reuse variable.
>     }
>     int c = 3;
>   }
> 
> You now only have two variables, but both of them coexist at the end of
> the function.  Unless the compiler applies a clever optimization, the
> compiler is now forced to allocate space for two variables on the stack.

Not necessarily. The optimizer uses a technique called "live range 
analysis" to determine if two variables have non-overlapping ranges. It 
uses this for register assignment, but it could just as well be used for 
minimizing stack usage.