Examples of DFA

[Walter Bright]

In order to do @nogc DFA, the following is a start:

1. construct an array of all the functions

2. each function gets allocated a bit vector with one bit for each function in (1)

3. each of those (2) bit vectors gets a corresponding bit set for every function 
that is called

4. a single bit vector gets a bit set for every function that does a gc operation

5. loop through all the bit vectors, and every function that calls a function 
that has a bit set in (4) also gets a bit set in (4)

6. keep looping until no more changes in (4)

7. mark all the functions that don't have a bit set as @nogc

Pruning of the number of functions that need a bit can be done by not 
considering any functions already marked as @nogc, and functions that only call 
@nogc functions and don't do any other allocations can be eagerly marked @nogc.