SSA backend

[Stephen Weeks]

> Do we really need a condition that says that any use of a label as a cont
> in a non-tail call must always be paired with the same handler?  

I don't think that we do.  In fact, I'm pretty sure that the
conditions that I have written down in my notes for the forthcoming
exceptions paper will happily accept your transformed program,
precisely because the use of Bug imposes no constraints.

> I was actually thinking about this on the way up this morning in
> relation to the contification pass.  If we contify a function at a
> continuation, under SSA we need to know the handler to rewrite
> Raise's -- which must mean that there is only one handler.

Yes.

> If this makes sense, then I would have remove-unused.new.fun rewrite the
> above to:
> 
>    L_3 ()
>      x_10: unit = Stdio_print (global_215)
>      L_1022a (exit_0 (global_0, env_0)) handle L_1023
>    L_166 ()
>      L_1022b (exit_0 (global_12, x_21)) handle L_91   
>    L_1023 (x_1614: exn)
>      L_2 ()
>    L_1022a () = L_1022 ()
>    L_1022b () = L_1022 ()
>    L_1022 ()
>      Bug
>    L_91 (x_43: exn)
>      SetHandler L_0
>      case x_43 of
>        Fail_0 => L_116 | SysErr_0 => L_101 | _ => L_100
> 
> Note, this means that the shrinker needs to be aware of that CPS IL
> condition, because it can't change occurences of L_1022a to L_1022.

Yeah, and that sounds like a bad idea to me.  If this is a condition
that we need just in the backend, then we shouldn't constrain the rest
of the optimizer, we should introduce extra wrapper labels when we
need them.