[tree-ssa] Lazy updating of stmt operands

[Zdenek Dvorak]

Hello,

>  >> Which needs to pass through every single statement in the program. Not
>  >> really terribly efficient.
>  >> 
>  >*shrug*, it's used by SSA-CCP.  Since def-use edges are only needed by
>  >some passes, I don't think it would be worth our while trying to
>  >maintain them in get_stmt_operands.
>  >
>  >But I'm ready to be convinced otherwise.  Say, with an implementation
>  >comparing both approaches with timings over a reasonable code base (a
>  >typical GCC bootstrap or one of the compile-time related PRs).
> IMHO, when we have the need, then we'll expand on the immediate uses
> interface -- either by having a way to incrementally update it, or 
> rebuild it.
> 
> While it's nice to think about a world where you always have immediate uses,
> you end up with FUD chains if you do that -- with ultimately a tangled nest
> of pointers that is bloody impossible to work with in the real world.

why? I have this written and it does not seem very complicated.  I
didn't get to measuring its speed yet, and of course there is unpleasant
memory overhead, but I don't see any other problems.

Zdenek