Re: SSA Iterators

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On Thu, 30 Jan 2020, 05:44 Nicholas Krause wrote:
>
> Greetings,
>
> I was looking into starting to cleaning up the SSA trees for various
> reasons and iterators
> seem to be the easiest to do. I searched the list to see if someone
> mentioned it before
> and I ran across this:
> https://gcc.gnu.org/ml/gcc-patches/2019-10/msg02031.html
>
> If your trying to get a second ++ working then why not override it
> prefix as your
> just doing it for postfix and those can be separate.

No no no.

No.

> Its not ideal to
> have two
> different versions for prefix and postfix in terms of overloading but it
> may be
> the simplest solution here.

No, making pre-increment and post-incrememt so different things (i.e.
iterate over different ranges) is a horrible idea. That's the kind of
thing that gives operator overloading a bad name. Overloaded operators
should do the thing you expect them to. They should not be used to
hide a non-obvious action behind an apparently simple syntax.


I would suggest avoiding "smart" iterators that contain all the state
and know their own end point. Instead create a type that holds all the
state and has begin/end member functions which return an iterator that
refers to the state. And have the iterator  dereference to some other
object that has the state for the second level of iteration, with its
own begin/end members returning the second iterator type. That would
end up looking like:

  imm_use_stmt_range r (SSAVAR);
  for (imm_use_stmt_iter it = r.begin (); it != r.end (); ++it)
    for (imm_use_on_stmt_iter it2 = it->begin (); it2 != it->end (); ++it2)
      ;

At some point when we move to C++11 or later that could become:

  imm_use_stmt_range r (SSAVAR);
  for (auto& stmt : r)
    for (auto& on_stmt : *stmt)
      ;