Re: [PATCH][RFC] C++-style iterators for FOR_EACH_IMM_USE_STMT

[Richard Biener <rguenther at suse dot de>]

On Tue, 29 Oct 2019, Oleg Endo wrote:

> On Tue, 2019-10-29 at 11:26 +0100, Richard Biener wrote:
> > While I converted other iterators requiring special BREAK_FROM_XYZ
> > a few years ago FOR_EACH_IMM_USE_STMT is remaining.  I've pondered
> > a bit but cannot arrive at a "nice" solution here with just one
> > iterator as the macros happen to use.  For reference, the macro use
> > is
> > 
> >   imm_use_iterator iter;
> >   gimple *use_stmt;
> >   FOR_EACH_IMM_USE_STMT (use_stmt, iter, name)
> >     {
> >       use_operand_p use_p;
> >       FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
> >         ;
> >     }
> > 
> > which expands to (w/o macros)
> > 
> >    imm_use_iterator iter; 
> >    for (gimple *use_stmt = first_imm_use_stmt (&iter, name);
> >         !end_imm_use_stmt_p (&iter);
> >         use_stmt = nest_imm_use_stmt (&iter))
> >      for (use_operand_p use_p = first_imm_use_on_stmt (&iter);
> >           !end_imm_use_on_stmt_p (&iter);
> >           use_p = next_imm_use_on_stmt (&iter))
> >        ;
> > 
> > and my foolish C++ attempt results in
> > 
> >      for (imm_use_stmt_iter it = SSAVAR; !it.end_p (); ++it)
> >        for (imm_use_stmt_iter::use_on_stmt it2 = it; !it2.end_p ();
> > ++it2)
> >          ;
> > 
> > with *it providing the gimple * USE_STMT and *it2 the use_operand_p.
> > The complication here is to map the two-level iteration to "the C++
> > way".
> > Are there any STL examples mimicing this?  Of course with C++11 we
> > could
> > do
> > 
> >   for (imm_use_stmt_iter it = SSAVAR; !it.end_p (); ++it)
> >     for (auto it2 = it.first_use_on_stmt (); !it2.end_p (); ++it2)
> >       ;
> > 
> > but that's not much nicer either.
> 
> Is there a way to put it in such a way that the iterators follow
> standard concepts for iterators?  It would increase chances of it
> becoming nicer by utilizing range based for loops.

Hmm, not sure - I'd like to write

 for (gimple *use_stmt : imm_stmt_uses (SSAVAR))
   for (use_operand_p use_p : <need to refer to the iterator object from 
above>)
     ...

I don't see how that's possible.  It would need to be "awkward" like

 for (auto it : imm_stmt_uses (SSAVAR))
   {
     gimple *use_stmt = *it;
     for (use_operand_p use_p : it)
       ...
   }

so the first loops iteration object are the actual iterator and you'd
have to do extra indirection to get at the actual stmt you iterated to.

So I'd extend C++ (hah) to allow

  for (gimple *use_stmt : imm_stmt_uses (SSAVAR))
    for (use_operand_p use_p : auto)
      ...

where 'auto' magically selects the next iterator object in scope
[that matches].

;)

But yeah, range-based for looks like a nice possibility for most
of our IL iteration besides this special one.  Guess nobody
thought of "multi-level" iteration (and yes, we sometimes terminate
the inner loop but continue the outer, so flattening the iteration
isn't a solution here).

Richard.