Re: C++ edge_iterator (was: Re: [SH] PR 53976 - Add RTL pass to eliminate clrt, sett insns)

[Trevor Saunders <tsaunders at mozilla dot com>]

On Fri, Dec 13, 2013 at 10:00:57PM +0100, Oleg Endo wrote:
> On Thu, 2013-12-12 at 03:13 -0500, Trevor Saunders wrote:
> > On Wed, Dec 11, 2013 at 06:47:37PM +0100, Oleg Endo wrote:
> > > On Thu, 2013-11-21 at 00:04 +0100, Steven Bosscher wrote:
> > > > Declaring the edge_iterator inside the for() is not a good argument
> > > > against FOR_EACH_EDGE. Of course, brownie points are up for grabs for
> > > > the brave soul daring enough to make edge iterators be proper C++
> > > > iterators... ;-)
> > 
> > so, as a first question why do we have a special edge iterator at all? it
> > seems like we could just have a vec iterator and use that removing a
> > bunch of indirection that seems pretty useless.
> 
> I don't know why it's there.  Looks like a remainder from the pre-C++
> code, as the conversion is being done step by step.
> 
> > 
> > > So, I gave it a try -- see the attached patch.
> > > It allows edge iteration to look more like STL container iteration:
> > > 
> > > for (basic_block::edge_iterator ei = bb->pred_edges ().begin ();
> > >      ei != bb->pred_edges ().end (); ++ei)
> > > {
> > >   basic_block pred_bb = (*ei)->src;
> > >   ...
> > > }
> > 
> > personally I'm not really a fan of overloading ++ / * that way, but I
> > can't speak for anyone else.  I'd prefer something like
> > 
> > for (vec_iterator i = vec.forward_iterator (); !i.done (); i.next ())
> > and
> > for (backward_vec_iterator i = vec.backward_iterator (); !i.done (); i.next ())
> > 
> > but that might break range base for loops?
> 
> Right, that doesn't work with range-based for loops, since it doesn't
> follow the standard concept of iteration.  For a more detailed
> explanation, see also for example
> http://www.codesynthesis.com/~boris/blog/2012/05/16/cxx11-range-based-for-loop/
> 
> BTW, if you look at the patch, I haven't overloaded any ++ operators:
> 
> Index: gcc/vec.h
> ===================================================================
> --- gcc/vec.h	(revision 205866)
> +++ gcc/vec.h	(working copy)
> @@ -482,6 +482,15 @@
>    void quick_grow (unsigned len);
>    void quick_grow_cleared (unsigned len);
>  
> +  /* STL like iterator interface.  */
> +  typedef T* iterator;
> +  typedef const T* const_iterator;
> +
> +  iterator begin (void) { return &m_vecdata[0]; }
> +  iterator end (void) { return &m_vecdata[m_vecpfx.m_num]; }
> +  const_iterator begin (void) const { return &m_vecdata[0]; }
> +  const_iterator end (void) const { &m_vecdata[m_vecpfx.m_num]; }
> 
> This is because raw pointers can be used as random access iterators.

yeah, somehow didn't occur to me T* just works (not that it wouldn't be
easier to see if the type had * in it)

> > > Then the
> > > typedef struct basic_block_def* basic_block;
> > > 
> > > is replaced with a wrapper class 'basic_block', which is just a simple
> > > POD wrapper around a basic_block_def*.  There should be no penalties
> > > compared to passing/storing raw pointers.  Because of the union with
> > > constructor restriction of C++98 an additional wrapper class
> > > 'basic_block_in_union' is required, which doesn't have any constructors
> > > defined.
> > > 
> > > Having 'basic_block' as a class allows putting typedefs for the edge
> > > iterator types in there (initially I tried putting the typedefs into
> > > struct basic_block_def, but gengtype would bail out).
> > 
> > namespacing like that seems a little messy, but so is vec_iterator or
> > such I guess.
> 
> I'm not sure which part of the namespacing you're referring to exactly.

I'm saying I'd rather have edge_iterator than
basic_blokc::edge_iterator.

> The basic_block::edge_iterator thing?  Usually the iterator type is
> defined in the container type.  In this case it would be vec<edge,
> va_gc>.  The choice of the container type for storing edges is done in
> basic_block_def.  Thus, ideally the iterator type should be obtained
> from the basic_block_def class somehow.  A more bureaucratic way would
> be to have a typedef inside basic_block_def (which is not possible
> because of gengtype as mentioned before, so let's assume it's in
> basic_block)... 
> 
> class basic_block
> {
> public:
>   typedef vec<edge, va_gc> edge_container;
> 
>   edge_container& pred_edges (void);
>   edge_container& succ_edges (void);
> 
> ...
> };
> 
> and then access the iterator via 
> for (basic_block::edge_container::iterator i = bb->bb->pred_edges
> ().begin (); ...)
> 
> Having to type out iterator types is a well known annoyance of C++98.
> Of course it's shorter to write
> for (edge_iterator i = ...)

actually what I really want to do is make returning a const vector part
of the api for getting the edge list from a basic block and then just
write

for (edge *e = vec.begin (); e != vec.end (); e++)

which imo makes it clear what's happening and is short, but I guess
richi doesn't like the interface being that a vector is returned :/

> but that means, that there can be only one type of edge container ever.

gcc seems to have survived for a long time with a single edge_iterator
so I don't see much reason to worry about allowing
basic_block::edge_iterator and foobar::edge_iterator.

> > > It would also be possible to have a free standing definition / typedef
> > > of edge_iterator, but it would conflict with the existing one and
> > > require too many changes at once.  Moreover, the iterator type actually
> > 
> > I bet it'll be a lot of work but changing everything seems nice so maybe
> > its worth just sitting down for a couple days and banging it out if it
> > gives nicer names?
> 
> Nicer names than "edge_iterator" you mean?  I can't think of any at the
> moment... 

well, I meant edge_iterator instead of basic_block::edge_iterator which
afaik is what you proposed?

> > > depends on the container type, which is vec<edge, ...>, and the
> > > container type is defined/selected by the basic_block class.
> > 
> > I don't see how this is relevent
> 
> I hope that the explanation above makes it somewhat clearer.
> 
> > 
> > > The following
> > >   basic_block pred_bb = (*ei)->src;
> > > 
> > > can also be written as
> > >   basic_block pred_bb = ei->src;
> > > 
> > > after converting the edge typedef to a wrapper of edge_def*.
> > 
> > this is assuming you overload operator -> on the iterator? I'm a c++ guy
> > not a stl guy, but that seems pretty dubious to me.
> 
> Yes, that requires overloading of "operator ->".  However, in this case
> not in the iterator, but in the pointer wrapper as I've done it already
> in the patch for class basic_block (in the file basic_block2.h).  This
> is common practice for pointer wrappers (see e.g. std::shared_ptr).

I know doing this for pointer wrappers is common and I think that's
fine.  However I'm not really convinced we need to or should make
basic_block a pointer wrapper class, I'd wrather see basic_block_def
become basic_block, and use basic_block * all over.

> Overloading "operator ->" is also required in iterators.  See
> http://www.cplusplus.com/reference/iterator/
> If raw pointers are used as iterators (as in my example patch), there's
> nothing to overload for those of course.

yeah, I'm not really a fan of that style of iterator :/

> > > The idea of the approach is to allow co-existence of the new
> > > edge_iterator and the old and thus be able to gradually convert code.
> > > The wrappers around raw pointers also helo encapsulating the underlying
> > > memory management issues.  For example, it would be much easier to
> > > replace garbage collected objects with intrusive reference counting.
> > 
> > I don't think there's actually a memory management issue here,
> > edge_iterator can only work if you allocate it on the stack since its
> > not marked for gty, and afaik ggc doesn't scan the stack so the
> > edge_iterator can't keep the vector alive.  Now I think it would be nice
> > if these vectors moved out of gc memory, but I don't think this is
> > particularly helpful for that.
> 
> Sorry, I think I caused a misunderstanding here.  By "memory management
> issue" I just meant the way a container stores its objects, like whether
> it's storing pointers to garbage collected objects, smart pointers like
> shared_ptr<edge> or whatever.  I didn't mean that the iterator should
> somehow influence the lifetime of the container.

 sure, I agree C++ makes memory management easier.

Trev

> Cheers,
> Oleg
>