Oleg Endo <oleg.e...@t-online.de> 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.
The fact that we iterate over a vector is an implementation detail that should
be hidden.
Richard.
>>
>> > 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.
>
>
>> > 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.
>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 = ...)
>
>but that means, that there can be only one type of edge container ever.
>
>
>> > 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...
>
>>
>> > 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).
>
>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.
>
>> > 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.
>
>Cheers,
>Oleg
