Fwd: [PATCH] Add iterator for PHINode value/BB pair

David Blaikie dblaikie at gmail.com
Wed Jul 22 17:10:30 PDT 2015


(oops, dropped the list by accident)
---------- Forwarded message ----------
From: David Blaikie <dblaikie at gmail.com>
Date: Wed, Jul 22, 2015 at 5:10 PM
Subject: Re: [PATCH] Add iterator for PHINode value/BB pair
To: "Duncan P. N. Exon Smith" <dexonsmith at apple.com>




On Wed, Jul 22, 2015 at 5:05 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2015-Jul-22, at 16:57, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Wed, Jul 22, 2015 at 4:50 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >
> > > On 2015-Jul-22, at 15:07, David Blaikie <dblaikie at gmail.com> wrote:
> > >
> > >
> > >
> > > On Wed, Jul 22, 2015 at 12:11 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> > >
> > > > On 2015-Jul-21, at 21:20, Pete Cooper <peter_cooper at apple.com>
> wrote:
> > > >
> > > > Thanks for all the feedback.  This is a patch which addresses all of
> it.
> > > >
> > > > <phinode.diff>
> > > >
> > >
> > > > +
> > > > +    const PHINodeEdge operator*() const {
> > >
> > > No reason for the const in `const PHINodeEdge` here.
> > >
> > > to support operator-> you have to return a pointer, which means you
> need the PHINodeEdge storage inside the iterator to point to (& then you
> can just return a const ref from op*)
> >
> > Do we need operator->()?
> >
> > Seems poor form not to provide it (someone'll trip over it pretty
> quickly, I'd imagine)
> >
> > If so, we can return a proxy object:
> >
> >     struct PHINodeEdgeArrowProxy {
> >       const PHINodeEdge RefProxy;
> >     public:
> >       PHINodeEdgeArrowProxy(PHINodeEdge Edge) : RefProxy(Edge) {}
> >       const PHINodeEdge *operator->() const { return &RefProxy; }
> >     };
> >
> >     PHINodeEdgeArrowProxy operator->() const { return operator*(); }
> >
> > Then we avoid bloating the iterator, and only make the copy when we
> > actually need it.
> >
> > Non-conforming in terms of the iterator traits, I would imagine
>
> AFAICT, iterators only require that `i->m` has the same semantics as
> `(*i).m`.  The return type isn't specified.
>

Fair enough - can't quite find the wording on what constitutes the
iterators value type (as is mentioned in the iterator traits) or how it
might relate, but the basic definition is as you've mentioned.


> > - but I take it that's the N1550 stuff you're talking about below? It
> makes these sort of proxy solutions valid?
> >
>
> IIRC, proxy solutions are always valid for InputIterator and for
> OutputIterator; it's ForwardIterator that prevents `operator*()`
> from returning a proxy.
>

*nod* I don't mind violating that too much, if that's the preference.


> (This makes `std::vector<bool>::iterator` invalid.)
>

Yep. Fails in so many ways.


>
> >
> > >
> > >
> > > > +      return { *Values, *Blocks };
> > > > +    }
> > > > +  };
> > > > +
> > >
> > > Otherwise LGTM.  Might want to pass it through clang-format; I noticed
> > > some minor whitespace oddities.
> > >
> > > > *snip*
> > > >> On Jul 21, 2015, at 8:21 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> > > >>
> > > >>
> > > >>
> > > >> If you drop the requirements from `forward_iterator` to
> > > >> `input_iterator`, then you're allowed to return a `PHINodeEdge`
> by-value
> > > >> here instead of by-reference (unfortunately this makes it illegal
> to use
> > > >> a bunch of STL algorithms; STL iterator traits are completely broken
> > > >> IMO).
> > > > I’m fine with this.  David, Chandler, please let me know how you
> feel about this.
> > > >
> > > > Also, i forgot to say that I considered doing a zip iterator and
> inheriting this from it.  This is something I think Chandler or David
> mentioned a few months ago.  If there’s been any progress in the C++
> committee on that then i’m happy to try implement something better.  If
> not, then i don’t think what I have here should be difficult to change in
> future.
> > >
> > > I guess a generalized version would return a
> > > `std::tuple<Value *const &, BasicBlock *const &>` or some such.  Not
> > > sure how to actually make zip iterators work well without something
> > > like N1550 though.
> > >
> > > What's N1550 offer to make zip work?
> >
> > s/work/& well/
> >
> > I think bloated iterators are bad, but without bloating them (so you
> > can return a T&), you can't call a zip_iterator a ForwardIterator,
> > which means you can't use it in a bunch of algorithms that you might
> > want to (such as the destination for `std::copy()`).
> >
> > N1550 let's you correctly identify the type of traversal the
> > zip_iterator can do, without requiring a T& return from operator*().
> >
> > > I wouldn't mind a slightly half-hearted version that works for
> basic/common cases...
>
>
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