[PATCH] Add iterator for PHINode value/BB pair

[Pete Cooper]

> On Jul 22, 2015, at 3:07 PM, 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 <mailto:dexonsmith at apple.com>> wrote:
> 
> > On 2015-Jul-21, at 21:20, Pete Cooper <peter_cooper at apple.com <mailto: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*)
Ah, good point.  I’m working on a version now with a mutable value we can return a reference to.
>  
> 
> > +      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 <mailto: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?
> 
> I wouldn't mind a slightly half-hearted version that works for basic/common cases…  
I’m working on it now.  Got most of it done, but am still working through the details of just how operator* should be implemented across a std::tuple<iterator1, iterator2, …>.

Pete

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