Re: [PATCH][RFC] Make iterating over hash-map elide copying/destructing

[Richard Biener <rguenther at suse dot de>]

On Tue, 10 Jul 2018, Trevor Saunders wrote:

> On Tue, Jul 10, 2018 at 10:43:20AM +0200, Richard Biener wrote:
> > 
> > The following makes the hash-map iterator dereference return a pair<Key, 
> > Value&> rather than a copy of Value.  This matches the hash-table iterator
> > behavior and avoids issues with
> > 
> >   hash_map<tree, auto_vec<..., 2> >
> 
> Eventually somebodies probably going to want
> hash_map<unique_ptr<x>>, auto_vec<tree>> too, so we might as well go ahead
> and make it pair<key &, value &>?
> 
> > where iterating over the hash-table will call the auto_vec destructor
> > when dereferencing the iterator.  I note that the copy ctor of
> > auto_vec should probably be deleted and the hash-table/map iterators
> > should possibly support an alternate "reference" type to the stored
> > Values so we can use vec<> for "references" and auto_vec<> for
> > stored members.
> 
> I think code somewhere uses the auto_vec copy ctor to return a auto_vec,
> this is pretty similar to the situation with unique_ptr in c++98 mode.
> 
> > But that's out of scope - the patch below seems to survive minimal
> > testing at least.
> > 
> > I suppose we still want to somehow hide the copy ctors of auto_vec?
> 
> I suspec the best we can do is delete it in c++11 mode and provide a
> auto_vec<T>(auto_vec<T> &&) move ctor instead.  Though I think for the
> case where auto_vec has inline storage we should be able to just delete
> the copy ctor?
> 
> > How does hash-map growth work here?  (I suppose it doesn't...?)
> 
> Yeah was going to ask, I think hash_table memcpy's the elements? in
> which case memcpying a pointer into yourself isn't going to work.

It doesn't work.  It uses assignment but auto_vec doesn't implement
that so auto-storage breaks.  So you say it should use
std::move<> where that's obviously not available for us :/

> However I think if you use the auto_vec specialization for 0 internal
> elements that should be able to work if we null out the old auto_vec or
> avoid running dtors on the old elements.

Well, then I don't really need auto_vec, I'm more interested in the
embedded storage than the destructor ;)

> > Any further comments?
> 
> other than using a reference for the key type seems good.

OK, I suppose it should be 'const Key&' then (hopefully that
works for Key == const X / X * as intended).

I guess given the expansion problem I'm going to re-think using
auto_vec for now :/

Can we please move to C++11? ;)

Richard.