[PATCH] vectorizer: add _bb_vec_info::const_iterator

[Jonathan Wakely]

On 16/06/20 15:14 +0100, Richard Sandiford wrote:
>Martin Li?ka <mliska@suse.cz> writes:
>> On 6/16/20 10:50 AM, Richard Sandiford wrote:
>> Hmm, sounds like a nice abstraction but I see 2 problems with that:
>>
>> 1) How can I define a constructor of the iterator_pair when I need something like:
>> iterator_pair<const_iterator> (region_begin, region_end)?
>
>Not sure if I'm answering the right question, sorry, but I meant
>something along the lines of:
>
>  template<typename T>
>  struct iterator_pair
>  {
>  public:
>    iterator_pair (const T &begin, const T &end)
>      : m_begin (begin), m_end (end) {}
>
>    T begin () const { return m_begin; }
>    T end () const { return m_end; }
>
>  private:
>    T m_begin;
>    T m_end;
>  };

You could also add an "object generator" for that type:

template<typename T>
iterator_range<T>
make_iterator_range(T begin, T end)
{
   return iterator_range<T>(begin, end);
}

This deduces the type T from the function arguments, so will allow you
to write:

   return make_iterator_range(x, y);

instead of:

   return iterator_range<const_iterator>(x, y);

i.e. you don't need to say <const_iterator> because the compiler can
deduce it from the arguments.

Does your reverse iterator work correctly? It looks to me like it will
fail to visit the region_begin statement, because this loop will
terminate when the reverse iterator reaches the end() value, and will
not actually process that gsi:

-      gimple_stmt_iterator si = bb_vinfo->region_end;
-      gimple *stmt;
-      do
+      for (gimple *stmt : bb_vinfo->reverse_region_stmts ())

Adding const_iterator::operator--() and using
std::reverse_iterator<const_iterator> would fix that problem. The
std::reverse_iterator utility already solves the problem of getting
the begin and end of a reversed range correct.

You might need to add a couple of things to your const_iterator to
make it more like a conventional STL iterator in order to use
std::reverse_iterator, but IMHO it would be worth it. I can look at
that in the morning.