Re: [RFC/PATCH] Use range-based for loops for traversing loops

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Tue, 20 Jul 2021 at 10:49, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> On Tue, 20 Jul 2021 at 09:58, Kewen.Lin <linkw@linux.ibm.com> wrote:
> >
> > on 2021/7/19 下午11:59, Martin Sebor wrote:
> > > On 7/19/21 12:20 AM, Kewen.Lin wrote:
> > >> Hi,
> > >>
> > >> This patch follows Martin's suggestion here[1], to support
> > >> range-based for loops for traversing loops, analogously to
> > >> the patch for vec[2].
> > >>
> > >> Bootstrapped and regtested on powerpc64le-linux-gnu P9,
> > >> x86_64-redhat-linux and aarch64-linux-gnu, also
> > >> bootstrapped on ppc64le P9 with bootstrap-O3 config.
> > >>
> > >> Any comments are appreciated.
> > >
> > > Thanks for this nice cleanup!  Just a few suggestions:
> > >
> > > I would recommend against introducing new macros unless they
> > > offer a significant advantage over alternatives (for the two
> > > macros the patch adds I don't think they do).
> > >
> > > If improving const-correctness is one of our a goals
> > > the loops_list iterator type would need to a corresponding
> > > const_iterator type, and const overloads of the begin()
> > > and end() member functions.
> > >
> > > Rather than introducing more instances of the loop_p typedef
> > > I'd suggest to use loop *.  It has at least two advantages:
> > > it's clearer (it's obvious it refers to a pointer), and lends
> > > itself more readily to making code const-correct by declaring
> > > the control variable const: for (const class loop *loop: ...)
> > > while avoiding the mistake of using const loop_p loop to
> > > declare a pointer to a const loop.
> > >
> >
> > Thanks for the suggestions, Martin!  Will update them in V2.
> >
> > With some experiments, I noticed that even provided const_iterator
> > like:
> >
> >    iterator
> >    begin ()
> >    {
> >      return iterator (*this, 0);
> >    }
> >
> > +  const_iterator
> > +  begin () const
> > +  {
> > +    return const_iterator (*this, 0);
> > +  }
> >
> > for (const class loop *loop: ...) will still use iterator instead
> > of const_iterator pair.  We have to make the code look like:
> >
> >   const auto& const_loops = loops_list (...);
> >   for (const class loop *loop: const_loops)
> >
> > or
> >   template<typename T> constexpr const T &as_const(T &t) noexcept { return t; }
> >   for (const class loop *loop: as_const(loops_list...))
> >
> > Does it look good to add below as_const along with loops_list in cfgloop.h?
> >
> > +/* Provide the functionality of std::as_const to support range-based for
> > +   to use const iterator.  (We can't use std::as_const itself because it's
> > +   a C++17 feature.)  */
> > +template <typename T>
> > +constexpr const T &
> > +as_const (T &t) noexcept
>
> The noexcept is not needed because GCC is built -fno-exceptions. For
> consistency with all the other code that doesn't use noexcept, it
> should probably not be there.
>
> > +{
> > +  return t;
> > +}
> > +
>
> That's one option. Another option (which could coexist with as_const)
> is to add cbegin() and cend() members, which are not overloaded for
> const and non-const, and so always return a const_iterator:
>
> const_iterator cbegin () const { return const_iterator (*this, 0); }
> iterator begin () const { return cbegin(); }
>
> And similarly for `end () const` and `cend () const`.

The range-based for loop would not use cbegin and cend, so you'd still
want to use as_const for that purpose.