Re: [09/23] Add a cut-down version of std::span (array_slice)

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Aug 10, 2022 at 6:04 PM Martin Jambor <mjambor@suse.cz> wrote:
>
> Hello,
>
> I have one more question/comment about array_slice.  Ever since I
> started to use it...
>
> On Fri, Nov 13 2020, Richard Sandiford via Gcc-patches wrote:
> > A later patch wants to be able to pass around subarray views of an
> > existing array.  The standard class to do that is std::span, but it's
> > a C++20 thing.  This patch just adds a cut-down version of it.
> >
> > The intention is just to provide what's currently needed.
> >
> > gcc/
> >       * vec.h (array_slice): New class.
> > ---
> >  gcc/vec.h | 120 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 120 insertions(+)
> >
> > diff --git a/gcc/vec.h b/gcc/vec.h
> > index f02beddc975..7768de9f518 100644
> > --- a/gcc/vec.h
> > +++ b/gcc/vec.h
> > @@ -2128,6 +2128,126 @@ release_vec_vec (vec<vec<T> > &vec)
> >    vec.release ();
> >  }
> >
> > +// Provide a subset of the std::span functionality.  (We can't use std::span
> > +// itself because it's a C++20 feature.)
> > +//
> > +// In addition, provide an invalid value that is distinct from all valid
> > +// sequences (including the empty sequence).  This can be used to return
> > +// failure without having to use std::optional.
> > +//
> > +// There is no operator bool because it would be ambiguous whether it is
> > +// testing for a valid value or an empty sequence.
> > +template<typename T>
> > +class array_slice
> > +{
> > +  template<typename OtherT> friend class array_slice;
> > +
> > +public:
> > +  using value_type = T;
> > +  using iterator = T *;
> > +  using const_iterator = const T *;
> > +
> > +  array_slice () : m_base (nullptr), m_size (0) {}
> > +
> > +  template<typename OtherT>
> > +  array_slice (array_slice<OtherT> other)
> > +    : m_base (other.m_base), m_size (other.m_size) {}
> > +
> > +  array_slice (iterator base, unsigned int size)
> > +    : m_base (base), m_size (size) {}
> > +
> > +  template<size_t N>
> > +  array_slice (T (&array)[N]) : m_base (array), m_size (N) {}
> > +
> > +  template<typename OtherT>
> > +  array_slice (const vec<OtherT> &v)
> > +    : m_base (v.address ()), m_size (v.length ()) {}
> > +
> > +  iterator begin () { return m_base; }
> > +  iterator end () { return m_base + m_size; }
> > +
> > +  const_iterator begin () const { return m_base; }
> > +  const_iterator end () const { return m_base + m_size; }
> > +
> > +  value_type &front ();
> > +  value_type &back ();
> > +  value_type &operator[] (unsigned int i);
> > +
> > +  const value_type &front () const;
> > +  const value_type &back () const;
> > +  const value_type &operator[] (unsigned int i) const;
> > +
> > +  size_t size () const { return m_size; }
>
> ...this has been a constant source of compile errors, because vectors
> have length () and this is size ().
>
> I understand that the motivation was consistency with std::span, but do
> we really want to add another inconsistency with ourselves?
>
> Given that array_slice is not that much used yet, I believe we can still
> change to be consistent with vectors.  I personally think we should but
> at the very least, if we keep it as it is, I'd like us to do so
> deliberately.

We could alternatively add length in addition to size (and maybe size to
vec<> if std::vector has size but not length) with a comment deprecating
the "non-standard" variant?

Richard.

>
> Thanks,
>
> Martin
>