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

[Martin Jambor <mjambor@suse.cz>]

Hi Richard,

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.

thanks a lot for introducing it.  I hope to use it as a unified view
into something which might be a GC vec or heap vec an an auto_vec.

But I have one question:

>
> 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 ()) {}
> +

What is the reason for making the parameter const here?

The problem is that if you do for example:

  auto_vec<bool, 16> test_base;
  test_base.quick_grow_cleared (10);
  array_slice<bool> test(test_base);

the constructor will get a const reference to test_base and so will
invoke the const variant of v.address() which returns a const bool *
which cannot be assigned into non-const qualified base.  AFAICS, the
constructor only works if the array_slice is array_slice<const bool>.

Is that intentional?  I am not a C++ expert and can be easily
overlooking something.  I understand that users need to be careful not
to cause reallocation of the underlying vector while the array_slice
exists but the const qualifier does not achieve that.  (A wild idea to
be to add a array_slice ref-counter to auto_vec, which seems to be less
space-efficiency-critical than other vecs, and assert on reallocation
when it is not zero, hehe).

Removing the const qualifier in the constructor parameter makes the
error go away - as does adding another constructor without it, which
might be the correct thing to do.

On a related note, would the following constructor be a good addition to
the class (I can make it const too)?

  template<typename OtherT>
  array_slice (vec<OtherT, va_gc> *v)
    : m_base (v ? v->address () : nullptr), m_size (v ? v->length (): 0) {}


Thanks,

Martin



> +  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; }
> +  size_t size_bytes () const { return m_size * sizeof (T); }
> +  bool empty () const { return m_size == 0; }
> +
> +  // An invalid array_slice that represents a failed operation.  This is
> +  // distinct from an empty slice, which is a valid result in some contexts.
> +  static array_slice invalid () { return { nullptr, ~0U }; }
> +
> +  // True if the array is valid, false if it is an array like INVALID.
> +  bool is_valid () const { return m_base || m_size == 0; }
> +
> +private:
> +  iterator m_base;
> +  unsigned int m_size;
> +};
> +
> +template<typename T>
> +inline typename array_slice<T>::value_type &
> +array_slice<T>::front ()
> +{
> +  gcc_checking_assert (m_size);
> +  return m_base[0];
> +}
> +
> +template<typename T>
> +inline const typename array_slice<T>::value_type &
> +array_slice<T>::front () const
> +{
> +  gcc_checking_assert (m_size);
> +  return m_base[0];
> +}
> +
> +template<typename T>
> +inline typename array_slice<T>::value_type &
> +array_slice<T>::back ()
> +{
> +  gcc_checking_assert (m_size);
> +  return m_base[m_size - 1];
> +}
> +
> +template<typename T>
> +inline const typename array_slice<T>::value_type &
> +array_slice<T>::back () const
> +{
> +  gcc_checking_assert (m_size);
> +  return m_base[m_size - 1];
> +}
> +
> +template<typename T>
> +inline typename array_slice<T>::value_type &
> +array_slice<T>::operator[] (unsigned int i)
> +{
> +  gcc_checking_assert (i < m_size);
> +  return m_base[i];
> +}
> +
> +template<typename T>
> +inline const typename array_slice<T>::value_type &
> +array_slice<T>::operator[] (unsigned int i) const
> +{
> +  gcc_checking_assert (i < m_size);
> +  return m_base[i];
> +}
> +
> +template<typename T>
> +array_slice<T>
> +make_array_slice (T *base, unsigned int size)
> +{
> +  return array_slice<T> (base, size);
> +}
> +
>  #if (GCC_VERSION >= 3000)
>  # pragma GCC poison m_vec m_vecpfx m_vecdata
>  #endif
> -- 
> 2.17.1