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

[Richard Sandiford <richard.sandiford@arm.com>]

Martin Jambor <mjambor@suse.cz> writes:
> 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.

Yeah, the latter sounds better to me.  (The existing uses of array_slice
are for const elements, which is why I didn't come across this.)

> 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) {}

LGTM.

Thanks,
Richard

> 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