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
