Martin Jambor <mjam...@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
