Re: [09/23] Add a cut-down version of std::span (array_slice)
Richard Biener writes:
> On Wed, Aug 10, 2022 at 6:04 PM Martin Jambor 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.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
>> > +class array_slice
>> > +{
>> > + template 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
>> > + array_slice (array_slice 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
>> > + array_slice (T ()[N]) : m_base (array), m_size (N) {}
>> > +
>> > + template
>> > + array_slice (const vec )
>> > +: 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 ();
>> > + value_type ();
>> > + value_type [] (unsigned int i);
>> > +
>> > + const value_type () const;
>> > + const value_type () const;
>> > + const value_type [] (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?
Yeah, I'd prefer to do the latter: add vec::size as a synonym of
vec::length, and deprecate length. Doing anything else seems like
it's going to increase the inconsistency rather than decrease it.
E.g. we already have uses of (hopefully) uncontroversial standard
containers like std::array (my fault).
(FWIW, I keep tripping up in the opposite direction: expecting
size to be available in vec, like for standard containers.)
Thanks,
Richard
Re: [09/23] Add a cut-down version of std::span (array_slice)
On Wed, Aug 10, 2022 at 6:04 PM Martin Jambor 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.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
> > +class array_slice
> > +{
> > + template 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
> > + array_slice (array_slice 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
> > + array_slice (T ()[N]) : m_base (array), m_size (N) {}
> > +
> > + template
> > + array_slice (const vec )
> > +: 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 ();
> > + value_type ();
> > + value_type [] (unsigned int i);
> > +
> > + const value_type () const;
> > + const value_type () const;
> > + const value_type [] (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
>
Re: [09/23] Add a cut-down version of std::span (array_slice)
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.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
> +class array_slice
> +{
> + template 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
> + array_slice (array_slice 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
> + array_slice (T ()[N]) : m_base (array), m_size (N) {}
> +
> + template
> + array_slice (const vec )
> +: 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 ();
> + value_type ();
> + value_type [] (unsigned int i);
> +
> + const value_type () const;
> + const value_type () const;
> + const value_type [] (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.
Thanks,
Martin
Re: [09/23] Add a cut-down version of std::span (array_slice)
Martin Jambor 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.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
>> +class array_slice
>> +{
>> + template 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
>> + array_slice (array_slice 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
>> + array_slice (T ()[N]) : m_base (array), m_size (N) {}
>> +
>> + template
>> + array_slice (const vec )
>> +: 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 test_base;
> test_base.quick_grow_cleared (10);
> array_slice 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.
>
> 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
> array_slice (vec *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 ();
>> + value_type ();
>> + value_type [] (unsigned int i);
>> +
>> + const value_type () const;
>> + const value_type () const;
>> + const value_type [] (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
>> +inline typename array_slice::value_type &
>> +array_slice::front ()
>> +{
>> + gcc_checking_assert (m_size);
>> + return m_base[0];
>> +}
>> +
>> +template
>> +inline const typename array_slice::value_type &
>> +array_slice::front () const
>> +{
>> + gcc_checking_assert (m_size);
>> + return m_base[0];
>> +}
>> +
>>
Re: [09/23] Add a cut-down version of std::span (array_slice)
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.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
> +class array_slice
> +{
> + template 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
> + array_slice (array_slice 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
> + array_slice (T ()[N]) : m_base (array), m_size (N) {}
> +
> + template
> + array_slice (const vec )
> +: 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 test_base;
test_base.quick_grow_cleared (10);
array_slice 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.
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
array_slice (vec *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 ();
> + value_type ();
> + value_type [] (unsigned int i);
> +
> + const value_type () const;
> + const value_type () const;
> + const value_type [] (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
> +inline typename array_slice::value_type &
> +array_slice::front ()
> +{
> + gcc_checking_assert (m_size);
> + return m_base[0];
> +}
> +
> +template
> +inline const typename array_slice::value_type &
> +array_slice::front () const
> +{
> + gcc_checking_assert (m_size);
> + return m_base[0];
> +}
> +
> +template
> +inline typename array_slice::value_type &
> +array_slice::back ()
> +{
> + gcc_checking_assert (m_size);
> + return m_base[m_size - 1];
> +}
> +
> +template
> +inline const typename array_slice::value_type &
> +array_slice::back () const
> +{
> + gcc_checking_assert (m_size);
> + return m_base[m_size - 1];
> +}
> +
> +template
> +inline typename
Re: [09/23] Add a cut-down version of std::span (array_slice)
On 11/13/20 1:15 AM, 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. OK. Obviously we can add more capabilities as we need them. jeff
[09/23] Add a cut-down version of std::span (array_slice)
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.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
+class array_slice
+{
+ template 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
+ array_slice (array_slice 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
+ array_slice (T ()[N]) : m_base (array), m_size (N) {}
+
+ template
+ array_slice (const vec )
+: 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 ();
+ value_type ();
+ value_type [] (unsigned int i);
+
+ const value_type () const;
+ const value_type () const;
+ const value_type [] (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
+inline typename array_slice::value_type &
+array_slice::front ()
+{
+ gcc_checking_assert (m_size);
+ return m_base[0];
+}
+
+template
+inline const typename array_slice::value_type &
+array_slice::front () const
+{
+ gcc_checking_assert (m_size);
+ return m_base[0];
+}
+
+template
+inline typename array_slice::value_type &
+array_slice::back ()
+{
+ gcc_checking_assert (m_size);
+ return m_base[m_size - 1];
+}
+
+template
+inline const typename array_slice::value_type &
+array_slice::back () const
+{
+ gcc_checking_assert (m_size);
+ return m_base[m_size - 1];
+}
+
+template
+inline typename array_slice::value_type &
+array_slice::operator[] (unsigned int i)
+{
+ gcc_checking_assert (i < m_size);
+ return m_base[i];
+}
+
+template
+inline const typename array_slice::value_type &
+array_slice::operator[] (unsigned int i) const
+{
+ gcc_checking_assert (i < m_size);
+ return m_base[i];
+}
+
+template
+array_slice
+make_array_slice (T *base, unsigned int size)
+{
+ return array_slice (base, size);
+}
+
#if (GCC_VERSION >= 3000)
# pragma GCC poison m_vec m_vecpfx m_vecdata
#endif
--
2.17.1
