[libc++][format][3/6] Adds a __container_buffer.

[Mordante]

Summary

Instead of writing every character directly into the container by using
a back_insert_iterator the data is buffered in an array. This buffer
is then inserted to the container by calling its insert member function.

Since there's no guarantee every container's insert behaves properly
containers need to opt-in to this behaviour. The appropriate standard
containers opt-in to this behaviour.

This change improves the performance of the format functions that use a
back_insert_iterator.

Depends on D110495

[Mordante]

Attempt to fix Clang-11 build issue.

[• Quuxplusone]

libcxx/include/__format/buffer.h

146

The machinery to __enable_insertable in a container right now is like 16 lines. You could get it down to 1 line, and no helper headers, and C++11-compatible, if you changed this to

is_same_v<_Container, typename _Container::__enable_format_insertable>

Then the containers (vector, deque, string, list, etc.) would just have to say e.g.

using __enable_format_insertable = vector<_Tp, _Alloc>;

without any extra headers or ifdef-guards or anything.

149

add_pointer_t<typename _Container::value_type> is invariably typename _Container::value_type*, right?

154–176

Rather than lines 129-140, I suggest that you add a member function container_type *back_insert_iterator::__get_container() const to libc++'s back_insert_iterator.
I suppose you still need a partial-specialization-based approach equivalent to lines 117-127, in order to tell you whether the user's iterator type is exactly back_insert_iterator (in which case this optimization is safe) or merely something that slices to back_insert_iterator (in which case this optimization is dubious). But once you know it's exactly back_insert_iterator, you can just ask for its container_type member typedef, and use your new member function to get a pointer to the container; you don't have to use tricks for that part.

208

Why not _CharT __buffer_[__buffer_size];? (What does the std::array dependency buy you? and the zero-initialization?)

libcxx/include/module.modulemap

442 ↗

(On Diff #375099)

/intertable/insertable/

[Mordante]

Needs to be updated due to the removal of __put in D110495.

[Mordante]

libcxx/include/__format/buffer.h

146

Thanks for the suggestion, this indeed is a lot less intrusive.

154–176

See D110573.

208

I prefer an array just for convenience. I need to know the last element and I like it better to use end().
The zero-initialization isn't needed.

libcxx/include/module.modulemap

442 ↗

(On Diff #375099)

This is no longer needed due to your earlier suggestions.

[Mordante]

Addresses the review comments.
Temporary include the used part of D110573 to avoid possible Buildkite dependency issues.

[• Quuxplusone]

I think it's up to @ldionne and/or @vitaut to say whether this PR is ultimately a good tradeoff, complexity-and-performance-wise. IMHO we've successfully reached the local optimum, code-wise. :)

libcxx/include/__format/buffer.h

275

Style nit: _CharT* __buffer_it_ = __buffer_.begin(); is easier to read, and equivalent. Could even say __buffer_.data() and then I wouldn't need to consciously avoid looking up on cppreference whether array::iterator is guaranteed to be _CharT*. ;)

libcxx/include/list

853

Style nit: _Tp, _Alloc with a space

libcxx/test/libcxx/utilities/format/enable_insertable.compile.pass.cpp

30–52

FWIW, I think it is not useful to test no_value_type, no_end, or no_insert, because we don't permit users to define such types and we don't expect to define any such types ourselves. Seeing a correct "specialization" of __enable_format_insertable should be enough to indicate that the type author intends the type to be insertable. If it's somehow not insertable, then that's a serious show-stopping bug in the type, not something that <format> should be silently working-around.

69

Also test

struct UserVector : public std::vector<char> {
    using std::vector<char>::vector;
};
static_assert(!std::__format::__insertable<UserVector>);

This is important to test because users are allowed to do stupid things like this. (They shouldn't; it's bad style and has many downsides; but they are allowed to.)

94

What do you expect to happen for std::vector<int> or std::vector<std::string>? On the one hand, int and string are not "char types," right? But on the other hand, vec.insert(vec.end(), charptr, charendptr) will work as expected, so we do want to do the optimization, right?

[Mordante]

Thanks for the feedback, I'll look at these issues when Buildkite is working properly again.

libcxx/test/libcxx/utilities/format/enable_insertable.compile.pass.cpp

94

The format specific functions require a char or wchar_t. So I expect them not to pass this trait. When we want to use this more generic, we can add an std::__insertable and rename the typedef in the containers. I didn't restrain the typedef in the classes; I looked at it but it became quite ugly quite fast. Instead it's constrained in the std::__format::__insertable concept. So it would be quite trivial to add std::__insertable, but I don't have a usecase for it.

[Mordante]

Addresses review comments.

libcxx/include/__format/buffer.h

275

I prefer the begin() since that conveys the intended usage better.

libcxx/include/list

853

Interesting clang-format didn't catch that for me.

libcxx/test/libcxx/utilities/format/enable_insertable.compile.pass.cpp

30–52

I not entirely convinced that removing them is a good idea. I'd like to hear @ldionne's opinion.

[Mordante]

Rebased to test with wchar_t changes.

[Mordante]

Fixes wide char build.

[vitaut]

Since there's no guarantee every container's insert behaves properly

What does behaving properly mean? Could you give an example of a container's insert that doesn't behave properly?

libcxx/include/__format/buffer.h

219

replace -> replaced

221

opt-in to -> "opt in to" or "opt into"

[vitaut]

Character-by-character output, especially type erased one introduced in D110494 doesn't seem efficient. As commented on the the previous diff the buffer and corresponding output iterator should provide access to a contiguous buffer and type erase the reallocation, not character output.

[• Quuxplusone]

Character by character output doesn't seem efficient

Right, IIUC, this would end up rebased on top of D112361 or the like.

libcxx/include/__format/buffer.h

221

In D110497#3098970, @vitaut wrote:

Since there's no guarantee every container's insert behaves properly

What does behaving properly mean? Could you give an example of a container's insert that doesn't behave properly?

User-defined containers can make their insert do arbitrary things, e.g.

struct Evil : std::string {
    void insert(std::string::iterator, char *, char *) { /*no-op*/ }
};
Evil e;
auto evil_it = std::back_inserter(e);  // uses e.push_back, which is fine

(Serendipitously, Charlie Barto, of the MSVC STL, also mentioned the need to guard against this possibility in his CppCon talk "Why does std::format do that?" on Thursday. https://cppcon.digital-medium.co.uk/thursday-recordings/ )

[vitaut]

libcxx/include/__format/buffer.h

221

Makes sense, thanks for clarifying.

[Mordante]

In D110497#3098973, @vitaut wrote:

Character-by-character output, especially type erased one introduced in D110494 doesn't seem efficient. As commented on the the previous diff the buffer and corresponding output iterator should provide access to a contiguous buffer and type erase the reallocation, not character output.

Yes I first want to get a good solution in D112361 and then update this patch with these changes.

libcxx/include/__format/buffer.h

221

In D110497#3098970, @vitaut wrote:

Since there's no guarantee every container's insert behaves properly

What does behaving properly mean? Could you give an example of a container's insert that doesn't behave properly?

User-defined containers can make their insert do arbitrary things, e.g.

struct Evil : std::string {
    void insert(std::string::iterator, char *, char *) { /*no-op*/ }
};
Evil e;
auto evil_it = std::back_inserter(e);  // uses e.push_back, which is fine

(Serendipitously, Charlie Barto, of the MSVC STL, also mentioned the need to guard against this possibility in his CppCon talk "Why does std::format do that?" on Thursday. https://cppcon.digital-medium.co.uk/thursday-recordings/ )

Yes that's the reason, interesting to see Charlie did the same. I'm looking forward to watching this talk.

[Mordante]

Rebased and make changes based on D112361.
Addresses review comments.

[Mordante]

Rebased.

[Mordante]

Rebased tp trigger CI.

[vitaut]

Same as D110495:

As commented, I'm very skeptical about this approach because it has extra buffering for common cases but I defer to libc++ maintainers the decision whether they want to land this as is or optimize buffering now.

[ldionne]

libcxx/include/__format/buffer.h

162–169

Is there any reason why this isn't implemented as a bool variable template that containers specialize or something similar? IMO it would be nice to keep containers free of a __enable_format_insertable typedef used for this sole purpose.

Requiring a specialization also makes it slightly less likely that users are going to do it for their own containers, which IMO is a good thing (we really don't want users to start depending on this unless the Standard commits to providing this functionality).

[Reading some other comments, I get the feeling this might have been how it was implemented in previous diffs, please let me know if that's the case]

[Mordante]

libcxx/include/__format/buffer.h

162–169

That was my initial approach https://reviews.llvm.org/D110497?vs=on&id=375096#toc
when you too like that better I can switch back to that approach.

[Mordante]

Rebase and remove concepts guards.

[ldionne]

LGTM with a comment I'd like to see addressed, but I don't think I need to see this again. IIUC, @vitaut 's concern about character-by-character output has been resolved since this was rebased. We are still buffering too much to @vitaut 's liking, but @Mordante 's telling me that he'll investigate that in the future, before we ship this.

Also, this should be rebased onto the __get_container() patch.

libcxx/include/__format/buffer.h

162–169

Yeah, I think it would be better to have a less intrusive mechanism. We can specialize it for various containers in this header instead of having to touch vector & friends directly.

[Mordante]

libcxx/include/__format/buffer.h

162–169

In that case format pulls in several unrelated headers. In the original approach these container headers pull in one small header. So I think that's the better solution.

[Mordante]

• Rebased
• Updated to recent upstream cheanges
• Address review comments

[Mordante]

Forgot to format last patch.

[Mordante]

Fix a typo in the module map.

[Mordante]

Fix CI.

libcxx/include/__format/buffer.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 

 /// Write policy for copying the buffer to the output.
 template <class _OutIt, __formatter::__char_type _CharT>
 class _LIBCPP_TEMPLATE_VIS __writer_iterator {
 public:
   _LIBCPP_HIDE_FROM_ABI explicit __writer_iterator(_OutIt __out_it)
       : __out_it_{_VSTD::move(__out_it)} {}
 
   _LIBCPP_HIDE_FROM_ABI auto out() { return __out_it_; }
 
   _LIBCPP_HIDE_FROM_ABI void flush(_CharT* __ptr, size_t __size) {
     __out_it_ = _VSTD::copy_n(__ptr, __size, _VSTD::move(__out_it_));
   }
 
 private:
   _OutIt __out_it_;
 };
 
+/// Concept to see whether a \a _Container is insertable.
+///
+/// The concept is used to validate whether multiple calls to a
+/// \ref back_insert_iterator can be replaced by a call to \c _Container::insert.
+///
+/// \note a \a _Container needs to opt into the concept by adding a typedef
+/// \ref __enable_format_insertable with the same type as \a _Container.
+template <class _Container>
+concept __insertable =
+    same_as<_Container, typename _Container::__enable_format_insertable> &&
+    __formatter::__char_type<typename _Container::value_type> &&
+    requires(_Container& __t, typename _Container::value_type* __first,
+             typename _Container::value_type* __last) {
+  __t.insert(__t.end(), __first, __last);
+};
+
+/// Extract the container type of a \ref back_insert_iterator.
+template <class _It>
+struct _LIBCPP_TEMPLATE_VIS __back_insert_iterator_container {
+  using type = void;
+};
+
+template <__insertable _Container>
+struct _LIBCPP_TEMPLATE_VIS
+    __back_insert_iterator_container<back_insert_iterator<_Container>> {
+  using type = _Container;
+};
+
+/// Write policy for inserting the buffer in a container.
+template <class _Container>
+class _LIBCPP_TEMPLATE_VIS __writer_container {
+public:
+  using _CharT = typename _Container::value_type;
+
+  _LIBCPP_HIDE_FROM_ABI explicit __writer_container(
+      back_insert_iterator<_Container> __out_it)
+      : __container_{__out_it.__get_container()} {}
+
+  _LIBCPP_HIDE_FROM_ABI auto out() { return back_inserter(*__container_); }
+
+  _LIBCPP_HIDE_FROM_ABI void flush(_CharT* __ptr, size_t __size) {
+    __container_->insert(__container_->end(), __ptr, __ptr + __size);
+  }
+
+private:
+  _Container* __container_;
+};
+
 /// Selects the type of the writer used for the output iterator.
 template <class _OutIt, class _CharT>
 class _LIBCPP_TEMPLATE_VIS __writer_selector {
+  using _Container = typename __back_insert_iterator_container<_OutIt>::type;
+
 public:
-  using type = conditional_t<__enable_direct_output<_OutIt, _CharT>,
+  using type =
+      conditional_t<!same_as<_Container, void>, __writer_container<_Container>,
+                    conditional_t<__enable_direct_output<_OutIt, _CharT>,
                                   __writer_direct<_OutIt, _CharT>,
-                             __writer_iterator<_OutIt, _CharT>>;
+                                  __writer_iterator<_OutIt, _CharT>>>;
 };
 
 /// The generic formatting buffer.
 template <class _OutIt, __formatter::__char_type _CharT>
 requires(output_iterator<_OutIt, const _CharT&>) class _LIBCPP_TEMPLATE_VIS
     __format_buffer {
   using _Storage =
       conditional_t<__enable_direct_output<_OutIt, _CharT>,
                     __direct_storage<_CharT>, __internal_storage<_CharT>>;
 
 public:
   _LIBCPP_HIDE_FROM_ABI explicit __format_buffer(_OutIt __out_it) requires(
       same_as<_Storage, __internal_storage<_CharT>>)
       : __output_(__storage_.begin(), __storage_.capacity(), this),
         __writer_(_VSTD::move(__out_it)) {}
 
private:
   typename __writer_selector<_OutIt, _CharT>::type __writer_;
 };
 } // namespace __format
 
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FORMAT_BUFFER_H

libcxx/include/__iterator/back_insert_iterator.h

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator=(const typename _Container::value_type& __value_)
         {container->push_back(__value_); return *this;}
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator=(typename _Container::value_type&& __value_)
         {container->push_back(_VSTD::move(__value_)); return *this;}
 #endif // _LIBCPP_CXX03_LANG
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator*()     {return *this;}
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator++()    {return *this;}
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator  operator++(int) {return *this;}
+    // This is not part of this patch but D110573.
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 _Container* __get_container() const { return container; }
 };
 
 template <class _Container>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
 back_insert_iterator<_Container>
 back_inserter(_Container& __x)
 {
     return back_insert_iterator<_Container>(__x);
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ITERATOR_BACK_INSERT_ITERATOR_H

libcxx/include/deque

public:
     typedef typename __base::const_iterator               const_iterator;
     typedef typename __base::size_type                    size_type;
     typedef typename __base::difference_type              difference_type;
 
     typedef typename __base::pointer                      pointer;
     typedef typename __base::const_pointer                const_pointer;
     typedef _VSTD::reverse_iterator<iterator>             reverse_iterator;
     typedef _VSTD::reverse_iterator<const_iterator>       const_reverse_iterator;
+    typedef deque<_Tp, _Allocator>                        __enable_format_insertable;
 
     using typename __base::__deque_range;
     using typename __base::__deque_block_range;
     using typename __base::_ConstructTransaction;
 
     // construct/copy/destroy:
     _LIBCPP_INLINE_VISIBILITY
     deque()

libcxx/include/list

public:
     typedef typename base::const_iterator                  const_iterator;
     typedef _VSTD::reverse_iterator<iterator>              reverse_iterator;
     typedef _VSTD::reverse_iterator<const_iterator>        const_reverse_iterator;
 #if _LIBCPP_STD_VER > 17
     typedef size_type                                      __remove_return_type;
 #else
     typedef void                                           __remove_return_type;
 #endif
+    typedef list<_Tp, _Alloc>                                        __enable_format_insertable;
 
     _LIBCPP_INLINE_VISIBILITY
     list()
         _NOEXCEPT_(is_nothrow_default_constructible<__node_allocator>::value)
     {
         _VSTD::__debug_db_insert_c(this);
     }
     _LIBCPP_INLINE_VISIBILITY

libcxx/include/string

public:
     typedef _Allocator                                   allocator_type;
     typedef allocator_traits<allocator_type>             __alloc_traits;
     typedef typename __alloc_traits::size_type           size_type;
     typedef typename __alloc_traits::difference_type     difference_type;
     typedef value_type&                                  reference;
     typedef const value_type&                            const_reference;
     typedef typename __alloc_traits::pointer             pointer;
     typedef typename __alloc_traits::const_pointer       const_pointer;
+    typedef basic_string<_CharT, _Traits, _Allocator>    __enable_format_insertable;
 
     static_assert((!is_array<value_type>::value), "Character type of basic_string must not be an array");
     static_assert(( is_standard_layout<value_type>::value), "Character type of basic_string must be standard-layout");
     static_assert(( is_trivial<value_type>::value), "Character type of basic_string must be trivial");
     static_assert(( is_same<_CharT, typename traits_type::char_type>::value),
                   "traits_type::char_type must be the same type as CharT");
     static_assert(( is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");

libcxx/include/vector

public:
     typedef typename __alloc_traits::size_type              size_type;
     typedef typename __alloc_traits::difference_type        difference_type;
     typedef typename __alloc_traits::pointer                pointer;
     typedef typename __alloc_traits::const_pointer          const_pointer;
     typedef __wrap_iter<pointer>                            iterator;
     typedef __wrap_iter<const_pointer>                      const_iterator;
     typedef _VSTD::reverse_iterator<iterator>               reverse_iterator;
     typedef _VSTD::reverse_iterator<const_iterator>         const_reverse_iterator;
+    typedef vector<_Tp, _Allocator>                         __enable_format_insertable;
 
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
 
     _LIBCPP_INLINE_VISIBILITY
     vector() _NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value)
     {
         _VSTD::__debug_db_insert_c(this);

libcxx/test/libcxx/utilities/format/enable_insertable.compile.pass.cpp

+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// <format>
+
+#include <format>
+#include <array>
+#include <deque>
+#include <forward_list>
+#include <list>
+#include <map>
+#include <queue>
+#include <set>
+#include <span>
+#include <stack>
+#include <string>
+#include <string_view>
+
+#include "test_macros.h"
+
+template <class CharT>
+struct no_value_type {
+  using __enable_format_insertable = no_value_type<CharT>;
+  using iterator = CharT*;
+  iterator end();
+  void insert(iterator, CharT*, CharT*);
+};
+
+template <class CharT>
+struct no_end {
+  using __enable_format_insertable = no_end<CharT>;
+  using value_type = CharT;
+  using iterator = CharT*;
+  void insert(iterator, CharT*, CharT*);
+};
+
+template <class CharT>
+struct no_insert {
+  using __enable_format_insertable = no_insert<CharT>;
+  using value_type = CharT;
+  using iterator = CharT*;
+  iterator end();
+};
+
+template <class CharT>
+struct no_specialization {
+  using value_type = CharT;
+  using iterator = CharT*;
+  iterator end();
+  void insert(iterator, CharT*, CharT*);
+};
+
+template <class CharT>
+struct valid {
+  using __enable_format_insertable = valid<CharT>;
+  using value_type = CharT;
+  using iterator = CharT*;
+  iterator end();
+  void insert(iterator, CharT*, CharT*);
+};
+
+static_assert(!std::__format::__insertable<no_value_type<char>>);
+static_assert(!std::__format::__insertable<no_end<char>>);
+static_assert(!std::__format::__insertable<no_insert<char>>);
+static_assert(!std::__format::__insertable<no_specialization<char>>);
+static_assert(std::__format::__insertable<valid<char>>);
+
+static_assert(std::__format::__insertable<std::string>);
+static_assert(!std::__format::__insertable<std::string_view>);
+
+static_assert(!std::__format::__insertable<std::array<char, 1>>);
+
+static_assert(std::__format::__insertable<std::vector<char>>);
+static_assert(std::__format::__insertable<std::deque<char>>);
+static_assert(!std::__format::__insertable<std::forward_list<char>>);
+static_assert(std::__format::__insertable<std::list<char>>);
+
+static_assert(!std::__format::__insertable<std::set<char>>);
+static_assert(!std::__format::__insertable<std::map<char, char>>);
+static_assert(!std::__format::__insertable<std::multiset<char>>);
+static_assert(!std::__format::__insertable<std::multimap<char, char>>);
+
+static_assert(!std::__format::__insertable<std::stack<char>>);
+static_assert(!std::__format::__insertable<std::queue<char>>);
+static_assert(!std::__format::__insertable<std::priority_queue<char>>);
+
+static_assert(!std::__format::__insertable<std::span<char>>);
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+static_assert(!std::__format::__insertable<no_value_type<wchar_t>>);
+static_assert(!std::__format::__insertable<no_end<wchar_t>>);
+static_assert(!std::__format::__insertable<no_insert<wchar_t>>);
+static_assert(!std::__format::__insertable<no_specialization<wchar_t>>);
+static_assert(std::__format::__insertable<valid<wchar_t>>);
+
+static_assert(std::__format::__insertable<std::wstring>);
+static_assert(!std::__format::__insertable<std::wstring_view>);
+
+static_assert(!std::__format::__insertable<std::array<wchar_t, 1>>);
+
+static_assert(std::__format::__insertable<std::vector<wchar_t>>);
+static_assert(std::__format::__insertable<std::deque<wchar_t>>);
+static_assert(!std::__format::__insertable<std::forward_list<wchar_t>>);
+static_assert(std::__format::__insertable<std::list<wchar_t>>);
+
+static_assert(!std::__format::__insertable<std::set<wchar_t>>);
+static_assert(!std::__format::__insertable<std::map<wchar_t, wchar_t>>);
+static_assert(!std::__format::__insertable<std::multiset<wchar_t>>);
+static_assert(!std::__format::__insertable<std::multimap<wchar_t, wchar_t>>);
+
+static_assert(!std::__format::__insertable<std::stack<wchar_t>>);
+static_assert(!std::__format::__insertable<std::queue<wchar_t>>);
+static_assert(!std::__format::__insertable<std::priority_queue<wchar_t>>);
+
+static_assert(!std::__format::__insertable<std::span<wchar_t>>);
+#endif
+
+static_assert(!std::__format::__insertable<valid<signed char>>);
+static_assert(!std::__format::__insertable<valid<unsigned char>>);
+
+struct UserVector : public std::vector<char> {
+  using std::vector<char>::vector;
+};
+static_assert(!std::__format::__insertable<UserVector>);