[libc++] Remove get_temporary_buffer and return_temporary_buffer (#10…

…0914)

Works towards P0619R4 / #99985.

The use of `std::get_temporary_buffer` and `std::return_temporary_buffer`
are replaced with `unique_ptr`-based RAII buffer holder.

Escape hatches:
- `_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER` restores
`std::get_temporary_buffer` and `std::return_temporary_buffer`.

Drive-by changes:
- In `<syncstream>`, states that `get_temporary_buffer` is now removed,
because `<syncstream>` is added in C++20.

libcxx/docs/ReleaseNotes/20.rst

@@ -49,6 +49,9 @@ Improvements and New Features
 - The ``lexicographical_compare`` and ``ranges::lexicographical_compare`` algorithms have been optimized for trivially
   equality comparable types, resulting in a performance improvement of up to 40x.
 
+- The ``_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER`` macro has been added to make ``std::get_temporary_buffer`` and
+  ``std::return_temporary_buffer`` available.
+
 - The ``_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception``
   available in C++20 and later modes.
 

libcxx/docs/Status/Cxx20Papers.csv

@@ -34,7 +34,7 @@
 "`P0528R3 <https://wg21.link/P0528R3>`__","The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange","2018-06 (Rapperswil)","","",""
 "`P0542R5 <https://wg21.link/P0542R5>`__","Support for contract based programming in C++","2018-06 (Rapperswil)","|Nothing To Do|","n/a","Pulled at the 2019-07 meeting in Cologne"
 "`P0556R3 <https://wg21.link/P0556R3>`__","Integral power-of-2 operations","2018-06 (Rapperswil)","|Complete|","9.0",""
-"`P0619R4 <https://wg21.link/P0619R4>`__","Reviewing Deprecated Facilities of C++17 for C++20","2018-06 (Rapperswil)","|Partial|","","Only sections D.7, D.8, D.9, D.10, D.11 and D.13 are implemented. Sections D.4 and D.12 remain undone."
+"`P0619R4 <https://wg21.link/P0619R4>`__","Reviewing Deprecated Facilities of C++17 for C++20","2018-06 (Rapperswil)","|Partial|","","Only sections D.7, D.8, D.9, D.10, D.11, D.12, and D.13 are implemented. Section D.4 remains undone."
 "`P0646R1 <https://wg21.link/P0646R1>`__","Improving the Return Value of Erase-Like Algorithms","2018-06 (Rapperswil)","|Complete|","10.0",""
 "`P0722R3 <https://wg21.link/P0722R3>`__","Efficient sized delete for variable sized classes","2018-06 (Rapperswil)","|Complete|","9.0",""
 "`P0758R1 <https://wg21.link/P0758R1>`__","Implicit conversion traits and utility functions","2018-06 (Rapperswil)","|Complete|","",""

libcxx/docs/UserDocumentation.rst

@@ -181,6 +181,9 @@ C++20 Specific Configuration Macros
 **_LIBCPP_ENABLE_CXX20_REMOVED_RAW_STORAGE_ITERATOR**:
   This macro is used to re-enable `raw_storage_iterator`.
 
+**_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER**:
+  This macro is used to re-enable `get_temporary_buffer` and `return_temporary_buffer`.
+
 **_LIBCPP_ENABLE_CXX20_REMOVED_TYPE_TRAITS**:
   This macro is used to re-enable `is_literal_type`, `is_literal_type_v`,
   `result_of` and `result_of_t`.

libcxx/include/CMakeLists.txt

@@ -556,6 +556,7 @@ set(files
   __memory/temporary_buffer.h
   __memory/uninitialized_algorithms.h
   __memory/unique_ptr.h
+  __memory/unique_temporary_buffer.h
   __memory/uses_allocator.h
   __memory/uses_allocator_construction.h
   __memory/voidify.h

libcxx/include/__algorithm/inplace_merge.h

@@ -24,8 +24,8 @@
 #include <__iterator/iterator_traits.h>
 #include <__iterator/reverse_iterator.h>
 #include <__memory/destruct_n.h>
-#include <__memory/temporary_buffer.h>
 #include <__memory/unique_ptr.h>
+#include <__memory/unique_temporary_buffer.h>
 #include <__utility/pair.h>
 #include <new>
 
@@ -208,16 +208,19 @@ _LIBCPP_HIDE_FROM_ABI void __inplace_merge(
     _BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last, _Compare&& __comp) {
   typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
   typedef typename iterator_traits<_BidirectionalIterator>::difference_type difference_type;
-  difference_type __len1     = _IterOps<_AlgPolicy>::distance(__first, __middle);
-  difference_type __len2     = _IterOps<_AlgPolicy>::distance(__middle, __last);
-  difference_type __buf_size = std::min(__len1, __len2);
-  // TODO: Remove the use of std::get_temporary_buffer
-  _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-  pair<value_type*, ptrdiff_t> __buf = std::get_temporary_buffer<value_type>(__buf_size);
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-  unique_ptr<value_type, __return_temporary_buffer> __h(__buf.first);
+  difference_type __len1                             = _IterOps<_AlgPolicy>::distance(__first, __middle);
+  difference_type __len2                             = _IterOps<_AlgPolicy>::distance(__middle, __last);
+  difference_type __buf_size                         = std::min(__len1, __len2);
+  __unique_temporary_buffer<value_type> __unique_buf = std::__allocate_unique_temporary_buffer<value_type>(__buf_size);
   return std::__inplace_merge<_AlgPolicy>(
-      std::move(__first), std::move(__middle), std::move(__last), __comp, __len1, __len2, __buf.first, __buf.second);
+      std::move(__first),
+      std::move(__middle),
+      std::move(__last),
+      __comp,
+      __len1,
+      __len2,
+      __unique_buf.get(),
+      __unique_buf.get_deleter().__count_);
 }
 
 template <class _BidirectionalIterator, class _Compare>

libcxx/include/__algorithm/stable_partition.h

@@ -16,8 +16,8 @@
 #include <__iterator/distance.h>
 #include <__iterator/iterator_traits.h>
 #include <__memory/destruct_n.h>
-#include <__memory/temporary_buffer.h>
 #include <__memory/unique_ptr.h>
+#include <__memory/unique_temporary_buffer.h>
 #include <__utility/move.h>
 #include <__utility/pair.h>
 #include <new>
@@ -132,14 +132,12 @@ __stable_partition_impl(_ForwardIterator __first, _ForwardIterator __last, _Pred
   // We now have a reduced range [__first, __last)
   // *__first is known to be false
   difference_type __len = _IterOps<_AlgPolicy>::distance(__first, __last);
+  __unique_temporary_buffer<value_type> __unique_buf;
   pair<value_type*, ptrdiff_t> __p(0, 0);
-  unique_ptr<value_type, __return_temporary_buffer> __h;
   if (__len >= __alloc_limit) {
-    // TODO: Remove the use of std::get_temporary_buffer
-    _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-    __p = std::get_temporary_buffer<value_type>(__len);
-    _LIBCPP_SUPPRESS_DEPRECATED_POP
-    __h.reset(__p.first);
+    __unique_buf = std::__allocate_unique_temporary_buffer<value_type>(__len);
+    __p.first    = __unique_buf.get();
+    __p.second   = __unique_buf.get_deleter().__count_;
   }
   return std::__stable_partition_impl<_AlgPolicy, _Predicate&>(
       std::move(__first), std::move(__last), __pred, __len, __p, forward_iterator_tag());
@@ -272,14 +270,12 @@ _LIBCPP_HIDE_FROM_ABI _BidirectionalIterator __stable_partition_impl(
   // *__last is known to be true
   // __len >= 2
   difference_type __len = _IterOps<_AlgPolicy>::distance(__first, __last) + 1;
+  __unique_temporary_buffer<value_type> __unique_buf;
   pair<value_type*, ptrdiff_t> __p(0, 0);
-  unique_ptr<value_type, __return_temporary_buffer> __h;
   if (__len >= __alloc_limit) {
-    // TODO: Remove the use of std::get_temporary_buffer
-    _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-    __p = std::get_temporary_buffer<value_type>(__len);
-    _LIBCPP_SUPPRESS_DEPRECATED_POP
-    __h.reset(__p.first);
+    __unique_buf = std::__allocate_unique_temporary_buffer<value_type>(__len);
+    __p.first    = __unique_buf.get();
+    __p.second   = __unique_buf.get_deleter().__count_;
   }
   return std::__stable_partition_impl<_AlgPolicy, _Predicate&>(
       std::move(__first), std::move(__last), __pred, __len, __p, bidirectional_iterator_tag());

libcxx/include/__algorithm/stable_sort.h

@@ -18,8 +18,8 @@
 #include <__debug_utils/strict_weak_ordering_check.h>
 #include <__iterator/iterator_traits.h>
 #include <__memory/destruct_n.h>
-#include <__memory/temporary_buffer.h>
 #include <__memory/unique_ptr.h>
+#include <__memory/unique_temporary_buffer.h>
 #include <__type_traits/is_trivially_assignable.h>
 #include <__utility/move.h>
 #include <__utility/pair.h>
@@ -241,14 +241,12 @@ __stable_sort_impl(_RandomAccessIterator __first, _RandomAccessIterator __last,
   using difference_type = typename iterator_traits<_RandomAccessIterator>::difference_type;
 
   difference_type __len = __last - __first;
+  __unique_temporary_buffer<value_type> __unique_buf;
   pair<value_type*, ptrdiff_t> __buf(0, 0);
-  unique_ptr<value_type, __return_temporary_buffer> __h;
   if (__len > static_cast<difference_type>(__stable_sort_switch<value_type>::value)) {
-    // TODO: Remove the use of std::get_temporary_buffer
-    _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-    __buf = std::get_temporary_buffer<value_type>(__len);
-    _LIBCPP_SUPPRESS_DEPRECATED_POP
-    __h.reset(__buf.first);
+    __unique_buf = std::__allocate_unique_temporary_buffer<value_type>(__len);
+    __buf.first  = __unique_buf.get();
+    __buf.second = __unique_buf.get_deleter().__count_;
   }
 
   std::__stable_sort<_AlgPolicy, __comp_ref_type<_Compare> >(__first, __last, __comp, __len, __buf.first, __buf.second);

libcxx/include/__memory/temporary_buffer.h

@@ -11,6 +11,7 @@
 #define _LIBCPP___MEMORY_TEMPORARY_BUFFER_H
 
 #include <__config>
+#include <__memory/unique_temporary_buffer.h>
 #include <__utility/pair.h>
 #include <cstddef>
 #include <new>
@@ -19,57 +20,27 @@
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER)
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>
 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_CFI _LIBCPP_DEPRECATED_IN_CXX17 pair<_Tp*, ptrdiff_t>
 get_temporary_buffer(ptrdiff_t __n) _NOEXCEPT {
-  pair<_Tp*, ptrdiff_t> __r(0, 0);
-  const ptrdiff_t __m =
-      (~ptrdiff_t(0) ^ ptrdiff_t(ptrdiff_t(1) << (sizeof(ptrdiff_t) * __CHAR_BIT__ - 1))) / sizeof(_Tp);
-  if (__n > __m)
-    __n = __m;
-  while (__n > 0) {
-#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
-    if (__is_overaligned_for_new(_LIBCPP_ALIGNOF(_Tp))) {
-      align_val_t __al = align_val_t(_LIBCPP_ALIGNOF(_Tp));
-      __r.first        = static_cast<_Tp*>(::operator new(__n * sizeof(_Tp), __al, nothrow));
-    } else {
-      __r.first = static_cast<_Tp*>(::operator new(__n * sizeof(_Tp), nothrow));
-    }
-#else
-    if (__is_overaligned_for_new(_LIBCPP_ALIGNOF(_Tp))) {
-      // Since aligned operator new is unavailable, return an empty
-      // buffer rather than one with invalid alignment.
-      return __r;
-    }
-
-    __r.first = static_cast<_Tp*>(::operator new(__n * sizeof(_Tp), nothrow));
-#endif
-
-    if (__r.first) {
-      __r.second = __n;
-      break;
-    }
-    __n /= 2;
-  }
-  return __r;
+  __unique_temporary_buffer<_Tp> __unique_buf = std::__allocate_unique_temporary_buffer<_Tp>(__n);
+  pair<_Tp*, ptrdiff_t> __result(__unique_buf.get(), __unique_buf.get_deleter().__count_);
+  __unique_buf.release();
+  return __result;
 }
 
 template <class _Tp>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_DEPRECATED_IN_CXX17 void return_temporary_buffer(_Tp* __p) _NOEXCEPT {
-  std::__libcpp_deallocate_unsized((void*)__p, _LIBCPP_ALIGNOF(_Tp));
+  __unique_temporary_buffer<_Tp> __unique_buf(__p);
+  (void)__unique_buf;
 }
 
-struct __return_temporary_buffer {
-  _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-  template <class _Tp>
-  _LIBCPP_HIDE_FROM_ABI void operator()(_Tp* __p) const {
-    std::return_temporary_buffer(__p);
-  }
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-};
-
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER)
+
 #endif // _LIBCPP___MEMORY_TEMPORARY_BUFFER_H

libcxx/include/__memory/unique_temporary_buffer.h

@@ -0,0 +1,92 @@
+// -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MEMORY_UNIQUE_TEMPORARY_BUFFER_H
+#define _LIBCPP___MEMORY_UNIQUE_TEMPORARY_BUFFER_H
+
+#include <__assert>
+#include <__config>
+
+#include <__memory/allocator.h>
+#include <__memory/unique_ptr.h>
+#include <__type_traits/is_constant_evaluated.h>
+#include <cstddef>
+#include <new>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Tp>
+struct __temporary_buffer_deleter {
+  ptrdiff_t __count_; // ignored in non-constant evaluation
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __temporary_buffer_deleter() _NOEXCEPT : __count_(0) {}
+  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_CONSTEXPR explicit __temporary_buffer_deleter(ptrdiff_t __count) _NOEXCEPT : __count_(__count) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 void operator()(_Tp* __ptr) _NOEXCEPT {
+    if (__libcpp_is_constant_evaluated()) {
+      allocator<_Tp>().deallocate(__ptr, __count_);
+      return;
+    }
+
+    std::__libcpp_deallocate_unsized((void*)__ptr, _LIBCPP_ALIGNOF(_Tp));
+  }
+};
+
+template <class _Tp>
+using __unique_temporary_buffer = unique_ptr<_Tp, __temporary_buffer_deleter<_Tp> >;
+
+template <class _Tp>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __unique_temporary_buffer<_Tp>
+__allocate_unique_temporary_buffer(ptrdiff_t __count) {
+  using __deleter_type       = __temporary_buffer_deleter<_Tp>;
+  using __unique_buffer_type = __unique_temporary_buffer<_Tp>;
+
+  if (__libcpp_is_constant_evaluated()) {
+    return __unique_buffer_type(allocator<_Tp>().allocate(__count), __deleter_type(__count));
+  }
+
+  _Tp* __ptr = nullptr;
+  const ptrdiff_t __max_count =
+      (~ptrdiff_t(0) ^ ptrdiff_t(ptrdiff_t(1) << (sizeof(ptrdiff_t) * __CHAR_BIT__ - 1))) / sizeof(_Tp);
+  if (__count > __max_count)
+    __count = __max_count;
+  while (__count > 0) {
+#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+    if (__is_overaligned_for_new(_LIBCPP_ALIGNOF(_Tp))) {
+      align_val_t __al = align_val_t(_LIBCPP_ALIGNOF(_Tp));
+      __ptr            = static_cast<_Tp*>(::operator new(__count * sizeof(_Tp), __al, nothrow));
+    } else {
+      __ptr = static_cast<_Tp*>(::operator new(__count * sizeof(_Tp), nothrow));
+    }
+#else
+    if (__is_overaligned_for_new(_LIBCPP_ALIGNOF(_Tp))) {
+      // Since aligned operator new is unavailable, constructs an empty buffer rather than one with invalid alignment.
+      return __unique_buffer_type();
+    }
+
+    __ptr = static_cast<_Tp*>(::operator new(__count * sizeof(_Tp), nothrow));
+#endif
+
+    if (__ptr) {
+      break;
+    }
+    __count /= 2;
+  }
+
+  return __unique_buffer_type(__ptr, __deleter_type(__count));
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___MEMORY_UNIQUE_TEMPORARY_BUFFER_H

libcxx/include/memory

@@ -182,8 +182,8 @@ public:
     raw_storage_iterator  operator++(int);
 };
 
-template <class T> pair<T*,ptrdiff_t> get_temporary_buffer(ptrdiff_t n) noexcept;
-template <class T> void               return_temporary_buffer(T* p) noexcept;
+template <class T> pair<T*,ptrdiff_t> get_temporary_buffer(ptrdiff_t n) noexcept; // deprecated in C++17, removed in C++20
+template <class T> void               return_temporary_buffer(T* p) noexcept;     // deprecated in C++17, removed in C++20
 
 template <class T> T* addressof(T& r) noexcept;
 template <class T> T* addressof(const T&& r) noexcept = delete;

libcxx/include/module.modulemap

@@ -1542,6 +1542,11 @@ module std_private_memory_unique_ptr                      [system] {
   export std_private_type_traits_is_pointer
   export std_private_type_traits_type_identity
 }
+module std_private_memory_unique_temporary_buffer         [system] {
+  header "__memory/unique_temporary_buffer.h"
+  export std_private_memory_unique_ptr
+  export std_private_type_traits_is_constant_evaluated
+}
 module std_private_memory_uses_allocator                  [system] { header "__memory/uses_allocator.h" }
 module std_private_memory_uses_allocator_construction     [system] { header "__memory/uses_allocator_construction.h" }
 module std_private_memory_voidify                         [system] { header "__memory/voidify.h" }

libcxx/include/syncstream

@@ -364,7 +364,7 @@ private:
   // TODO Use a more generic buffer.
   // That buffer should be light with almost no additional headers. Then
   // it can be use here, the __retarget_buffer, and place that use
-  // the now deprecated get_temporary_buffer
+  // the now removed get_temporary_buffer
 
   basic_string<_CharT, _Traits, _Allocator> __str_;
   bool __emit_on_sync_{false};

libcxx/test/libcxx/diagnostics/memory.nodiscard.verify.cpp

@@ -10,6 +10,7 @@
 
 // check that <memory> functions are marked [[nodiscard]]
 
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
 
 // clang-format off

libcxx/test/std/algorithms/alg.modifying.operations/alg.partitions/stable_partition.pass.cpp

@@ -282,9 +282,9 @@ test()
     assert(array[9] == P(0, 2));
   }
 #if TEST_STD_VER >= 11 && !defined(TEST_HAS_NO_EXCEPTIONS)
-  // TODO: Re-enable this test once we're no longer using get_temporary_buffer().
+  // TODO: Re-enable this test once we get recursive inlining fixed.
   // For now it trips up GCC due to the use of always_inline.
-#if 0
+#  if 0
   { // check that the algorithm still works when no memory is available
     std::vector<int> vec(150, 3);
     vec[5]                             = 6;
@@ -300,7 +300,7 @@ test()
     assert(std::is_partitioned(vec.begin(), vec.end(), [](int i) { return i < 5; }));
     getGlobalMemCounter()->reset();
   }
-#endif
+#  endif
 #endif // TEST_STD_VER >= 11 && !defined(TEST_HAS_NO_EXCEPTIONS)
 }
 

libcxx/test/std/utilities/memory/temporary.buffer/overaligned.pass.cpp

@@ -8,6 +8,7 @@
 
 // UNSUPPORTED: c++03
 
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
 
 // <memory>

libcxx/test/std/utilities/memory/temporary.buffer/temporary_buffer.pass.cpp

@@ -8,6 +8,7 @@
 
 // <memory>
 
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
 
 // template <class T>