[libcxx-commits] [libcxx] [llvm] [libc++] Remove `get_temporary_buffer`/`return_temporary_buffer` (PR #100914)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,122 @@
+// -*- 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_SCOPED_TEMPORARY_BUFFER_H
+#define _LIBCPP___MEMORY_SCOPED_TEMPORARY_BUFFER_H
+
+#include <__config>
+#include <__memory/allocator.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_allocation_result {
+  _Tp* __ptr;
+  ptrdiff_t __count;
+};
+
+template <class _Tp>
+class __scoped_temporary_buffer {
+public:
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __scoped_temporary_buffer() _NOEXCEPT
+      : __ptr_(NULL),
+        __count_(0) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __scoped_temporary_buffer(ptrdiff_t __count) _NOEXCEPT
+      : __ptr_(NULL),
+        __count_(0) {
+    __try_allocate(__count);
+  }
+
+  // This constructor is only needed to implement get_temporary_buffer().
+  // pre: __buf_ptr points to the beginning of a previously allocated scoped temporary buffer or is null
+  // notes: __count_ is ignored in non-constant evaluation
+  _LIBCPP_HIDE_FROM_ABI explicit __scoped_temporary_buffer(_Tp* __buf_ptr) _NOEXCEPT : __ptr_(__buf_ptr), __count_(0) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__scoped_temporary_buffer() _NOEXCEPT {
+    if (__libcpp_is_constant_evaluated()) {
+      if (__ptr_)
+        allocator<_Tp>().deallocate(__ptr_, __count_);
+      return;
+    }
+
+    std::__libcpp_deallocate_unsized((void*)__ptr_, _LIBCPP_ALIGNOF(_Tp));
+  }
+
+  __scoped_temporary_buffer(const __scoped_temporary_buffer&)            = delete;
+  __scoped_temporary_buffer& operator=(const __scoped_temporary_buffer&) = delete;
+
+  // pre: __ptr_ == nullptr && __count_ == 0
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __try_allocate(ptrdiff_t __count) _NOEXCEPT {
+    if (__libcpp_is_constant_evaluated()) {
+      __ptr_   = allocator<_Tp>().allocate(__count);
+      __count_ = __count;
+      return;
+    }
+
+    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;
+      }
+
+      __ptr_ = static_cast<_Tp*>(::operator new(__count * sizeof(_Tp), nothrow));
+#endif
----------------
ldionne wrote:

FWIW I'd be in favour of copy-pasting the implementation of `get_temporary_buffer()` (what was done here) and then refactoring it, to avoid any unintended fallout from this patch.

https://github.com/llvm/llvm-project/pull/100914