Danny Robson
b23c274742
We can't use null for the proxy view passed to the successor allocators because it triggers undefined behaviour.
118 lines
4.1 KiB
C++
118 lines
4.1 KiB
C++
/*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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* Copyright 2016 Danny Robson <danny@nerdcruft.net>
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*/
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#ifndef CRUFT_UTIL_ALLOC_RAW_ALIGNED_OFFSET_HPP
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#define CRUFT_UTIL_ALLOC_RAW_ALIGNED_OFFSET_HPP
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#include "direct.hpp"
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#include "../../../pointer.hpp"
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#include "../../../debug.hpp"
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#include <cstddef>
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#include <utility>
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namespace util::alloc::raw::aligned {
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/// wraps a child allocator and enforces a fixed alignment that is
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/// independant of the alignment of the provided source buffer.
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///
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/// we use the approach of:
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/// * shifting the source buffer's alignment to satisfy the requested
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/// alignment
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/// * using a direct alignment child to service the requests
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/// * then applying the reverse offset to values as we return the values
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///
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/// This approach will explode if a child allocator wants to write to the
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/// range, so the user is mostly restricted to very simple allocators
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/// (like 'linear').
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///
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/// We supply a proxy view at the (probably) invalid address:
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/// (void*)alignment
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/// This removes any potential bias in the successor allocators. We very
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/// specifically cannot use nullptr because it breaks under optimisations
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/// due to undefined behaviour of null pointers.
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///
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/// The proxy view will _probably_ result in a segfault if the successor
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/// allocator attempts to read/write directly to it given typical
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/// alignments will give addresses far below typical mappings.
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template <typename ChildT>
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class foreign {
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public:
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template <typename ...Args>
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foreign (util::view<std::byte*> _data, std::size_t _alignment, Args &&...args):
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m_successor (
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view<std::byte*> {
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reinterpret_cast<std::byte*> (_alignment),
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reinterpret_cast<std::byte*> (_alignment + _data.size ()),
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},
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_alignment,
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std::forward<Args> (args)...
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),
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m_offset (_data.data () - m_successor.data ()),
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m_alignment (_alignment)
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{ ; }
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void*
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allocate (std::size_t size)
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{
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auto ptr= reinterpret_cast<std::byte*> (
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m_successor.allocate (size)
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);
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return ptr + m_offset;
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}
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auto
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deallocate (void *ptr, std::size_t size)
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{
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return m_successor.deallocate (
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reinterpret_cast<std::byte*> (ptr) - m_offset, size
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);
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}
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constexpr auto alignment (void) const noexcept { return m_alignment; }
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auto offset (const void *ptr) const
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{
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return m_successor.offset (
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reinterpret_cast<const std::byte*> (ptr) - m_offset
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);
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}
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auto data (void) { return m_successor.data () + m_offset; }
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auto data (void) const { return m_successor.data () + m_offset; }
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auto begin (void) { return m_successor.begin () + m_offset; }
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auto begin (void) const { return m_successor.begin () + m_offset; }
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auto end (void) { return m_successor.end () + m_offset; }
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auto end (void) const { return m_successor.end () + m_offset; }
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auto reset (void) { return m_successor.reset (); }
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auto capacity (void) const { return m_successor.capacity (); }
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auto used (void) const { return m_successor.used (); }
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auto remain (void) const { return m_successor.remain (); }
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private:
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direct<ChildT> m_successor;
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std::ptrdiff_t m_offset;
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std::size_t m_alignment;
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};
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}
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#endif
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