libcruft-util/alloc/raw/aligned/foreign.hpp
Danny Robson b23c274742 alloc/foreign: don't use a null base pointer
We can't use null for the proxy view passed to the successor allocators
because it triggers undefined behaviour.
2018-03-05 15:58:36 +11:00

118 lines
4.1 KiB
C++

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