unaligned.hh

/*
 * This file is open source software, licensed to you under the terms
 * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
 * distributed with this work for additional information regarding copyright
 * ownership.  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 (C) 2015 Cloudius Systems, Ltd.
 */
 
#pragma once
 
// The following unaligned_cast<T*>(p) is a portable replacement for
// reinterpret_cast<T*>(p) which should be used every time address p
// is not guaranteed to be properly aligned to alignof(T).
//
// On architectures like x86 and ARM, where unaligned access is allowed,
// unaligned_cast will behave the same as reinterpret_cast and will generate
// the same code.
//
// Certain architectures (e.g., MIPS) make it extremely slow or outright
// forbidden to use ordinary machine instructions on a primitive type at an
// unaligned addresses - e.g., access a uint32_t at an address which is not
// a multiple of 4. Gcc's "undefined behavior sanitizer" (enabled in our debug
// build) also catches such unaligned accesses and reports them as errors,
// even when running on x86.
//
// Therefore, reinterpret_cast<int32_t*> on an address which is not guaranteed
// to be a multiple of 4 may generate extremely slow code or runtime errors,
// and must be avoided. The compiler needs to be told about the unaligned
// access, so it can generate reasonably-efficient code for the access
// (in MIPS, this means generating two instructions "lwl" and "lwr", instead
// of the one instruction "lw" which faults on unaligned/ access). The way to
// tell the compiler this is with __attribute__((packed)). This will also
// cause the sanitizer not to generate runtime alignment checks for this
// access.
 
#ifndef SEASTAR_MODULE
#include <type_traits>
#include <seastar/util/modules.hh>
#endif
 
namespace seastar {
 
SEASTAR_MODULE_EXPORT
template <typename T>
struct unaligned {
    // This is made to support only simple types, so it is fine to
    // require them to be trivially copy constructible.
    static_assert(std::is_trivially_copy_constructible_v<T>);
    T raw;
    unaligned() noexcept = default;
    unaligned(T x) noexcept : raw(x) {}
    unaligned& operator=(const T& x) noexcept { raw = x; return *this; }
    operator T() const noexcept { return raw; }
} __attribute__((packed));
 
 
template <typename T, typename F>
[[deprecated("violates strict aliasing rules. See issue #165.")]]
inline auto unaligned_cast(F* p) noexcept {
    return reinterpret_cast<unaligned<std::remove_pointer_t<T>>*>(p);
}
 
template <typename T, typename F>
[[deprecated("violates strict aliasing rules. See issue #165.")]]
inline auto unaligned_cast(const F* p) noexcept {
    return reinterpret_cast<const unaligned<std::remove_pointer_t<T>>*>(p);
}
 
}