ydb/util/system/unaligned_mem.h

#pragma once

#include "defaults.h"
#include "yassert.h"

#include <string.h>
#include <type_traits>

// The following code used to have smart tricks assuming that unaligned reads and writes are OK on x86. This assumption
// is wrong because compiler may emit alignment-sensitive x86 instructions e.g. movaps. See IGNIETFERRO-735.

template <class T>
inline T ReadUnaligned(const void* from) noexcept {
    T ret;
    memcpy(&ret, from, sizeof(T));
    return ret;
}

// std::remove_reference_t for non-deduced context to prevent such code to blow below:
// ui8 first = f(); ui8 second = g();
// WriteUnaligned(to, first - second) (int will be deduced)
template <class T>
inline void WriteUnaligned(void* to, const std::remove_reference_t<T>& t) noexcept {
    memcpy(to, &t, sizeof(T));
}

template <class T, unsigned Align = sizeof(T)>
class TUnalignedMemoryIterator {
public:
    inline TUnalignedMemoryIterator(const void* buf, size_t len)
        : Current_((const unsigned char*)buf)
        , End_(Current_ + len)
        , AlignedEnd_(End_ - (len % Align))
    {
        Y_FAKE_READ(buf);
    }

    inline bool AtEnd() const noexcept {
        return Current_ == AlignedEnd_;
    }

    inline T Cur() const noexcept {
        Y_ASSERT(Current_ < AlignedEnd_ || sizeof(T) < Align);
        return ::ReadUnaligned<T>(Current_);
    }

    inline T Next() noexcept {
        T ret(Cur());

        Current_ += sizeof(T);

        return ret;
    }

    inline const unsigned char* Last() const noexcept {
        return Current_;
    }

    inline size_t Left() const noexcept {
        return End_ - Current_;
    }

private:
    const unsigned char* Current_;
    const unsigned char* End_;
    const unsigned char* AlignedEnd_;
};