Fwd: __attribute__((packed)) seems to break pointer arithmetic

[Maxim Yegorushkin <maxim dot yegorushkin at gmail dot com>]

Hi gcc-help,

I stumbled upon strange behaviour of pointer arithmetic when
__attribute__((packed)) is used. Please consider the following:

    $ cat test.cc
    #include <cstdio>
    #include <cstdint>

    struct __attribute__((packed)) Xyz {
        uint32_t seq_no;
        uint64_t end_offset;
    };

    struct Mapping {
        void* mem;
        size_t len;

        template<class T>
        T* begin() {
            return static_cast<T*>(mem);
        }

        template<class T>
        T* end() {
            auto e = reinterpret_cast<uintptr_t>(static_cast<char*>(mem) + len);
            return reinterpret_cast<T*>(e - e % sizeof(T));
        }
    };

    int main() {
        Mapping m{reinterpret_cast<void*>(0x40000u), 0x1000u};
        Xyz* beg = m.begin<Xyz>();
        Xyz* end = m.end<Xyz>();
        size_t len = end - beg;
        printf("%p\n", beg);
        printf("%p\n", end);
        printf("%zx\n", len);
    }

    $ g++ -o test -std=gnu++11 -Wall -Wextra test.cc

    $ ./test
    0x40000
    0x40ff8
    55555555555556aa <---- wrong

When __attribute__((packed)) is removed it correctly outputs:

    $ ./test
    0x40000
    0x41000
    100

I checked the documentation for __attribute__((packed)) and it says
nothing about it changing the behaviour of pointer arithmetic. Could
anyone shed some light on this strange behaviour please?

-- Maxim