[Bug c++/56394] pointer arithmetic breaks with __attribute__((packed))

[maxim.yegorushkin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56394

Maxim Yegorushkin <maxim.yegorushkin at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |

--- Comment #3 from Maxim Yegorushkin <maxim.yegorushkin at gmail dot com> 2013-02-19 14:20:44 UTC ---
I can reproduce this without __attribute__((packed)) actually:

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

    struct Xyz
    {
        uint32_t seq_no;
        uint32_t end_offset_lo, end_offset_hi;
    };

    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("beg = %p\n", beg);
        printf("end = %p\n", end);
        printf("end - beg = %zx\n", len);
        printf("sizeof(Xyz) == %zu\n", sizeof(Xyz));
    }

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

    $ ./test
    beg = 0x40000
    end = 0x40ff8
    end - beg = 55555555555556aa <---- wrong
    sizeof(Xyz) == 12

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

    $ ./test
    beg = 0x40000
    end = 0x40ff8
    end - beg = 154 <---- correct
    sizeof(Xyz) == 12