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

[maxim.yegorushkin at gmail dot com]

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

             Bug #: 56394
           Summary: pointer arithmetic breaks with __attribute__((packed))
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: maxim.yegorushkin@gmail.com


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

If -O optimization switch added the output is correct:

    $ g++ -o test -O -std=gnu++11 -Wall -Wextra test.cc
    $ ./test
    0x40000
    0x40ff8
    154 <---- correct


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

    $ ./test
    0x40000
    0x41000
    100 <---- correct

I looked at the documentation for __attribute__((packed)) and it says nothing
about it changing the behaviour of pointer arithmetic.