Also noticed that with -O3 it produces correctly working code in both cases.
Maxim
On 19 February 2013 12:02, Maxim Yegorushkin
<maxim.yegorushkin@xxxxxxxxx> wrote:
> 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
