[csmith-dev] Alignment of struct members cause target dependent checksum

[John Regehr]

Thanks for the clarification Arthur and Derek!

John



On 06/05/2013 12:48 PM, Arthur O'Dwyer wrote:
> On Wed, Jun 5, 2013 at 11:14 AM, John Regehr <regehr at cs.utah.edu> wrote:
>>> described the problem correctly, the test program was definitely UB.)
>>
>> Actually I'm not sure this is true.  Can you justify this claim, Arthur?  My
>> read of 6.2.6.1.7 in C99 and C11 is that it's unspecified.
>
> Hm. My kneejerk "definitely UB" was based on the situation before DR
> 283. (Yikes, I'm turning into one of those language pedants whose
> pedantry ossified ten years ago!)  The C11 standard incorporates the
> footnote issued in response to DR 283:
>
> 6.5.2.3#3, footnote 95:
>> If the member used to read the contents of a union object is not
>> the same as the member last used to store a value in the object,
>> the appropriate part of the object representation of the value is
>> reinterpreted as an object representation in the new type as
>> described in 6.2.6 (a process sometimes called ‘‘type punning’’).
>> This might be a trap representation.
>
> So I retract my "definitely UB" claim. I think the actual situation is:
> - the byte-sequence at that location is unspecified;
> - for any given byte-sequence, its interpretation is implementation-defined;
> - certain implementation-defined byte-sequences ("trap
> representations") will give you undefined behavior;
> - but there are no trap representations on sane platforms such as x86.
>
> The fact that the stored byte-sequence is "unspecified" (6.2.6.1#6)
> means that the implementation doesn't have to make a consistent
> choice; it's totally okay for the behavior of Kees' test case at -O1
> to differ from its behavior at -O2. However, it's also true that the
> GCC or Clang developers might consider any such differences to be
> fix-worthy bugs.
>
> Derek sent his reply as I was composing this one... Derek, unless
> you're referring to trap representations (which we can agree don't
> apply in this case) or using "bite" as a synonym for something less
> severe than UB, I actually agree more with John than with you.
>
> –Arthur
>