Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference

[Jonathan Wakely]

On 20 February 2015 at 11:06, Florian Weimer wrote:
> On 02/19/2015 09:56 PM, Sandra Loosemore wrote:
>> Hmmmm,  Passing the additional option in user code would be one thing,
>> but what about library code?  E.g., using memcpy (either explicitly or
>> implicitly for a structure copy)?
>
> The memcpy problem isn't restricted to embedded architectures.
>
>   size_t size;
>   const unsigned char *source;
>   std::vector<char> vec;
>   …
>   vec.resize(size);
>   memcpy(vec.data(), source, size);
>
> std::vector<T>::data() can return a null pointer if the vector is empty,
> which means that this code is invalid for empty inputs.
>
> I think the C standard is wrong here.  We should extend it, as a QoI
> matter, and support null pointers for variable-length inputs and outputs
> if the size is 0.  But I suspect this is still a minority view.

I'm inclined to agree.

Most developers aren't aware of the preconditions on memcpy, but GCC
optimizes aggressively based on those preconditions, so we have a
large and potentially dangerous gap between what developers expect and
what actually happens.