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

On 02/20/2015 10:01 AM, Jeff Law wrote:
On 02/20/15 05:10, Jakub Jelinek wrote:
On Fri, Feb 20, 2015 at 12:06:28PM +0100, 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;
   memcpy(, 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 disagree.  If you want a function that will have that different
don't call it memcpy.
Right.  If someone wants to take it up with the Austin group, that's
fine. But until/unless the Austin group blesses, I don't think we should
extend as a QoI matter.

As a data point(*) it might be interesting to note that GCC itself
relies on memcpy providing stronger guarantees than the C standard
requires it to by emitting calls to the function for large structure
self-assignments (which are strictly conforming, as discussed in bug


[*] IMO, one in favor of tightening up the memcpy specification
to require implementations to provide the expected semantics.

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