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

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Mar 4, 2015 at 12:38 AM, Jeff Law <law@redhat.com> wrote:
> On 03/03/15 12:57, Martin Sebor wrote:
>>
>>
>> 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
>> 65029).
>
> Right.  I actually spent quite a bit of time struggling with this a while
> back in a different context.   The only case I could come up with where GCC
> would generate an overlapping memcpy was self assignment, but even that was
> bad and while we ultimately punted, I've always considered it a wart.

?

struct A { int large[100]; };
void foo (struct A *x, struct A *y)
{
  *x = *y;
}

call it as foo (&a, &a);  (on x86 you need -mstringop-strategy=libcall,
even at -O0, to emit a memcpy call)

The self-assignment doesn't have to be visible to the compiler - so
to fix this we'd have to assume pointer equality everywhere and
either emit a conditional call to memcpy or always emit a call to
memmove.

Richard.

>
>  [*] IMO, one in favor of tightening up the memcpy specification
>>
>> to require implementations to provide the expected semantics.
>
> That works for me :-)
>
> The things done in glibc's memcpy are a bit on the absurd side and the pain
> caused by the changes over time is almost impossible to overstate.  If the
> Austin group tightens memcpy to require fewer surprises I think most
> developers would ultimately be happy with the result -- a few would complain
> about the performance impacts for specific workloads, but I suspect they'd
> be in the minority.
>
>
> jeff
>