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Re: GCC extension for atomic access to members


On 09/19/2017 03:32 PM, Torvald Riegel wrote:
On Tue, 2017-09-19 at 07:19 +0200, Florian Weimer wrote:
On 09/18/2017 10:07 PM, Torvald Riegel wrote:
On Mon, 2017-09-18 at 14:19 +0200, Florian Weimer wrote:
I would like to see the GCC project to document that if the address of a
member is taken, this does not constitute an access to the object as a
whole.

That is, in the following code:

#include <stdatomic.h>

struct S {
     _Atomic int a;
     int b;
};

int
load_a (struct S *p)
{
     return atomic_load_explicit (&p->a, memory_order_relaxed);
}

int
store_b (struct S *p, int b)
{
     p->b = b;
}

If one thread calls load_a and another thread calls store_b on the same
struct S *, no data race happens.

There is no data race in this example; a and b are separate objects as
far as the memory model is concerned.  That's my understanding and I
believe also the understanding in the C++ committee.

I'm not sure what the C++ standard says in this matter.

C++ may indeed have addressed this and related issues (e.g., that a
memory location which is passed by reference to a function is not
actually read in the caller).

The memory models of C and C++ are intended to be the same.  In the
example above, the values of p in both calls are supposed to be equal,
so accesses go to the same object of type S; as far as the memory model
is concerned, a and b are separate objects.

The definition of “access” is quite different in C and C++, and this affects the definition of a data race. The core model is hopefully very similar, but the languages around that are quite different, so similar syntax can have very different behavior.

Thanks,
Florian


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