Re: [RFC] tools/memory-model: Rule out OOTA

[Jonas Oberhauser]

Am 1/7/2025 um 11:06 AM schrieb Peter Zijlstra:
> On Mon, Jan 06, 2025 at 10:40:03PM +0100, Jonas Oberhauser wrote:
>
> > We first discuss why the more standard definition does not work well for
> > language models like LKMM. For example, consider
> >
> >      r1 = *a;
> >      *b = 1;
> >      if (*a == 1)
>
>        if (r1 == 1)
>
> ?
>
> >        *b = 1;
> >      *c = *b;
> >
> > In the execution where r1 == 1, there is a control dependency from
> > the load of *a to the second store to *b, from which the load to *b reads,
> > and the store to *c has a data dependency on this load from *b. Nevertheless
> > there is no semantic dependency from the load of *a to the store to *c; the
> > compiler could easily replace the last line with *c = 1 and move this line to
> > the top as follows:
> >
> >      *c = 1;
> >      r1 = *a;
> >      *b = 1;
> >
> > Since there is no order imposed by this sequence of syntactic dependencies
> > and reads, syntactic dependencies can not by themselves form an acyclic
> > relation.
> >
> > In turn, there are some sequences of syntactic dependencies and reads that do
> > form semantic dependencies, such as
> >
> >      r1 = *a;
> >      *b = 2;
> >      if (*a == 1)
>
>   r1 again?
>
> >        *b = 1;
> >      *c = *b;
> >
> > Here we would consider that the store to *c has a semantic data dependency on
> > the read from *a, given that depending on the result of that read, we store
> > either the value 1 or 2 to *c.


Yes on both counts, thanks!

   jonas