Re: [PATCH] tools/memory-model: Document herd7 (internal) representation

[Jonas Oberhauser]

Am 5/28/2024 um 7:58 PM schrieb Boqun Feng:
> On Mon, May 27, 2024 at 03:40:13PM +0200, Jonas Oberhauser wrote:
> > Am 5/27/2024 um 3:28 PM schrieb Andrea Parri:
> > > > > +    |                smp_store_mb | W[once] ->po F[mb]                        |
> > > >
> > > > I expect this one to be hard-coded in herd7 source code, but I cannot find
> > > > it. Can you give me a pointer?
> > >
> > > smp_store_mb() is currently mapped to { __store{once}(X,V); __fence{mb}; } in
> > > the .def file, so it's semantically equivalent to "WRITE_ONCE(); smp_mb();".
> >
> > By the way, I experimented a little with these kind of mappings to see if we
> > can just explicitly encode the mapping there. E.g., I had an idea to use
> >      { __fence{mb-successful-rmw}; __cmpxchg{once}...;
> > __fence{mb-successful-rmw}; }
> >
> > for defining (almost) the current mapping of cmpxchg explicitly.
> >
> > But none of the changes I made were accepted by herd7.
> >
> > Do you know how the syntax works?
>
> This may not be trivial. Note that cmpxchg() is an expression (it has a
> value), so in .def, we want to define it as an expression. However, the
> C-like multiple-statement expression is not supported by herd parser, in
> other words we want:
>
> 	{
> 		__fence{mb-successful-rmw};
> 		int tmp = __cmpxchg{once}(...);
> 		__fence{mb-successful-rmw};
> 		tmp;
> 	}

Oh, you're right. Then probably the rule I was violating is that 
value-returning macros can not be defined with {} at all.

Given herd's other syntactic limitations, perhaps the best way would be 
to introduce these macros as

	x = cmpxchg(...) {
		__fence{mb-successful-rmw};
 		x = __cmpxchg{once}(...);
 		__fence{mb-successful-rmw};
	}

since I think x = M(...) is the only way we are allowed to use these 
macros anyways.