Re: shm_toc_lookup API

On 2017-06-05 14:57:10 -0400, Tom Lane wrote:
> > If it doesn't prevent both the hardware and the compiler from
> > reordering, it's broken.  See the comments for pg_read_barrier() in
> > atomics.h.
> 
> Meh.  Without volatile, I think that the compiler would be within its
> rights to elide the nentry local variable and re-fetch toc->toc_nentry
> each time through the loop.

I don't think that's true. Excerption from the docs of the macros:
About pg_read_barrier()
 * A read barrier must act as a compiler barrier, and in addition must
About pg_compiler_barrier():
 * A compiler barrier need not (and preferably should not) emit any actual
 * machine code, but must act as an optimization fence: the compiler must not
 * reorder loads or stores to main memory around the barrier.  However, the
 * CPU may still reorder loads or stores at runtime, if the architecture's
 * memory model permits this.
 */

Given that I don't see how it'd be permissible to elide the local
variable.  Are you saying that's permitted, or that our implementations
don't guarantee that?

- Andres