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Date: Wed, 10 May 2017 09:45:16 -0700 (PDT)
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Subject: Re: Portable memory barrier?
From: Robert Eachus <rieachus@comcast.net>
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Xref: news.eternal-september.org comp.lang.ada:46747
Date: 2017-05-10T09:45:16-07:00
List-Id: <comp.lang.ada>

On Wednesday, May 10, 2017 at 3:13:27 AM UTC-4, Dmitry A. Kazakov wrote:

> But you don't need elements atomic only index has to. If I understood=20
> Randy's explanation correctly atomic access to the buffer index will=20
> give you the barrier between index and element accesses which is all=20
> that is required.

That is my reading as well, and should result in efficient implementations.=
  Also, for portable code, there are times when Volatile is required.  But =
you really need ancient (in Internet years) hardware for Volatile to have a=
ny effect at all. (At least in non-erroneous programs.)  In general, Atomic=
 should be implemented as a RMW (read-modify-write) instruction on x86, and=
 will add fences on any hardware that requires them.

The other part, which occurs in this code, is that (user) Atomic variables =
should only be written by one task/thread.  Hmm.  Compiler back-ends should=
 probably insert a RMW of a non-program variable to cause synchronization w=
here an Atomic variable is only being read.  Of course, this would only eli=
minate most erroneous executions.  Best is for programmers to use protected=
 objects which can correctly (in an interrupt protected way) use RMWs for s=
ynchronization. =20