Am Mittwoch, den 14.04.2021, 21:51 +0300 schrieb Ville Voutilainen:
> On Wed, 14 Apr 2021 at 21:47, Jens Gustedt via Liaison
> <liaison_at_[hidden]> wrote:
> > Am 14. April 2021 20:07:18 MESZ schrieb JF Bastien <cxx_at_jfbastien.com>:
> > > On Wed, Apr 14, 2021 at 11:00 AM Uecker, Martin <Martin.Uecker_at_med.uni-goettingen.de> wrote:
> > > > Am Mittwoch, den 14.04.2021, 08:54 -0700 schrieb JF Bastien via Liaison:
> > > > > On Tue, Apr 13, 2021 at 11:40 AM Peter Sewell <Peter.Sewell_at_cl.cam.ac.uk>
> > > > > wrote:
> > > > > > - reading uninitialised representation bytes and padding bytes is also
> > > > > > necessary for other bytewise polymorphic operations: memcmp, marshalling,
> > > > > > encryption, and hashing (deferring what one knows about the results of
> > > > > > such reads for a moment). It's not clear how generally these operations
> > > > > > have to be supported, and we would like more data. Atomic cmpxchg on large
> > > > > > structs, implemented with locks, would do a memcmp/memcpy combination (in
> > > > > > fact is described as such in the standard).
> > > > > >
> > > > >
> > > > > For atomics with padding, C++20 adopted the following change (and I expect
> > > > > that compilers will implement it in previous versions as well):
> > > > > http://wg21.link/P0528
> > > >
> > > > I am not terribly excited about this solution.
> > > >
> > > > I think C should stick to the memcmp/memcpy semantics of cmpxchg
> > > > which operate on the representation including padding. This fits
> > > > to the hardware instructions, simplifies compiler design (no
> > > > need to look into each type), is easy to explain, handles all
> > > > cases consistently including unions, and is what most
> > > > C programmers would expect.
> > >
> > > OK, but it doesn't work, as explained in the paper.
> >
> > Well, there is actually not much of an explanation in the paper.
> >
> > And doesn't work isn't much of a description
>
> You need to look at the R0 revision
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0528r0.html

The closest thing which could apply to C is the following:

Padded infloop_maybe(Atomic* atomic) {
  Padded desired; // Padding unknown.
  Padded expected; // Could be different.
  peek("desired before", &desired);
  peek("expected before", &expected);
  peek("atomic before", atomic);
  while (
    !atomic->compare_exchange_strong(
      expected,
      desired // Padding bits added and removed here ˙ ͜ʟ˙
  ));
  peek("expected after", &expected);
  peek("atomic after", atomic);
  return expected; // Maybe changed here as well.
}

But the claim that this can loop indefinitely seems wrong.

The padding of desired is irrelevant.

If the padding of 'expected' is different from
the padding of 'atomic', then there is one additional
executation of the loop where 'expected' is
updated to a version with the right padding. Then
in the next round the compare exchange succeeds.


Best,
Martin