Re: LKMM: Making RMW barriers explicit

[Andrea Parri <parri.andrea@gmail.com>]

Alan, all,

("randomly" picking a recent post in the thread, after having observed
this discussion for a while...)

> It would be better if there was a way to tell herd7 not to add the 'mb 
> tag to failed instructions in the first place.  This approach is 
> brittle; see below.

AFAIU, changing the herd representation to generate mb-accesses in place
of certain mb-fences...

> If you do want to use this approach, it should be simplified.  All you 
> need is:
> 
> 	[M] ; po ; [RMW_MB]
> 
> 	[RMW_MB] ; po ; [M]
> 
> This is because events tagged with RMW_MB always are memory accesses, 
> and accesses that aren't part of the RMW are already covered by the 
> fencerel(Mb) thing above.

... and updating the .cat file to the effects of something like

  -let mb = ([M] ; fencerel(Mb) ; [M]) |
  +let mb = (([M] ; po? ; [Mb] ; po? ; [M]) \ id) |

... can hardly be called "making RMW barriers explicit".  (So much so
that the first commit in PR #865 was titled "Remove explicit barriers
from RMWs".  :-))

Overall, this discussion rather seems to confirm the close link between
tools/memory-model/ and herdtools7.  (After all, to what extent could
any putative RMW_MB be considered "explicit" without _knowing the under-
lying representation of the RMW operations...)  My understanding is that
this discussion was at least in part motivated by a desire to experiment
and familiarize with the current herd representation (that does indeed
require some getting-used-to...); this suggests, as some of you already
mentioned, to add some comments or a .txt in tools/memory-model/ in order
to document such representation and ameliorate that experience.  OTOH, I
must admit, I'm unable to see here sufficient motivation(tm) for changing
the current representation (and model): not the how, but the why...

  Andrea