Re: reordering of trapping operations and volatile

[Michael Matz <matz@suse.de>]

Hello,

On Sat, 15 Jan 2022, Martin Uecker wrote:

> > Because it interferes with existing optimisations. An explicit 
> > checkpoint has a clear meaning. Using every volatile access that way 
> > will hurt performance of code that doesn't require that behaviour for 
> > correctness.
> 
> This is why I would like to understand better what real use cases of 
> performance sensitive code actually make use of volatile and are 
> negatively affected. Then one could discuss the tradeoffs.

But you seem to ignore whatever we say in this thread.  There are now 
multiple examples that demonstrate problems with your proposal as imagined 
(for lack of a _concrete_ proposal with wording from you), problems that 
don't involve volatile at all.  They all stem from the fact that you order 
UB with respect to all side effects (because you haven't said how you want 
to avoid such total ordering with all side effects).

As I said upthread: you need to define a concept of time at whose 
granularity you want to limit the effects of UB, and the borders of each 
time step can't simply be (all) the existing side effects.  Then you need 
to have wording of what it means for UB to occur within such time step, in 
particular if multiple UB happens within one (for best results it should 
simply be UB, not individual instances of different UBs).

If you look at the C++ proposal (thanks Jonathan) I think you will find 
that if you replace 'std::observable' with 'sequence point containing a 
volatile access' that you basically end up with what you wanted.  The 
crucial point being that the time steps (epochs in that proposal) aren't 
defined by all side effects but by a specific and explicit thing only (new 
function in the proposal, volatile accesses in an alternative).

FWIW: I think for a new language feature reusing volatile accesses as the 
clock ticks are the worse choice: if you intend that feature to be used 
for writing safer programs (a reasonable thing) I think being explicit and 
at the same time null-overhead is better (i.e. a new internal 
function/keyword/builtin, specified to have no effects except moving the 
clock forward).  volatile accesses obviously already exist and hence are 
easier to integrate into the standard, but in a given new/safe program, 
whenever you see a volatile access you would always need to ask 'is thise 
for clock ticks, or is it a "real" volatile access for memmap IO'.


Ciao,
Michael.