Re: reordering of trapping operations and volatile

[Martin Uecker <ma.uecker@gmail.com>]

Am Dienstag, den 11.01.2022, 08:11 +0100 schrieb Richard Biener:
> On Mon, Jan 10, 2022 at 6:36 PM Martin Uecker <ma.uecker@gmail.com> wrote:
> > Am Montag, den 10.01.2022, 10:04 +0100 schrieb Richard Biener:

Hi Richard,

> > > > For volatile, it seems this would need some tweaks.
> > > 
> > > Yes, likewise when re-ordering (observable) traps like
> > > 
> > >   r = a / b;
> > >   q = c / d;
> > 
> > I think this could also be useful. But at the moment I am
> > concerned about the effect previous defined behavior
> > being affected. For this, reordering traps is OK.  Also
> > sinking traps across observable behavior is OK. Just
> > hoisting it up across observable behavior would
> > be a problem.
> 
> But in general that would apply to all UB.  Consider
> 
> int foo (int a, int b)
> {
>    if (a < b)
>      return a + b;
>    bar ();
>    return a + b;
> }
> 
> we are happily hoisting a + b to the start of the function
> but of course a + b can invoke UB.  We consider that to
> not matter because we eventually invoke this UB anyway.
> Unless of course bar() does not return.

Yes.

> I realize that UB in a + b isn't (usually) observable but
> UB resulting in traps are.

Code motion for UB which then does not cause
a change in observable behavior would still be ok.

So my understanding is that you can not hoist a potentially
trapping operation across a function call, but if it is
UB which is implemented in way that just produces some
random result but does not trap then this is ok.

It would also be wrong if it affects the arguments for
the function call. Here MSVC seems to do this:

https://godbolt.org/z/8a8fTW8qP

This seems incorect because if the call does not
return there is no UB. I did not observe this with
GCC or another compiler.

> So I'm still wondering why you think that 'volatile' makes
> a critical difference we ought to honor?  I don't remember
> 'volatile' being special in the definition of the abstract
> machine with regarding to observability (as opposed to
> sequence points).

It is because it is used for I/O.   Sequence points only
matter for the semantics of the abstract machine, so
according to "as-if" rule optimizers can do whatever
they want as long as the observable behavior is the same
"as-if" it followed the rules of the abstract machine.

This observable behavior that needs to be preserved is
defined as I/O and volatile accesses. The relevant
part o the standard is this:

"5.1.2.3 Program execution" paragraph 6

The least requirements on a conforming implementation are:

— Accesses to volatile objects are evaluated strictly
according to the rules of the abstract machine.
— At program termination, all data written into files
shall be identical to the result that execution
of the program according to the abstract semantics would 
have produced.
— The input and output dynamics of interactive devices 
shall take place as specified in 7.21.3.

The intent of these requirements is that unbuffered or
line-buffered output appear as soon as possible, to 
ensure that prompting messages actually appear prior
to a program waiting for input.

This is the observable behavior of the program."



Martin

> > > > I am trying to figure out whether this is feasible.
> > > 
> > > For PRE yes, you'd just need to include the observable stmts you
> > > care in the set of stmts that cause PRE to set BB_MAY_NOTRETURN.
> > > In general this is of course harder.
> > 
> > What other passes would need to be checked?
> 
> All that do code motion by design or by accident.  The difficulty is
> that the resulting "wrong IL" is not wrong per se, just the difference is
> which is hard to write a checker for (well, in priciple you could copy the
> IL before passes and compare to the IL after)
> 
> > And do you think there is any negative impact on
> > an important optimization (considering this affects
> > only volatile accesses)?
> 
> Probably not.  But then semantics of 'volatile' are very weak defined
> so I'd like
> to see a reference to a part of the standard that supports declaring this
> (and only this - the 'volatile' case) a bug.
> 
> > > > > GCC assumes by default that divide is trappable but stores not are not
> > > > > observable. This is where -fnon-call-exceptions come into play.
> > > > 
> > > > Ok, thanks! I will look at this!
> > > > 
> > > > > In the second case, GCC assumes reducing trappable instructions are
> > > > > fine.
> > > > 
> > > > -fnon-call-exceptions would treat trapping instructions
> > > > as defined (and trapping) instead of UB? This is
> > > > then probably even stronger than the requirement above.
> > > 
> > > No, I don't think it turns UB into defined behavior.  Some frontends might
> > > expect that to some extent.  So even with -fnon-call-exceptions we'd
> > > happily do the re-ordering unless the exception is catched in the same
> > > function.
> > 
> > Thanks,
> > Martin
> > 
> > > > > Note I thought -fno-delete-dead-exceptions would fix the sink
> > > > > but it didn't.
> > > > 
> > > > Martin
> > > > 
> > > >