Re: [c++std-parallel-1632] Re: Compilers and RCU readers: Once more unto the breach!

["Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>]

On Thu, May 21, 2015 at 06:17:43PM +0200, Michael Matz wrote:
> Hi,
> 
> On Thu, 21 May 2015, Paul E. McKenney wrote:
> 
> > The point is -exactly- to codify the current state of affairs.
> 
> Ah, I see, so it's not yet about creating a more useful (for compilers, 
> that is) model.

There are several approaches being considered for that as well, but we
do need to codify current usage.

> > >   char * fancy_assign (char *in) { return in; }
> > >   ...
> > >   char *x, *y;
> > >   
> > >   x = atomic_load_explicit(p, memory_order_consume);
> > >   y = fancy_assign (x);
> > >   atomic_store_explicit(q, y, memory_order_relaxed);
> > > 
> > > So, is there, or is there not a dependency carried from x to y in your 
> > > proposed model (and which rule in your document states so)?  Clearly, 
> > > without any other language the compiler would have to assume that there is 
> > > (because the equivalent 'y = x' assignment would carry the dependency).  
> > 
> > The dependency is not carried, though this is due to the current set
> > of rules not covering atomic loads and stores, which I need to fix.
> 
> Okay, so with the current regime(s), the dependency carries ...

Yes, that is the intent.

> > o	Rule 14 says that if a value is part of a dependency chain and
> > 	is used as the actual parameter of a function call, then the
> > 	dependency chain extends to the corresponding formal parameter,
> > 	namely "in" of fancy_assign().
> > 
> > o	Rule 15 says that if a value is part of a dependency chain and
> > 	is returned from a function, then the dependency chain extends
> > 	to the returned value in the calling function.
> > 
> > o	And you are right.  I need to make the first and second rules
> > 	cover the relaxed atomic operations, or at least atomic loads and
> > 	stores.  Not that this is an issue for existing Linux-kernel code.
> > 
> > 	But given such a change, the new version of rule 2 would
> > 	extend the dependency chain to cover the atomic_store_explicit().
> 
> ... (if this detail would be fixed).  Okay, that's quite awful ...
> 
> > > If it has to assume this, then the whole model is not going to work 
> > > very well, as usual with models that assume a certain less-optimal 
> > > fact ("carries-dep" is less optimal for code generation purposes that 
> > > "not-carries-dep") unless very specific circumstances say it can be 
> > > ignored.
> > 
> > Although that is a good general rule of thumb, I do not believe that it 
> > applies to this situation, with the exception that I do indeed assume 
> > that no one is insane enough to do value-speculation optimizations for 
> > non-NULL values on loads from pointers.
> > 
> > So what am I missing here?
> 
> ... because you are then missing that if "carries-dep" can flow through 
> function calls from arguments to return values by default, the compiler 
> has to assume this in fact always happens when it can't see the function 
> body, or can't analyze it.  In effect that's making the whole "carries-dep 
> stops at these and those uses" a useless excercise because a malicious 
> user (malicious in the sense of abusing the model to show that it's 
> hindering optimizations), i.e. me, can hide all such carries-dep stopping 
> effects inside a function, et voila, the dependecy carries through.  So 
> for a slightly more simple example:
> 
>   extern void *foo (void *);  // body not available
>   x = load
>   y = foo (x);
>   store (y)
> 
> the compiler has to assume that there's a dep-chain from x to y; always.  

Yes, the compiler does have to make this assumption.  And the intent
behind the rules is to ensure that this assumption does not get in the
way of reasonable optimizations.  So although I am sure that you are as
busy as the rest of us, I really do need you to go through the rules in
detail before you get too much more excited about this.

> What's worse, it also has to assume a carries-dep for this:
> 
>   extern void foo (void *in, void **out1, void **out2);
>   x = load
>   foo (x, &o1, &o2);
>   store (o1);
>   store (o2);
> 
> Now the compiler has to assume that the body of 'foo' is just mean enough 
> to make the dep-chain carry from in to *out1 or *out2 (i.e. it has to 
> assume that for both).  This extends to _all_ memory accessible from foo's 
> body, i.e. generally all global and all local address-taken variables, so 
> as soon as you have a function call into which a dep-chain value flows 
> you're creating a dep-chain extension from that value to each and every 
> global piece of memory, because the compiler cannot assume that the 
> black box called foo is not mean.  This could conceivably be stopped by 
> making normal stores not to carry the dependency; then only the return 
> value might be infected; but I don't see that in your rules, as a "normal 
> store" is just an assigment in your model and hence rules 1 and 2 apply 
> (that is, carries-dep flows through all assignments, incl. loads and 
> stores).
> 
> Basically whenever you can construct black boxes for the compiler, you 
> have to limit their effects on such transitive relations like carries-dep 
> by default, at the border of such black boxes; otherwise that transitive 
> relation quickly becomes an most-x-everything relation (i.e. 
> mostthings carries-dep to everything), and as such is then totally 
> useless, because such a universally filled relation (like an empty one) 
> doesn't bear any interesting information, at which point it then is 
> questionably why the compiler should jump through hoops to analyse the few 
> cases that would be allowed to stop the carries-dep flow, when it more 
> often than not have to give up anyway and generate slow code.

Furthermore, in general, if the compiler loads a pointer (only a
pointer, not any other type) from some random location, it needs to
assume that the value loaded is part of a dependency chain.  Again,
the intent behind the rules is to make sure that the developers get the
guarantees that they need from the compiler without getting in the way
of reasonable optimizations.  (Value speculation of pointer loads being
the only example unreasonable optimization that I am aware of.)

> > Do you have a specific example where the compiler would need to suppress 
> > a production-quality optimization?
> 
> I can't say; I haven't completely grokked all details of the different sub 
> mem-models and their interaction with compiler optimizations, and when to 
> convert consume to aquire.  But I do know that transitive relations that 
> carry a code generation cost (i.e. that when two entities are related 
> you're limited in what you can do), let's call them infections :), have to 
> be quite strictly limited in scope, otherwise they become useless.

When I get the rules right (and I am sure that they need additional help),
it should never be necessary to promote consume to acquire.

> Real world examples that are quite terrible to get right, and get good 
> code out of at the same time, is aliasing and effective-type of memory 
> cells, which have some similar properties to the current carries-dep.  
> Both concepts added to the language after-the-fact (to capture some 
> assumptions that were used in practice, and that seemed sensible), but 
> then in a way that weren't limiting their scope very well.  For instance, 
> if you follow the c++ language strictly, you can't assume that a 
> anonymuous cell that was holding an int before a function call is still 
> holding an int afterwards, even without any stores in between, because the 
> function could use placement new to change the cells type.  Guess how GCC 
> deals with this?  We've designed and redesigned our memory model to 
> accomodate for this: It's conservatively assuming that crap-happens-here 
> is part of most function calls (because in the real world, it indeed is 
> the case that crap-happens-there) :)

Agreed, crap-happens-everywhere is a good approximation for real-world
code.  ;-)

But again, I believe that the rules do what needs to be done, although
some additional adjustments are no doubt needed.  I have updated the
document:

	http://www2.rdrop.com/users/paulmck/RCU/consume.2015.05.21b.pdf

Section 7.9 starts on page 28.

							Thanx, Paul