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SubjectRe: [RFC][PATCH 0/5] arch: atomic rework
On Mon, Feb 24, 2014 at 02:55:07PM +0100, Michael Matz wrote:
> Hi,
>
> On Fri, 21 Feb 2014, Paul E. McKenney wrote:
>
> > > And with conservative I mean "everything is a source of a dependency, and
> > > hence can't be removed, reordered or otherwise fiddled with", and that
> > > includes code sequences where no atomic objects are anywhere in sight [1].
> > > In the light of that the only realistic way (meaning to not have to
> > > disable optimization everywhere) to implement consume as currently
> > > specified is to map it to acquire. At which point it becomes pointless.
> >
> > No, only memory_order_consume loads and [[carries_dependency]]
> > function arguments are sources of dependency chains.
>
> I don't see [[carries_dependency]] in the C11 final draft (yeah, should
> get a real copy, I know, but let's assume it's the same language as the
> standard). Therefore, yes, only consume loads are sources of
> dependencies. The problem with the definition of the "carries a
> dependency" relation is not the sources, but rather where it stops.
> It's transitively closed over "value of evaluation A is used as operand in
> evaluation B", with very few exceptions as per 5.1.2.4#14. Evaluations
> can contain function calls, so if there's _any_ chance that an operand of
> an evaluation might even indirectly use something resulting from a consume
> load then that evaluation must be compiled in a way to not break
> dependency chains.
>
> I don't see a way to generally assume that e.g. the value of a function
> argument can impossibly result from a consume load, therefore the compiler
> must assume that all function arguments _can_ result from such loads, and
> so must disable all depchain breaking optimization (which are many).
>
> > > [1] Simple example of what type of transformations would be disallowed:
> > >
> > > int getzero (int i) { return i - i; }
> >
> > This needs to be as follows:
> >
> > [[carries_dependency]] int getzero(int i [[carries_dependency]])
> > {
> > return i - i;
> > }
> >
> > Otherwise dependencies won't get carried through it.
>
> So, with the above do you agree that in absense of any other magic (see
> below) the compiler is not allowed to transform my initial getzero()
> (without the carries_dependency markers) implementation into "return 0;"
> because of the C11 rules for "carries-a-dependency"?
>
> If so, do you then also agree that the specification of "carries a
> dependency" is somewhat, err, shall we say, overbroad?

From what I can see, overbroad. The problem is that the C++11 standard
defines how carries-dependency interacts with function calls and returns
in 7.6.4, which describes the [[carries_dependency]] attribute. For example,
7.6.4p6 says:

Function g’s second parameter has a carries_dependency
attribute, but its first parameter does not. Therefore, function
h’s first call to g carries a dependency into g, but its second
call does not. The implementation might need to insert a fence
prior to the second call to g.

When C11 declined to take attributes, they also left out the part saying
how carries-dependency interacts with functions. :-/

Might be fixed by now, checking up on it.

One could argue that the bit about emitting fence instructions at
function calls and returns is implied by the as-if rule even without
this wording, but...

> > > depchains don't matter, could _then_ optmize it to zero. But that's
> > > insane, especially considering that it's hard to detect if a given context
> > > doesn't care for depchains, after all the depchain relation is constructed
> > > exactly so that it bleeds into nearly everywhere. So we would most of
> > > the time have to assume that the ultimate context will be depchain-aware
> > > and therefore disable many transformations.
> >
> > Any function that does not contain a memory_order_consume load and that
> > doesn't have any arguments marked [[carries_dependency]] can be
> > optimized just as before.
>
> And as such marker doesn't exist we must conservatively assume that it's
> on _all_ parameters, so I'll stand by my claim.

Or that you have to emit a fence instruction when a dependency chain
enters or leaves a function in cases where all callers/calles are not
visible to the compiler.

My preference is that the ordering properties of a carries-dependency
chain is implementation defined at the point that it enters or leaves
a function without the marker, but others strongly disagreed. ;-)

> > > Then inlining getzero would merely add another "# j.dep = i.dep"
> > > relation, so depchains are still there but the value optimization can
> > > happen before inlining. Having to do something like that I'd find
> > > disgusting, and rather rewrite consume into acquire :) Or make the
> > > depchain relation somehow realistically implementable.
> >
> > I was actually OK with arithmetic cancellation breaking the dependency
> > chains. Others on the committee felt otherwise, and I figured that (1)
> > I wouldn't be writing that kind of function anyway and (2) they knew
> > more about writing compilers than I. I would still be OK saying that
> > things like "i-i", "i*0", "i%1", "i&0", "i|~0" and so on just break the
> > dependency chain.
>
> Exactly. I can see the problem that people had with that, though. There
> are very many ways to write conceiled zeros (or generally neutral elements
> of the function in question). My getzero() function is one (it could e.g.
> be an assembler implementation). The allowance to break dependency chains
> would have to apply to such cancellation as well, and so can't simply
> itemize all cases in which cancellation is allowed. Rather it would have
> had to argue about something like "value dependency", ala "evaluation B
> depends on A, if there exist at least two different values A1 and A2
> (results from A), for which evaluation B (with otherwise same operands)
> yields different values B1 and B2".

And that was in fact one of the arguments used against me. ;-)

> Alas, it doesn't, except if you want to understand the term "the value of
> A is used as an operand of B" in that way. Even then you'd still have the
> second case of the depchain definition, via intermediate not even atomic
> memory stores and loads to make two evaluations be ordered per
> carries-a-dependency.
>
> And even that understanding of "is used" wouldn't be enough, because there
> are cases where the cancellation happens in steps, and where it interacts
> with the third clause (transitiveness): Assume this:
>
> a = something() // evaluation A
> b = 1 - a // evaluation B
> c = a - 1 + b // evaluation C
>
> Now, clearly B depends on A. Also C depends on B (because with otherwise
> same operands changing just B also changes C), because of transitiveness C
> then also depends on A. But equally cleary C was just an elaborate way to
> write "0", and so depends on nothing. The problem was of course that A
> and B weren't independent when determining the dependencies of C. But
> allowing cancellation to break dependency chains would have to allow for
> these cases as well.
>
> So, now, that leaves us basically with depchains forcing us to disable
> many useful transformation or finding some other magic. One would be to
> just regard all consume loads as acquire loads and be done (and
> effectively remove the ill-advised "carries a dependency" relation from
> consideration).
>
> You say downthread that it'd also be possible to just emit barriers before
> all function calls (I say "all" because the compiler will generally
> have applied some transformation that broke depchains if they existed).
> That seems to me to be a bigger hammer than just ignoring depchains and
> emit acquires instead of consumes (because the latter changes only exactly
> where atomics are used, the former seems to me to have unbounded effect).

Yep, converting the acquire to a consume is a valid alternative to
emitting a memory-barrier instruction prior to entering/exiting the
function in question.

> So, am still missing something or is my understanding of the
> carries-a-dependency relation correct and my conclusions are merely too
> pessimistic?

Given the definition as it is, I believe you understand it.

Thanx, Paul

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