> On Dec 1, 2016, at 4:55 PM, Jiho Choi <jray...@gmail.com> wrote:
> Thanks for the explanation.
> One last question is why non-atomic ARC operations still use atomic load and
> store. Wouldn't regular memory operations be enough?
They use atomic operations with __ATOMIC_RELAXED, which basically means that
they should load/store the counts as one single entity (i.e. a single 64bit
word) and not as multiple parts. Aside from that, it does not involve any
further overheads known from CAS/fetch-and-add type of atomic operations.
You can actually see the assembly for this functions to see which machine
instructions are generated.
> On Thu, Dec 1, 2016 at 11:46 AM Roman Levenstein <rlevenst...@apple.com
> <mailto:rlevenst...@apple.com>> wrote:
>> On Nov 30, 2016, at 9:40 PM, Jiho Choi <jray...@gmail.com
>> <mailto:jray...@gmail.com>> wrote:
>> Thanks for providing the pointer.
>> Do you have any preliminary result or goal (e.g. the replacement ratio) of
>> the optimization?
>> Is it going to replace all ARC operations with non-atomic ones for
>> single-threaded applications?
> In the ideal world, it would be nice to replace all ARC operations with
> non-atomic ones for single-threaded applications.
> But in reality, it is way more difficult as it may seem at the first glance.
> If this needs to happen without any hints from the developer, just by means
> of a static analysis of a program, then it is rather difficult. The main
> problem is that the compiler needs to reason whether a given reference may
> escape to another thread. For references created inside a function, we have
> rather good chances to figure out if a reference escapes the thread. But if
> the origin (i.e. how it was created or if it has escaped before) of a given
> reference is unknown, which is a typical case with function parameters or
> references inside class instances, then the compiler has to assume that any
> such reference has escaped its original thread and thus it needs to use
> atomic ARC-operations. Some sort of a global, whole-module/whole-program
> analysis may help here somewhat. But even if we would introduce such kind of
> analysis, it is likely to remain a problem for dynamic libraries and
> frameworks, because they don’t know and cannot reason which parameters
> required by their exposed APIs escaped in the user-code.
> Alternatively , a developer could provide a hint and assure that compiler
> that the app is single-threaded. One simple possibility could be to have a
> special -single-threaded compiler option, which would basically claim that
> the app being developed is single threaded and thus there is no need for
> performing the atomic ARC operations. In this case, all ARC operations would
> be marked non-atomic by default in the code emitted from the user-code. The
> problem with this option could be that if a user app starts multiple threads
> directly or indirectly (e.g. it calls a library API, which starts a new
> thread), even though the option claimed the app would not do it, and some
> references will be shared between threads, then the execution of such an app
> may become unpredictable and end up with hard to find crashes. Mixing object
> files and libraries where a subset is compiled with this option and another
> part without is another receipt for a disaster. So, one would need to be
> extremely cautious when using this option.
> There could be also something in between, where one would use special
> attributes indicating something related to thread-safety of a given
> reference/type/function/etc. These hints could help a compiler to reason
> about references and check if they may escape to a different thread.
>> On Wed, Nov 30, 2016 at 8:50 PM Roman Levenstein <rlevenst...@apple.com
>> <mailto:rlevenst...@apple.com>> wrote:
>>> On Nov 30, 2016, at 6:25 PM, Jiho Choi via swift-dev <email@example.com
>>> <mailto:firstname.lastname@example.org>> wrote:
>>> Thanks for clarifications. I have a couple of follow-up questions.
>>> 1. Could you please provide more information (e.g. source code location)
>>> about the optimization applying non-atomic reference counting? What's the
>>> scope of the optimization? Is it method-based?
>> The optimization itself is not merged yet. But all the required machinery,
>> e.g. non-atomic versions of the ARC operations, special non-atomic flag on
>> SIL instructions, etc is in place already.
>> As for the prototype implementation, you can find it here, on my local
>>> 2. Looking at the source code, I assume Swift implements immediate
>>> reference counting (i.e. immediate reclamation of dead objects) without
>>> requiring explicit garbage collection phase for techniques, such as
>>> deferred reference counting or coalescing multiple updates. Is it right?
>>> If so, is there any plan to implement such techniques?
>> Yes. It is a correct understanding.
>> Different extensions like deferred reference counting were discussed, but
>> there are no short-term plans to implement it anytime soon.
>>> On Wed, Nov 30, 2016 at 11:41 AM John McCall <rjmcc...@apple.com
>>> <mailto:rjmcc...@apple.com>> wrote:
>>>> On Nov 30, 2016, at 8:33 AM, Jiho Choi via swift-dev <email@example.com
>>>> <mailto:firstname.lastname@example.org>> wrote:
>>>> I am new to Swift, and I have several questions about how ARC works in
>>>> 1. I read from one of the previous discussions in the swift-evolution list
>>>> that ARC operations are currently not atomic as Swift has no memory model
>>>> and concurrency model. Does it mean that the compiler generates
>>>> non-atomic instructions for updating reference counts (e.g. using
>>>> incrementNonAtomic() instead of increment() in RefCount.h)?
>>> No. We have the ability to do non-atomic reference counting as an
>>> optimization, but we only trigger it when we can prove that an object
>>> hasn't escaped yet. Therefore, at the user level, retain counts are atomic.
>>> Swift ARC is non-atomic in the sense that a read/write or write/write race
>>> on an individual property/variable/whatever has undefined behavior and can
>>> lead to crashes or leaks. This differs from Objective-C ARC only in that a
>>> (synthesized) atomic strong or weak property in Objective-C does promise
>>> correctness even in the face of race conditions. But this guarantee is not
>>> worth much in practice because a failure to adequately synchronize accesses
>>> to a class's instance variables is likely to have all sorts of other
>>> unpleasant effects, and it is quite expensive, so we decided not to make it
>>> in Swift.
>>>> 2. If not, when does it use non-atomic ARC operations? Is there an
>>>> optimization pass to recognize local objects?
>>>> 3. Without the concurrency model in the language, if not using GCD (e.g.
>>>> all Swift benchmark applications), I assume Swift applications are
>>>> single-threaded. Then, I think we can safely use non-atomic ARC
>>>> operations. Am I right?
>>> When we say that we don't have a concurrency model, we mean that (1) we
>>> aren't providing a more complete language solution than the options
>>> available to C programmers and (2) like C pre-C11/C++11, we have not yet
>>> formalized a memory model for concurrency that provides formal guarantees
>>> about what accesses are guaranteed to not conflict if they do race. (For
>>> example, we are unlikely to guarantee that accesses to different properties
>>> of a struct can occur in parallel, but we may choose to make that guarantee
>>> for different properties of a class.)
>>>> 4. Lastly, is there a way to measure the overhead of ARC (e.g. a compiler
>>>> flag to disable ARC)?
>>> No, because ARC is generally necessary for correctness.
>>> swift-dev mailing list
>>> email@example.com <mailto:firstname.lastname@example.org>
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