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?
On Thu, Dec 1, 2016 at 11:46 AM Roman Levenstein <rlevenst...@apple.com> wrote: > On Nov 30, 2016, at 9:40 PM, Jiho Choi <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. > > -Roman > > > On Wed, Nov 30, 2016 at 8:50 PM Roman Levenstein <rlevenst...@apple.com> > wrote: > > On Nov 30, 2016, at 6:25 PM, Jiho Choi via swift-dev <swift-dev@swift.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 > branch: > > https://github.com/swiftix/swift/blob/30409865ff49a4268363cd359f82f29c9a90cce8/lib/SILOptimizer/Transforms/NonAtomicRC.cpp > > > 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. > > -Roman > > > On Wed, Nov 30, 2016 at 11:41 AM John McCall <rjmcc...@apple.com> wrote: > > On Nov 30, 2016, at 8:33 AM, Jiho Choi via swift-dev <swift-dev@swift.org> > wrote: > Hi, > > I am new to Swift, and I have several questions about how ARC works in > Swift. > > 1. I read from one of the previous discussions in the swift-evolution list > ( > https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160208/009422.html) > 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. > > John. > > _______________________________________________ > swift-dev mailing list > swift-dev@swift.org > https://lists.swift.org/mailman/listinfo/swift-dev > > >
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