On Nov 30, 2016, at 9:40 PM, Jiho Choi <email@example.com> wrote:
Thanks for providing the pointer.
Do you have any preliminary result or goal (e.g. the replacement ratio) of
Is it going to replace all ARC operations with non-atomic ones for
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
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 <firstname.lastname@example.org> > wrote:
On Nov 30, 2016, at 6:25 PM, Jiho Choi via swift-dev <email@example.com> > 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 <firstname.lastname@example.org> wrote:
On Nov 30, 2016, at 8:33 AM, Jiho Choi via swift-dev <email@example.com> > 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
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.
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