[swift-users] Atomics and Memory Fences in Swift

[Anders Ha]

What about `withUnsafeMutablePointer` on a stored object property, marked with `final`? e.g. `withUnsafeMutablePointer(to: &object.lock, os_unfair_lock_lock)`.

The generated object code with `-Owmo` shows that it is optimised to an address calculation immediately followed by a call to `os_unfair_lock_lock`. But I am not quite sure about its behaviour with no optimisation flag.


Regards,
Anders

> On 6 Dec 2016, at 1:27 AM, Joe Groff <jgroff at apple.com> wrote:
> 
>> 
>> On Dec 4, 2016, at 4:53 PM, Andrew Trick via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> 
>>> On Nov 30, 2016, at 5:40 AM, Anders Ha via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>> 
>>> Hi guys
>>> 
>>> I have recently started adopting lock-free atomics with memory fences, but it seems Swift at this moment does not have any native instruments.
>>> 
>>> Then I read a thread in the Apple Developer Forum (https://forums.developer.apple.com/thread/49334 <https://forums.developer.apple.com/thread/49334>), which an Apple staff claimed that all imported atomic operations are "not guaranteed to be atomic". But for my tests with all optimizations enabled (-Owholemodule and -O), the OSAtomic primitives and stdatomic fences do not seem going wild.
>>> 
>>> Is these `atomic_*` and `OSAtomic*` primitives really unsafe in Swift as claimed? It doesn't seem like the Swift compiler would reorder memory accesses around a C function call that it wouldn't be able to see through.
>> 
>> Did you get an answer to this? I’m not sure what led you to believe the primitives are unsafe in Swift. Importing them doesn’t change their semantics.
> 
> If you apply them to memory you allocated manually with malloc/free on UnsafeMutablePointer's allocation methods, then yeah, they should work as they do in C. That's the safest way to use these functions today. Passing a Swift `var` inout to one of these functions does not guarantee that accesses to that var will maintain atomicity, since there may be bridging or reabstracting conversions happening under the hood.
> 
> -Joe

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