I’ve been looking a lot into how Swift and Objective C collections
inter-convert because a bunch of necessary cleanup for ABI stability interacts
with it. Unfortunately I’ve run into some conflicting information with respect
to how stuff should work, and how it seems to actually work. Hoping y'all could
help me clear this up.
Here’s the literal state of the Swift codebase, as I currently understand it:
* String, Array, Dictionary, and Set are all effectively tagged unions of
"Native || Objc”.
* The ObjC case is the result of lazy bridging, and is basically just storing
an NSWhatever pointer.
* If any of these collections are in the ObjC state, then bridging to ObjC is
obvious and trivial. (yay!)
* If any of these collections are in the Native state:
Array: If the storage is verbatim bridgeable to ObjC (~it’s a class or objc
existential), just return the buffer (toll free). Otherwise, wrap the storage
in a _SwiftDeferredNSArray (not toll free). The first time someone tries to
access the contents of the _SwiftDeferredNSArray, a CAS-loop race occurs to
create a new buffer containing all the bridged values. The only alternative to
this would be bridging each element as it’s requested, but that’s ultimately
just a trade-off (and has issues if people are relying on pointer equality).
There has to be some toll here.
However the construction of the _SwiftDeferredNSArray is hypothetically
unnecessary, as alluded to in the comments on _SDNSArray. The class and its CAS
pointer could be embedded in the native array buffer. This would presumably
incur some bloat on all native Arrays, but it might not be too bad (and
platforms which don’t support _runtime(ObjC) presumably can omit it)? That
said, I’m not 100% clear if we have the machinery to accomplish this yet or
not. If anyone knows this, it would help a lot!
(there’s also some special case singleton for empty arrays, that all seems fine)
See:
ContiguousArrayBuffer.swift: _asCocoaArray
ContiguousArrayBuffer.swift: _getNonVerbatimBridgedHeapBuffer
SwiftNativeNSArray: _SwiftDeferredNSArray (the class that wraps the
storage)
Dictionary/Set: Looks to be pretty much the same thing as Array, except that
the old indexing model led to a double-indirection being baked into the design,
so a wrapper class doesn’t need to be constructed in the non-verbatim case.
(_NativeDictionaryStorageOwner contains the CAS-pointer). (toll free as can be!)
But this means that cleaning up all the gunk from the old indexing model and
removing this indirection will lead to a regression in bridging performance
*unless* Dictionary/Set gets the kind of optimizations discussed for Array. The
class inlining optimization also seem more acceptable for Dictionary, since
it’s necessarily a more bloated allocation than Array (less overhead by %).
See:
HashedCollections.swift.gyb: _bridgeToObjectiveCImpl
HashedCollections.swift.gyb: _Native${Self}StorageOwner (the “outer”
class)
String: Unconditionally construct a class that wraps the String’s storage. (not
toll free)
This just seems bad, and as far as I can tell isn’t expected. It seems to be
the result of _StringBuffer (the lowest-level type in the String abstraction
stack that still actually knows it’s a string) being a struct, and not a class
that inherits from _HeapBuffer due to some problems with deriving from generic
classes. I’m not 100% sure what the “fix” for this is supposed to be.
I think any fix will necessarily lead to String becoming pointer-sized, which
appears to be a desirable ABI feature anyway.
However this has tricky consequences for Strings which are actually sub-slices
of other Strings. At the limit, this will definitely require some slices which
don’t allocate (because they just create a new String pointing at the old
buffer with different start/length values) to start requiring an allocation
(because those fields will be in a class, and not a struct). Maybe stack
promotion and careful pointer-tagging can eliminate most allocations in
practice.
See:
StringBridge.swift: _stdlib_binary_bridgeToObjectiveCImpl
StringBridge.swift: _NSContiguousString (the class that wraps the
storage)
StringBuffer.swift: _StringBuffer (the type that wants to subclass
_HeapBuffer)
So that’s the situation as I understand it. Did I get anything wrong? Are there
any details I’m missing?
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