to use it.
ConcurrentReadableHashMap is lock-free for readers, with writers using a lock to
ensure mutual exclusion amongst each other. The intent is to eventually replace
all uses ConcurrentMap with ConcurrentReadableHashMap.
ConcurrentReadableHashMap provides for relatively quick lookups by using a hash
table. Rearders perform an atomic increment/decrement in order to inform writers
that there are active readers. The design attempts to minimize wasted memory by
storing the actual elements out-of-line, and having the table store indices into
a separate array of elements.
The protocol conformance cache now uses ConcurrentReadableHashMap, which
provides faster lookups and less memory use than the previous ConcurrentMap
implementation. The previous implementation caches
ProtocolConformanceDescriptors and extracts the WitnessTable after the cache
lookup. The new implementation directly caches the WitnessTable, removing an
extra step (potentially a quite slow one) from the fast path.
The previous implementation used a generational scheme to detect when negative
cache entries became obsolete due to new dynamic libraries being loaded, and
update them in place. The new implementation just clears the entire cache when
libraries are loaded, greatly simplifying the code and saving the memory needed
to track the current generation in each negative cache entry. This means we need
to re-cache all requested conformances after loading a dynamic library, but
loading libraries at runtime is rare and slow anyway.
rdar://problem/67268325
Note that I've called out a couple of suspicious places where we
are requesting abstract metadata for superclasses but probably
need to be requesting something more complete.
Currently ignored, but this will allow future compilers to pass down source location information for cast
failure runtime errors without backward deployment constraints.
This is a funnel point for looking up the protocol conformance descriptor
for a given conforming type + conformance. Make it overridable in case we
need to back-deploy changes or fixes.
Implements rdar://problem/46281660.
Rename the funnel points for demangling strings/nodes to metadata to
swift_getTypeByMangled(Name|Node) and make them overridable. This will let
us back-deploy mangling improvements and bug fixes.
Simplify the signature of the internal _getTypeByMangledName() used by the
standard library to what we actually (currently) use. Drop it as a
compatibility override, because it’s not a useful place to introduce
customization.
