swift234

Introduce two new compiler-known library functions for performing bridging casts when conformances are known statically.

We define two new library functions _knownForceBridgeFromObjectiveC/_knownConditionallyBridgeFromObjectiveC, similar to _forceBridgeFromObjectiveC/_conditionallyBridgeFromObjectiveC. The main difference is that they require their arguments to conform to _BridgedToObjectiveC and _BridgedToObjectiveC. _ObjectiveCType accordingly. With this change, it is now possible to invoke the _BridgedToObjectiveC._forceBridgeFromObjectiveC witness directly, without going via the inefficient swift_bridgeNonVerbatimFromObjectiveC.

So now, for a cast O -> S, if it can be statically proven that an ObjC type O is bridgeable to a Swift type S implementing the _BridgedToObjectiveC protocol (i.e. O is the class (or its subclass) defined by the S._ObjectiveCType alias), we can generate a code to invoke the newly defined library function _knownForceBridgeFromObjectiveC/_knownConditionallyBridgeFromObjectiveC instead of _forceBridgeFromObjectiveC/_conditionallyBridgeFromObjectiveC.

After inlining, this will end-up invoking  S._forceBridgeFromObjectiveC directly instead of invoking a more general, but less effective swift_bridgeNonVerbatimFromObjectiveC, which always performs conformance checks at runtime, even if conformances are known statically. As a result, no conformance checks are performed at run-time if conformances are known statically.

The client code making use of these new APIs and the tests are coming in the subsequent commits.

The naming of the two new helper library functions was discussed with Dmitri.

This is part of the bridging casts optimization effort. And it is specifically useful for e.g. rdar://19081345.

Swift SVN r27100