[swift-dev] [Swift 2.2] Request to merge pull request: Perform a dynamic method call if a class has objc ancestry

[Arnold Schwaighofer]

https://github.com/apple/swift/pull/1159

Perform a dynamic method call if a class has objc ancestry in specula
tive devirt as fallback.

If a class has an @objc ancestry this class can be dynamically overridden and
therefore we don't know the default case even if we see the full class
hierarchy.

rdar://23228386

Explanation:

Before this change we would devirtualize a method call to static calls of the potential call targets without a fallback to a class method lookup if we believed to have the full class hierarchy e.g in WMO mode. But during runtime this assumption can be violated because an objective-c class can be dynamically extended and so we would end up calling through the wrong method.

private class A : NSObject {
func foo() {...}
}
private class B : A { 
override foo() {...}
}

Before:

callAnA(a : A) {
 if (a isa A) {
  A.foo(a)
 } else {
  B.foo(a)
 }
}

After:

callAnA(a : A) {
 if (a isa A) {
  A.foo(a)
 } else if (a isa B) {
  B.foo(a)
 } else a.foo(a) // call through class method table.
}

Scope:

The change only effects whether we emit a default case that calls through the class method table. Emitting the call through the class method table is always safe. This risk is low.

Testing:

There is a unit test testing the change, furthermore the change was tested in the project reported in rdar://23228386 and only with this change the test scenario in the project works.

Reviewed by:
Roman, the author of the speculative virtualization pass, and Slava also took a look at it.