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

[Arnold Schwaighofer]

Yes we could do something like this as an optimization I imagine. That should be done separately from this correctness bug, though.


> On Feb 2, 2016, at 10:02 AM, Joe Groff <jgroff at apple.com> wrote:
> 
> Could we use the method implementation pointer as the speculation key instead of the isa pointer? That makes the load chain a little longer, but should give you a guaranteed exhaustive key for Swift methods even with mixed ObjC heritage, since artificial ObjC subclasses still aren't allowed to override Swift methods that aren't explicitly `dynamic`.
> 
> -Joe
> 
>> On Feb 2, 2016, at 9:54 AM, Arnold Schwaighofer via swift-dev <swift-dev at swift.org> wrote:
>> 
>> 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.
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>