Are there any conditions where the compiler can optimize away the call to objc_msgSend? Or is it always used for any call between 2 ObjC/Swift methods?
There is never a way for the compiler to do this safely and automatically.
That said, there are ways for the programmer to do this manually if they know that it is safe to do so.
It would be nice if the compiler would help in these situations.
The interesting thing is that by inlining, for the inline case you will often gain additional type information. E.g. to take an example from Ruby, since I don't know Objective C very well. In isolation you have no way of telling what type "foo + 1 + 2 + 3" will return, as it depends entirely on "foo". But lets say most call-sites calling the method where this expression is found passes an integer.
If I can guarantee that "foo + 1" is an Integer addition, then I know it will return an Integer, and so I know the same addition method will be used for the next addition (and by extension the next as well), so I can turn the above into the following Ruby-ish pseudo-code instead:
if foo.is_a?(Integer)
# By recursively inlining, I know not just the type of foo, but the type of the full expression.
inlined foo + 1 + 2 + 3
else
foo.send(:+, 1).send(:+, 2).send(:+, 3)
end
The polymorphic inline caching from Self for example is guided by collecting simple stats. Tracing does the same. A JIT ensures those stats are always completely up to date, but nothing stops you from saving it and using it for an AOT compiler as well.
But often even that is overkill, as you can often statically deduce a lot about the types a method is likely to get called with by simply looking at the call sites, and most programs have very static call profiles.
In my Ruby compiler project which has to deal with the same level of dynamic behaviour, I handle dynamic overriding of methods with C++-style vtables, which turns method calls into the equivalent of this C-ish pseudo-code:
(* ob->vtable[some_method_offset])(args...)
Since Ruby classes can have methods overridden at any time, if class Bar inherits from class Foo, inherits from class Object, and I override a method in class Object, that again has previously been explicitly overridden in class Bar, this would happen:
- Store pointer to method in Object in ptr.
- Replace method pointer in Object's vtable.
- Iterate over all direct sub-classes of Object (but here we only care about Foo)
- Compare the same method offset against ptr. Since Foo has not overridden the method, it matches.
- Replace the pointer in Foo, and iterate over all direct sub-classes of Foo (but here we only care about Bar)
- Compare the method offset against ptr. Since Bar has overridden the method, it doesn't match, so leave it alone.
Method calls on the other hand are about as cheap as virtual method calls in C++, except when you hit method_missing where this approach gives you a very low extra overhead of tweaking the stack to add the symbol and jumping to the method_missing implementation.
This overall approach works for most dynamic languages. The caveat is memory - if you have an application with very large class hierarchies in a language where they are all singly rooted (as in Ruby where they all ultimately inherit from SimpleObject), each vtable will cost you at last pointer_size*global_number_of_method_names. In practice so far I've not seen all that many cases where this is a problem, and it's always possible for the compiler to set a roof above which it will resort to a slow send mechanism (because you'll need to support that anyway in any language that allows dynamic send mechanics; e.g. in Ruby you can always send a message to an object by a dynamically obtained symbol so you still need the equivalent of objc_msgSend as well).
A slightly cheaper approach in terms of memory was described by Michael Franz[1]. His approach was to group methods in interfaces, so instead of a vtable of method pointers, you had a vtable of pointers to interfaces with pointers to methods. You save memory as most classes would typically implement most or none of the methods of an interface; it provides potential namespacing of the methods if you want to do that, and you can cut memory further by re-using the same vtable for an interface until someone tries to override at least one method in it. The cost is one extra indirection at call-sites.
[1] Protocol Extension: A Technique for Structuring Large Extensible SoftwareSystems (1994) https://pdfs.semanticscholar.org/60d3/045d924dfb912e184014aa...
Apple ObjC used to have a simplified version of this which used a vtable for the most frequent 16 selectors. It wasn't considered profitable after sufficient optimizations on the hash-table based method cache.
What I saw when looking at this before choosing to go that route years ago is that most dynamic language implementations seems to have just rejected vtable based approaches out of hand or never considered them at all because it's become seen as a "write once" approach unsuitable for updates and the default assumption has become that a complex lookup is needed.
It's been a few years since I looked, but last time I looked Franz' paper was the only one I found that investigated dynamic changes to objects at runtime using a vtable-like approach at all, and ironically he did so with a statically typed language... It seems like a curious blindness to me - maybe it genuinely is unsuitable for Objective-C, but most dynamic dispatch mechanisms I've looked at have been in environments where the number of names being looked up tends to be too small for even Franz' approach to be necessary.
(For Ruby the method name count tends to remain quite small, to the point where Franz' approach doesn't seem worth the cost of that extra indirection most of the time)