Apple’s A15 chip will ship with an objc_msgSend branch predictor (opens in new tab)

Does this mean that the compiler can embed smart hints for branch prediction directly in the bytecode, by way of setting very specific dedicated registers?

Pretty much every method call, unless it is optimized—will go through `objc_msgSend`. The runtime looks up the code for a particular object using `objc_msgSend.` The function signature varies depending on how many parameters the method has but it always has at least 2 arguments—the instantiated object and the method name. Objc calls this the selector. :)

If you write a message send in Objective-C, like [object doSomething], behind the scenes the compiler translates this to a call to the objc_msgSend function in the Objective-C runtime library. This function looks up what code needs to run, taking into account inheritance and all the other dynamic features and runs it. So this function runs really often and is an important target for optimization.

It is a codepath that is used a lot, in system frameworks that many apps use. So, speeding it up can yield a systemwide performance improvement for all sorts of apps.

It’s not so much the apps as the system frameworks they’re built upon, which are still almost universally Obj-C (or some other C-family language). Even if every third-party or user-facing first-party app was 100% Swift, there’d still be massive wins from speeding up objc_msgSend.

(Not to mention that even pure-Swift apps use a lot of Obj-C-isms when touching said system frameworks, like the target/selector setup for most UI controls).

Most new Apple project are still started with ObjC, you can tell from those hiring. I mean a little less than two years ago Apple Pay team were "suggesting" doing something new written in ObjC.

My guess is that Apple dont mandate Swift no new project. And the team gets to decide. It will probably be another few years when Swift 6.0 is out before a gradual transition of news apps to Swift by default.

And I am still skeptical of Swift being a system programming language. Sometimes I wonder why Apple dont make some more minor changes and improvement to ObjC.

I wouldn't be surprised if Swift also calls objc_msgSend for accessing macOS APIs, or at least a practically identical function.

The Objective-C runtime library (which objc_msgSend is a part of) allows to access the "Objective-C object model" from other languages than Objective-C via a C-API.

Metal is a mix of Objective-C and C++14 subset as shading language.

Same applies to many system Frameworks written before Swift happened.

I bet they aren't running X rewritten in Swift blog posts inside Apple.

The functionality seems to be general-purpose, with the branch target being hinted at by providing the location of the branch and the input registers that can be used to calculate the address. However, based on how this is implemented it looks like there is only one place you can use this and Apple is going to use it for objc_msgSend, although its use in WebKit might mean that JavaScriptCore has uses planned as well.

Anything that calls into ObjC benefits as well, and if you built another programming language that used a very similar mechanism, presumably you could also use it.

AFAIK all language bindings for the macOS system APIs go through the ObjC runtime library and call objc_msgSend() to invoke object or class methods (not sure about Swift though).