Oh, I guess I didn’t explain what I was talking about well. I’m saying that the
compiler would do a full method inline but put it behind a check to see if it’s
legal to continue executing. For example, code like this:
@interface Foo
- (void)bar;
@end
// Another method in some random class
- (void)baz {
Foo *foo = // whatever
[foo bar];
}
would end up being compiled to something like this:
- (void)baz {
Foo *foo = // whatever
if (bar_is_unswizzled()) {
// Inlined version of -[Foo bar]
} else {
// Fall back to going through objc_msgSend
}
}
where bar_is_unswizzled() is some sort of runtime check that makes sure that
the actual target is what we had thought it’d be at compile time. I’d hope that
a branch predictor would be able to do a pretty good job on this considering
that the guessing “true” would work 99% of the time.
Saagar Jha
> On Nov 20, 2019, at 17:01, Jens Alfke <j...@mooseyard.com> wrote:
>
>
>
>> On Nov 20, 2019, at 2:46 PM, Saagar Jha <saa...@saagarjha.com
>> <mailto:saa...@saagarjha.com>> wrote:
>>
>> I am curious why this optimization went in instead of guarded speculative
>> inlining, which would let you keep dynamism.
>
> If I understand it correctly, that only 'inlines' (really caches) the
> resolved method address for the call site. That's not much of a win in Obj-C
> where method lookup is already quite fast.
>
> The real win comes with literally inlining the method at compile time.
> Link-Time Optimization allows _any_ method anywhere in the program to be
> inlined, provided the call is monomorphic. And this new feature allows
> monomorphic method calls in Obj-C. This is a big win for small method like
> getters/setters, and for methods with only one call site (i.e. where you
> factor out a method for readability even though it's only used in one place.)
>
> —Jens
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