forked from OSchip/llvm-project
d18fbfc097
Today the optimization is limited to:
- `[ClassName alloc]`
- `[self alloc]` when within a class method
However it means that when code is written this way:
```
@interface MyObject
- (id)copyWithZone:(NSZone *)zone
{
return [[self.class alloc] _initWith...];
}
@end
```
... then the optimization doesn't kick in and `+[NSObject alloc]` ends
up in IMP caches where it could have been avoided. It turns out that
`+alloc` -> `+[NSObject alloc]` is the most cached SEL/IMP pair in the
entire platform which is rather silly).
There's two theoretical risks allowing this optimization:
1. if the receiver is nil (which it can't be today), but it turns out
that `objc_alloc()`/`objc_alloc_init()` cope with a nil receiver,
2. if the `Clas` type for the receiver is a lie. However, for such a
code to work today (and not fail witn an unrecognized selector
anyway) you'd have to have implemented the `-alloc` **instance
method**.
Fortunately, `objc_alloc()` doesn't assume that the receiver is a
Class, it basically starts with a test that is similar to
`if (receiver->isa->bits & hasDefaultAWZ) { /* fastpath */ }`.
This bit is only set on metaclasses by the runtime, so if an instance
is passed to this function by accident, its isa will fail this test,
and `objc_alloc()` will gracefully fallback to `objc_msgSend()`.
The one thing `objc_alloc()` doesn't support is tagged pointer
instances. None of the tagged pointer classes implement an instance
method called `'alloc'` (actually there's a single class in the
entire Apple codebase that has such a method).
Differential Revision: https://reviews.llvm.org/D71682
Radar-Id: rdar://problem/58058316
Reviewed-By: Akira Hatanaka
Signed-off-by: Pierre Habouzit <phabouzit@apple.com>
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