Relax the rules around objc_alloc and objc_alloc_init optimizations.

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>

1 files changed, 10 insertions, 1 deletions

diff --git a/clang/test/CodeGenObjC/objc-alloc-init.m b/clang/test/CodeGenObjC/objc-alloc-init.m
index c5c1a763b7c..8370202748e 100644
--- a/clang/test/CodeGenObjC/objc-alloc-init.m
+++ b/clang/test/CodeGenObjC/objc-alloc-init.m
@@ -22,21 +22,30 @@ void f() {
 }
 
 @interface Y : X
++(Class)class;
 +(void)meth;
 -(void)instanceMeth;
 @end
 
 @implementation Y
++(Class)class {
+  return self;
+}
 +(void)meth {
   [[self alloc] init];
   // OPTIMIZED: call i8* @objc_alloc_init(
   // NOT_OPTIMIZED: call i8* @objc_alloc(
 }
++ (void)meth2 {
+  [[[self class] alloc] init];
+  // OPTIMIZED: call i8* @objc_alloc_init(
+  // NOT_OPTIMIZED: call i8* @objc_alloc(
+}
 -(void)instanceMeth {
   // EITHER-NOT: call i8* @objc_alloc
   // EITHER: call {{.*}} @objc_msgSend
   // EITHER: call {{.*}} @objc_msgSend
-  [[self alloc] init];
+  [[(id)self alloc] init];
 }
 @end