Question

Consider the following code:

@interface ClassA : NSObject
@property (nonatomic, copy) void(^blockCopy)();
@end

@implementation ClassA

@synthesize blockCopy;

- (void)giveBlock:(void(^)())inBlock {
    blockCopy = inBlock;
}

@end

Then use it in a class which has a strong property of type ClassA called someA:

self.someA = [[ClassA alloc] init];
[self.someA giveBlock:^{
    NSLog(@"self = %@", self);
}];
dispatch_async(dispatch_get_main_queue(), ^{
    self.someA.blockCopy();
    self.someA = nil;
});

If I run that built O3 with ARC enabled, on iOS, it crashes during the self.someA.blockCopy(); call inside objc_retain. Why?

Now I realise that people are probably going to say I should be setting it with self.blockCopy = inBlock but I did kind of think that ARC should be doing the right thing here. If I look at the assembly (ARMv7) produced from the giveBlock: method it looks like this:

        .align  2
        .code   16
        .thumb_func     "-[ClassA giveBlock:]"
"-[ClassA giveBlock:]":
        push    {r7, lr}
        movw    r1, :lower16:(_OBJC_IVAR_$_ClassA.blockCopy-(LPC0_0+4))
        mov     r7, sp
        movt    r1, :upper16:(_OBJC_IVAR_$_ClassA.blockCopy-(LPC0_0+4))
LPC0_0:
        add     r1, pc
        ldr     r1, [r1]
        add     r0, r1
        mov     r1, r2
        blx     _objc_storeStrong
        pop     {r7, pc}

That is calling objc_storeStrong which in turn does a retain on the block and a release on the old block. My guess is that ARC is not properly noticing it’s a block property as I think it should be calling objc_retainBlock instead of the normal objc_retain.

Or, am I just totally wrong and actually ARC is doing what it documents and I’ve just read it the wrong way?

Discussion very much welcome on this – I find this to be rather intriguing.

Points to note:

• It doesn’t crash on OS X.
• It doesn’t crash built O0.

Answer 1

- (void)giveBlock:(void(^)())inBlock {
    blockCopy = inBlock;
}

You need to copy the block either on assignment or when passed into this function. While ARC solves the auto-move-to-heap-on-return problem, it does not do so for arguments (can’t do to idiosyncrasies of C).

That it doesn’t crash in certain environments is merely coincidental; it won’t crash as long as the stack version of the block hasn’t been overwritten. A sure sign of this is when you have a crash that goes away with optimization turned off. With optimization off, the compiler won’t reuse stack memory within any given scope, causing the memory to be “valid” for long after it should be.

---

I still don’t quite understand why it can’t do a objc_blockRetain
rather than a normal objc_retain, though. The compiler knows the type
after all.

I’m pretty sure that the issue is the potential cost of the assignment. If the block captures a lot of state, including, potentially, other blocks, then the Block_copy() might be really really really expensive.

I.e. if you had something like:

BlockType b = ^(...) { ... capture lots of gunk ... };
SomeRandomFunc(b);

… and that implied a Block_copy() merely because of the assignment, it would make it impossible to use blocks consistently without risk of pathological performance issues. Because there is no way for the compiler to know whether SomeRandomFunc() is synchronous or asynchronous, there is no way to manage this automatically (at this time — I’m sure getting rid of this potential tripwire is desirable).