> On May 23, 2015, at 9:20 AM, Greg Parker <gpar...@apple.com> wrote:
> 
> free() does that sometimes. If zombies doesn't find anything then try guard 
> malloc.

Good suggestion! I’d forgotten about guard malloc. This changes the crash; now 
the parameter to objc_release points to unmapped memory, implying that the 
object has been dealloced:

(lldb) p/x $rdi
(unsigned long) $2 = 0x000061000d5a3fd0
(lldb) x $rdi
error: memory read failed for 0x61000d5a3e00

Still no clue what object this is/was, though. And it seems weird that it got 
freed instead of turned into a zombie, since I still have zombies enabled too.

I’m not at all good at reading IA64 assembly, but so far my suspicion is that 
this object is a temporary NSString being created from a Swift String in order 
to pass to -[NSObject addObserver:…] as the keyPath parameter. But since this 
NSString never appears in the source code at all, I don’t see how I could be 
messing up its refcount.

The method and its disassembly can be found here in case anyone feels like 
reading them:
        https://gist.github.com/snej/f6c0e1ba9230b3591a9f 
<https://gist.github.com/snej/f6c0e1ba9230b3591a9f>
(I tried to get a backtrace, but Xcode crashed when I entered the “bt” command…)

—Jens

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