[jsc-dev] Tracking down GC issue

[Geoffrey Garen]

The GC often nulls out the first word of an object after running its destructor.

The most likely cause of your bug is an object lifetime issue.

Geoff

> On Jan 16, 2017, at 7:04 PM, Dan Zimmerman <daniel.zimmerman at me.com> wrote:
> 
> Hey,
> 
> I apologize in advanced if this mailing list is inappropriate for this content.
> 
> I've ran into the bug that appears to be the root issue for both https://bugs.webkit.org/show_bug.cgi?id=160027 <https://bugs.webkit.org/show_bug.cgi?id=160027> and https://bugs.webkit.org/show_bug.cgi?id=149957 <https://bugs.webkit.org/show_bug.cgi?id=149957>. I'd like to try and figure out if I'm misusing the C API/if there's anything I can do to prevent the crash (as it seems like there are plenty of clients of JSC that don't run into this issue, it makes me believe either I'm doing something wrong or there's something that I can change). Unfortunately the codebase is under an NDA so I cannot share snippets here. It appears that the JSCell that are referenced in certain code blocks are being nulled out, as the JSCell at crash looks like this:
> 
> m_structureID: 0
> m_indexingType: 0
> m_type: UnspecifiedType (0)
> m_flags: 0
> m_cellState: AnthraciteOrBlack (0)
> 
> If I set the environment variable JSC_useZombieMode=1 then the crash doesn't occur (and no unexpected behavior occurs - the application actually works as desired). It feels wrong to ship anything with useZombieMode enabled so I was wondering if there's any guidance to figuring out the source of the issue.
> 
> Thanks,
> 
> Dan
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