[PATCH] D21803: [libcxxabi] Provide a fallback __cxa_thread_atexit() implementation

Tavian Barnes via cfe-commits cfe-commits at lists.llvm.org
Thu Sep 8 13:18:31 PDT 2016

tavianator added inline comments.

Comment at: src/cxa_thread_atexit.cpp:70
@@ +69,3 @@
+    while (auto head = dtors) {
+      dtors = head->next;
+      head->dtor(head->obj);
EricWF wrote:
> tavianator wrote:
> > EricWF wrote:
> > > There is a bug here. If `head->next == nullptr` and if `head->dtor(head->obj))` creates a TL variable in the destructor then that destructor will not be invoked.
> > > 
> > > Here's an updated test case which catches the bug: https://gist.github.com/EricWF/3bb50d4f28b91aa28d2adefea0e94a0e
> > I can't reproduce that failure here, your exact test case passes (even with `#undef HAVE___CXA_THREAD_ATEXIT_IMPL` and the weak symbol test commented out).
> > 
> > Tracing the implementation logic, it seems correct.  If `head->next == nullptr` then this line does `dtors = nullptr`.  Then if `head->dtor(head->obj)` registers a new `thread_local`, `__cxa_thread_atexit()` does `head = malloc(...); ... dtors = head;`.  Then the next iteration of the loop `while (auto head = dtors) {` picks up that new node.
> > 
> > Have I missed something?
> I can't reproduce this morning either, I must have been doing something funny. I'll look at this with a fresh head tomorrow. If I can't find anything this will be good to go. Thanks for working on this. 
No problem!  I can integrate your updated test case anyway if you want.


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