[LLVMdev] Darwin vs exceptions

[Chris Lattner]

On Dec 7, 2007, at 4:40 PM, Dale Johannesen wrote:

> So I couldn't get exceptions to work on PPC darwin.  After much  
> digging and confusion, there seem
> to be two separate issues.

Ok.

> The gcc testsuite is running the version of the unwinding code that  
> was built with the local (llvm-)gcc,
> which doesn't work because nobody has implemented  
> builtin_return_address for that target.
> So that's one problem.

That's an easy problem.  I just implemented __builtin_return_address  
on PPC, at least builtin_return_address(0), please let me know if that  
is sufficient.

> More seriously, the version of the unwinding code installed on the  
> system doesn't work either, for a
> different reason (and also doesn't work on x86 Darwin).   It turns  
> out the installed code has the
> following patch, which is not in the llvm-gcc source (or Apple's gcc  
> sources for that matter; libgcc
> lives in a separate repository these days):

Yuck :)

> Index: gcc/unwind-dw2.c
> ===================================================================
> --- gcc/unwind-dw2.c    (revision 124749)
> +++ gcc/unwind-dw2.c    (working copy)
> @@ -1526,7 +1526,7 @@
>  static inline _Unwind_Ptr
>  uw_identify_context (struct _Unwind_Context *context)
>  {
> -  return _Unwind_GetIP (context);
> +  return _Unwind_GetCFA (context);
>  }
>
> I'm uncertain of the merit of this patch, but it's in shipping  
> Leopard and works with gcc, so we're
> stuck with it.

Right, this is effectively part of the darwin ABI now.

> The effect of this is that llvm's strategy of making every C++  
> selector claim that it has a catch-all
> handler at the end, as described here
> http://llvm.org/docs/ExceptionHandling.html#restrictions
> does not work in this environment.   Claiming that a function has a  
> catch-all handler when it does
> not causes the unwinder to go into a loop.

Okay, you're beyond my knowledge here :)

> I tried simply removing the code that does this:
>
> Index: cp/except.c
> ===================================================================
> --- cp/except.c	(revision 44663)
> +++ cp/except.c	(working copy)
> @@ -104,7 +104,7 @@
>    lang_eh_runtime_type = build_eh_type_type;
>    lang_protect_cleanup_actions = &cp_protect_cleanup_actions;
>    /* LLVM local */
> -  lang_eh_catch_all = return_null_tree;
> +/*  lang_eh_catch_all = return_null_tree;*/
>  }
>
>  /* Returns an expression to be executed if an unhandled exception is
>
> and that works well for me; i.e., the gcc tests that exercise  
> exceptions work as well with the
> two patches above (on x86) as they did before.

Ok, cool :)  Duncan, Anton, what do you guys think about this?

> Now I need some help:
> - Why was C++ claiming that every selector has a catch-all handler?   
> The documentation above just says
> the C++ personality function will terminate the program if it  
> detects that unwinding the exception only results in matches with  
> cleanups.
> Isn't that what's supposed to happen if there's no handler anywhere?
> - What is the right thing for Linux?  Is the deletion above safe  
> there?

Unsure,

-Chris
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