[LLVMdev] x86 unwind support[MESSAGE NOT SCANNED]

Mark Shannon marks at dcs.gla.ac.uk
Mon Jul 20 06:00:22 PDT 2009

Andrew Haley wrote:
> Mark Shannon wrote:
>> Andrew Haley wrote:
>>> Mark Shannon wrote:
>>>> Nick Johnson wrote:
>>>>>> probably there should be a switch to choose whether codegen should turn
>>>>>> unwind/invoke into dwarf or setjmp/longjmp style code.
>>>> It seems to me that there is an implicit, and undocumented, assumption 
>>>> that unwinding needs to handle stack-allocated objects.
>>>> In languages without stack-allocated objects (ie. most languages that 
>>>> support exceptions) there is no need to unwind frame-by-frame, the 
>>>> unwind simply needs to make a single jump to the invoke instruction and 
>>>> restore the context (which in x86 is just 6 registers).
>>> Not quite.  It's also necessary to execute all the pending POSIX
>>> pthread_cleanup_pop() actions.
>> POSIX pthread_cleanup_pop() can only be called directly from C++/C.
>> C doesn't haven't exceptions.
> But it does have pthread_exit().
>> So yet again, this is a C++ issue.
> No, it isn't:
>        The effect of calling longjmp() or siglongjmp() is undefined  if  there
>        have  been any calls to pthread_cleanup_push() or pthread_cleanup_pop()
>        made without the matching call since the jump buffer  was  filled.

We are not talking about longjmp() or siglongjmp() we are talking about 

> Andrew.
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