[LLVMdev] x86 unwind support[MESSAGE NOT SCANNED]

[Andrew Haley]

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.

Andrew.