[LLVMdev] x86 unwind support[MESSAGE NOT SCANNED]

Mark Shannon marks at dcs.gla.ac.uk
Mon Jul 20 05:15:47 PDT 2009

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.
So yet again, this is a C++ issue.

>> It is possible to implement invoke/unwind in such a way that both invoke
>>   *and* unwind are fast, when  unwind just unwinds and doesn't perform
>> any magic behind-the-scenes operations.
> I don't think so.  Not for pthreads, anyway.
This is C++ specific.

> Andrew.
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