[LLVMdev] x86 unwind support[MESSAGE NOT SCANNED]

[Mark Shannon]

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).

> 
> There is, but it happens before codegen and is slow.
> -enable-correct-eh-support will translate invoke/unwind into
> setjmp/longjmp pairs for the correct behavior. See:
> http://llvm.org/docs/Passes.html#lowerinvoke

*Begin rant*

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.

After all, isn't it the job of the front-end to insert all the clean-up 
code for stack-allocated objects?
Java, C#, Python, and Ruby have destructors(finalizers), but they are 
managed by the  garbage collector.
C++ is the odd one out, so why do the semantics of an llvm instruction 
depend on C++ semantics?

*End rant* ;)

Mark.

> 
> Nick
> 
> 
> 
>> Ciao,
>>
>> Duncan.
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