[LLVMdev] About unwind...

[Gordon Henriksen]

Hi Sebastien.

On Dec 15, 2007, at 19:00, Sebastien Loisel <loisel at temple.edu> wrote:

> I have started playing with LLVM a little bit and I am thinking of  
> using it to write some linear algebra software for my class.
>
> I was reading the documentation and I am very excited by the  
> possibilities of "invoke" and "unwind". I have two questions.
>
> 1) How does the performance of "invoke" compare to the performance  
> of "call" on a typical x86, 32 or 64 bit platform? How many isns and  
> such?

With the exception of the "setjmp/longjmp" codegen that LLVM supports,  
invoke has no dynamic cost. It is used to implement C++ "zero-cost"  
exception-handling, which combine the dynamic execution stack itself  
and static metadata emitted by the compiler in order to implement  
unwinding.

> 2) When I unwind, the stack gets popped but it would make sense to  
> pass some data to the exception handler. I guess I could use a  
> global variable for this, but is there a way to do it on the stack?  
> (Wouldn't that make more sense also?)

Indeed, you should use a global (or thread-local global).

- Gordon

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