[LLVMdev] Integrating LLVM in an existing project

Nicolas Geoffray nicolas.geoffray at lip6.fr
Fri Apr 6 00:50:18 PDT 2007

Hi Chris,

Chris Lattner wrote:
>> We can not use current exception handling in llvm, see
>> http://www.nondot.org/sabre/LLVMNotes/ExceptionHandlingChanges.txt.
> Why not?

Like you say, it's not functional for non-calls instructions. Besides, 
having to change
all CalInst to InvokeInst is just too much pain in our current vm.

> There are two separate issues here:
> 1. catching exceptions from function calls
> 2. catching exceptions from "non call" instructions like divides, etc.
> The LLVM IR, as it stands now, is expressive enough to describe #1, but 
> not #2.  The notes above talk solely about how to extend the IR to support 
> #2.  Reid has an interest in implementing #2 at some point, which will be 
> a great improvement for some applications.
> There is an extra wrinkle here though.  Zero-cost C++ exception handling 
> (i.e. #1) is about 3/4 of the way implemented in the C front-end and code 
> generator, but it is not yet completed (notably, JIT support is missing). 
> This work is also currently stalled.

Actually, why is it missing? What's the difference between the code 
and the JIT?

> If you'd be interested in helping with any of these projects, please let 
> me know!

The thing is, I know very little on exception handling (our 
setjmp/longjmp mechanism in our vms
is functional but not really optimized) and zip on LLVM. I looked at 
Reid's notes for implementing #2,
and I'm not sure I can help without having a month or two (of spare 
time) to learn LLVM's internals.

However, I'd be interested to look at how exception handling can be 
implemented in the JIT.
What's the current situation?


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