Re: Dynamically generated code and DWARF exception handling

[jacob navia <jacob at jacob dot remcomp dot fr>]

Tom Tromey a écrit :

> > > > > > "jacob" == jacob navia <jacob@jacob.remcomp.fr> writes:
> > > > > >            
>
> jacob> This application generates dynamically code and executes it, using a
> jacob> JIT, a Just In time Compiler. Everything is working OK until the C++
> jacob> code generates a throw.
>
> Fun!
>
> I looked at this a little bit with libgcj.
>
> In some ways for libgcj it is simpler than it is for C++, since in the
> gcj context we know that the only objects thrown will be pointers.
> So, if we were so inclined, we could give the JIT its own exception
> handling approach and have little trampoline functions to handle the
> boundary cases.
>  
Well, there is no exception handling in the generated machine code, and 
we want not to have any exception handling but to let the C++ exception 
handling PASS THROUGH our stack frames to find its eventual catch. This 
means that we have to give the run time function that implements the 
throw some way of getting into the higher up frames without crashing. 
The problem is that any throw that encounters intermediate assembler 
frames will inevitably CRASH.

> Unfortunately things are also worse for libgcj, in that we need to be
> able to generate stack traces as well, and the trampoline function
> approach won't work there.
>  

????? Sorry I do not follow here

> Still, if you know something about the uses of 'throw' in your
> program, maybe this would work for you.
>
> Longer term, yeah, gcc's unwinder needs a JIT API, and then the
> various JITs need to be updated to use it.  At least LLVM appears to
> be headed this direction.
>  
Very interesting but maybe you could be more specific?

I browsed that "llvm" and seems a huge project, as libgcj is. To go 
through all that code to find how they implement this, will be extremely 
difficult. If you could give me some hints as to where is the needle in 
those haystacks I would be really thankful.

> Tom
>  
jacob