llvm dev - Mar 2013 - [LLVMdev] How to initiate/throw an exception (resume just continues)

I'm trying to add some basic exception handling to my language now and
I'm uncertain of how I actually start the exception propagation. It
appears that "resume" will continue the exception, but I see no
mention
of how the exception actually starts.

If I look at the IR output of a simple C++ program I see that it calls
@__cxa_throw, but that is a library function. Somehow I must be able to
initiate the exception in LLVM?

-- 
edA-qa mort-ora-y
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
Sign: Please digitally sign your emails.
Encrypt: I'm also happy to receive encrypted mail.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 261 bytes
Desc: OpenPGP digital signature
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20130329/76eb9bbf/attachment.sig>

On 29 Mar 2013, at 17:15, edA-qa mort-ora-y <eda-qa at disemia.com> wrote:
> I'm trying to add some basic exception handling to my language now and
> I'm uncertain of how I actually start the exception propagation. It
> appears that "resume" will continue the exception, but I see no
mention
> of how the exception actually starts.
> 
> If I look at the IR output of a simple C++ program I see that it calls
> @__cxa_throw, but that is a library function. Somehow I must be able to
> initiate the exception in LLVM?
Yes, by calling a library function.  You will need to implement this for your
language, along with a personality function (or reuse ones from some existing
language).  The __cxa_throw() function is responsible for allocating an
exception structure that contains two things, the C++ information (such as the
thrown object, its type_info, the current values of the terminate and unexpected
handlers, and so on), and a generic structure for the unwind library.  It then
passes it to _Unwind_RaiseException(), which is the generic unwind routine.

The _Unwind_RaiseException() function walks up the stack, invoking the
personality routine once to find a landing pad for a catch.  If it fails to find
one then it returns an error.  If it succeeds, then it invokes them again to run
cleanup code.  The personality function is passed the thrown object and also a
reference to the language-specific data area.  This area is filled in by LLVM
via landingpad instructions.

The resume instruction is just a convenience.  It will be expanded to a call to
another library function (_Unwind_Resume(), I think, but possible
_Unwind_ResumeOrRethrow()), and it exists so that the IR doesn't have to
have hard-coded knowledge of the semantics of one these functions.

The exact implementation of the personality function differs slightly between
ARM and pretty much everything else for *NIX platforms.  Win64 also has a very
similar mechanism for exceptions, but I don't believe it is currently
supported by LLVM.  ARM now works in some cases, but is still quite buggy.

The personality function is probably the least fun code I've ever written
(I've now done it three times, for three different languages, and it
doesn't become any more fun).  If you get it wrong, the stack is gone by the
time you get into the debugger, so lots and lots of logging statements are your
friend...

David

On 29/03/13 18:34, David Chisnall wrote:
> handlers, and so on), and a generic structure for the unwind library.
> It then passes it to _Unwind_RaiseException(), which is the generic
> unwind routine.
Excuse me if this sounds silly, but what "library" does the _Unwind_
series of functions come from? Is this a kind of target ABI instrinsic
library, or must I link with a particular runtime library to get these
functions?

I assume using the JIT there will somehow already be there, so at least
I can start experimenting. Perhaps another question, since this does
generate ABI compliant EH-tables, could my throw function just invoke a
native throw? That is, for really quick testing can I just create a c++
function which does a throw?
> The personality function is probably the least fun code I've ever
> written (I've now done it three times, for three different languages,
> and it doesn't become any more fun).  If you get it wrong, the stack
> is gone by the time you get into the debugger, so lots and lots of
> logging statements are your friend...
I feel excited already. :)
I'm still quite unclear on this part as well. Don't suppose you ever
wrote a tutorial on this aspect, or have some example code you could show?


-- 
edA-qa mort-ora-y
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
Sign: Please digitally sign your emails.
Encrypt: I'm also happy to receive encrypted mail.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 261 bytes
Desc: OpenPGP digital signature
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20130329/d9e31333/attachment.sig>