[D-runtime] druntime commit, revision 459

Don Clugston dclugston at googlemail.com
Tue Dec 28 12:17:21 PST 2010

On 28 December 2010 19:34, Sean Kelly <sean at invisibleduck.org> wrote:
> This one was discussed a while ago in private email and stalled for lack of an executive decision.  I'll paste some pertinent bits of the discussion below (all stuff I said).  The rest of this message is pasted from various old emails.

That's good to know.  So the Linux behaviour is also believed to be
incorrect, but in a different way?

Still, none of this SEH stuff seems right to me. localunwind returns a
CollidedUnwind, but it doesn't seem to actually be doing a collided
unwind.  If it does a collided unwind, it should be changing the catch
clause which it's ultimately trying to reach, but continuing to
execute any finally clauses en route. If it does exception chaining,
it should continue to execute any finally clauses en route.
But it does neither of these things; instead, it just silently fails!
(This would seem to be wrong on D1, regardless of what it is supposed
to do on D2).

For something which is so complicated and undocumented by Microsoft,
the lack of comments in the code is terrible. I added a lot of
comments out of frustration. Please fix any which are wrong, and
change the function names where appropriate.
Regardless of when bug 1513 gets fixed, it seemed to me an
embarrassment that a C compiler was required for the D runtime.

> ----------
> On Sep 2, 2010, at 3:44 PM, Sean Kelly wrote:
>> I spent some time investigating the test suite failures that caused Walter to revert the chaining code and ran into an interesting problem.  Consider this (from test4.d test55):
>> void funcA()
>> {
>>   try {
>>       try {
>>           throw new Exception( "A" );
>>       } finally {
>>           funcB();
>>       }
>>   } catch( Exception e ) {
>>       assert( e.msg == "A" );
>>   }
>> }
>> void funcB()
>> {
>>   try {
>>       throw new Exception( "B" );
>>   } catch( Exception e ) {
>>       assert( e.msg == "B" );
>>   }
>> }
>> In this case, the assertion in funcB() will fail because exception A is already in flight so exception B will be considered collateral damage and be chained to the existing exception A.  In an ideal world, proper behavior would probably be to make exception chaining scope-aware so an exception is only considered collateral if it escapes the top finally block called during stack unwinding.  Practically speaking however, this seems like it could be a very complicated thing to actually do.  What are the alternatives?  As it is, I don't feel comfortable rolling the chaining code back in.
> Okay, I think this could be done without too much work, but it would require adding a pointer to the handler table for each stack frame (for the current exception).  I'm unsure whether the chaining behavior is worth it... thoughts?
> On Sep 21, 2010, at 3:43 PM, Sean Kelly wrote:
>>> On 9/21/10 17:01 CDT, Sean Kelly wrote:
>>>> What's really convinced me that the simple TDPL approach isn't
>>>> sufficient is that it can violate the nothrow guarantee.  You'd think
>>>> it should be safe to put a nothrow label on funcB(), right?  But with
>>>> the previously implemented TDPL model, the exception "thrown" out of
>>>> funcB() is exception A because exception B is considered collateral
>>>> damage.  I think you're right that the language in TDPL isn't wrong
>>>> though.  It's just that a naive implementation of the described TDPL
>>>> model (as I did before) doesn't work.
>> I think doing so will require either a codegen change to add a void* to the DHandlerInfo struct (for saving a reference to the in-flight exception that caused the handler to be called), or a dynamic stack to store this info.
> I realized last night that the dynamic stack can be a linked-list of structs on the stack, so no need for DMA.  So store the Throwable reference and a pointer to the finally block being called or perhaps a stack position.  I'll experiment with it today.  The details are still fuzzy but I'm reasonably certain it will work.
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