Re: C++ PATCH for c++/58678 (devirt vs. KDE)

[Jan Hubicka <hubicka at ucw dot cz>]

> On 03/17/2014 04:39 AM, Jan Hubicka wrote:
> >Thank you!  would preffer different marker than cxa_pure_virtual in the vtable,
> >most probably simply NULL.
> >
> >The reason is that __cxa_pure_virtual will appear as a possible target in the
> >list and it will prevent devirtualization to happen when we end up with
> >__cxa_pure_virtual and real destructor in the list of possible targets.
> 
> Hmm?  __cxa_pure_virtual is not considered likely, so why wouldn't
> devirtualization choose the real function instead?

If you get list like ~foo(), __cxa_pure_virtual, you will get speculative devirtualization
to ~foo.
If you get ~foo(), NULL, the NULL will get translated to BUILTIN_UNREACHABLE and
that will be dropped from the list, so you will end up with unconditional call of ~foo().

I think in general we can not skip cxa_pure_virtual, since people want friendly
diagnostics on broken programs insted of getting devirtualized call to random other
function. I was under impression in this case we know that the virtual table entry won't
be used, so full devirtualization would be possible.
> 
> >gimple_get_virt_method_for_vtable knows that lookup in vtable that do not
> >result in FUNCTION_DECL should be translated to BUILTIN_UNREACHABLE and
> >ipa-devirt drops these from list of targets, unlike __cxa_pure_virtual that
> >stays.
> 
> I don't see the reason for that distinction; either way you get
> undefined behavior.  The only purpose of __cxa_pure_virtual is to
> give a friendly diagnostic before terminating the program.

I can drop the handling of cxa_pure_virtual if unconditoinal devirtualization is
desirable, or perhaps do it under some switch.
Targets list containing one cxa_pure_virtual and one extra function are common.
> 
> >Other problem with cxa_pure_virtual is that it needs external relocation.
> >I sort of wondered if we don't want to produce hidden comdat wrapper for
> >it, so C++ programs are easier to relocate.
> 
> Sure, that would make sense.
> 
> >What do you think of the following patch that makes ipa-devirt to conclude
> >that destructor calls are never done on types in construction.
> >If effect of doing so is undefined, I think it is safe to drop them from
> >list of targets and that really helps to reduce lists down.
> 
> That looks good to me.

Thanks, I am away for next 4 days to allaska hut w/o electricity, will check my email afterwards,
> 
> Jason