[cfe-dev] typeinfo visibility for pointer to incomplete type

[Richard Smith via cfe-dev]

On 24 Jan 2017 12:09 pm, "Vedant Kumar via cfe-dev" <cfe-dev at lists.llvm.org>
wrote:


> On Jan 24, 2017, at 10:31 AM, Douwe Gelling <douwegelling at gmail.com>
wrote:
>
> Ok, that makes sense. I wonder how g++ works around that problem..
> Any chance of warning on taking the typeinfo of an incomplete type? It's
only got limited use since you can't really use it across compilation units.
> Just for some context: I ran into this issue by storing a pointer to
incomplete type in a boost::any, and I assume the same issue would arise
with std::any

There is an error for that (err_incomplete_typeid), but it doesn't look
like it
complains about taking the typeinfo of a pointer to an incomplete type. I
think
it makes sense to extend the error.


No, it doesn't make any sense to add an error for valid code. Douwe, what
problems are you having using type_info objects across translation units?
It could be that there's a bug there (but at this point we don't have
enough information to discount user error).

vedant

>
> On Tue, Jan 24, 2017 at 6:43 PM David Majnemer <david.majnemer at gmail.com>
wrote:
> The reason why they cannot be combined is that "SomeClass" may have
bases. The typeinfo for the incomplete type cannot know which bases the
type might have while other translation units may know. We want to make
sure that the typeinfo for a complete type doesn't get replaced with the
incomplete type information, this is important to ensure that things like
dynamic_cast work.
>
> On Tue, Jan 24, 2017 at 6:39 AM, Douwe Gelling <douwegelling at gmail.com>
wrote:
> I see, thanks for the explanation.. maybe some warning can be issued when
this happens? It makes the typeinfo
> of ptr to incomplete type basically useless in clang, whereas it works
fine with other compilers
> (gcc makes it weak external for example)
>
> I feel it's a rather weird pitfall that will cause runtime errors and
it's very easy to diagnose for compilers, though
> there might be usecases I don't understand of course
>
>
>
> On Mon, Jan 23, 2017 at 8:06 PM Vedant Kumar <vsk at apple.com> wrote:
> It looks like getTypeInfoLinkage (ItaniumCXXABI.cpp) can return
'linkonce_odr'
> if SomeClass has a definition, and returns 'internal' otherwise. The
rationale
> is that type info for incomplete types must be kept separate from the
type info
> for complete types. IIUC we do this because inkonce_odr definitions can
be used
> by optimizations at any point, so the correct/full definition needs to be
> available up front.
>
> I'm CC'ing David since he wrote the comment, and can probably correct me
if I
> got this wrong.
>
> best,
> vedant
>
> > On Jan 22, 2017, at 5:52 AM, Douwe Gelling <douwegelling at gmail.com>
wrote:
> >
> > I've reproduced it both on the system clang:
> >
> > Apple LLVM version 8.0.0 (clang-800.0.42.1)
> >
> > and on a newer clang installed with homebrew:
> >
> > clang version 3.9.1 (tags/RELEASE_391/final)
> > Target: x86_64-apple-darwin16.3.0
> >
> > In llvm ir, it's marked internal constant with both compilers as well
> >
> > On Fri, Jan 20, 2017 at 6:23 PM Vedant Kumar <vsk at apple.com> wrote:
> > What version of clang are you using?
> >
> > I tried referring to the typeid of SomeClass* from two different
compilation
> > units and ended up with just one copy of __ZTSP9SomeClass in my binary.
The
> > symbol is marked as 'linkonce_odr constant' in LLVM IR, which checks
out.
> >
> > $ nm -m t | grep ZTSP9SomeClass
> > 0000000100001f60 (__TEXT,__const) weak external __ZTSP9SomeClass
> >
> > best,
> > vedant
> >
> > > On Jan 20, 2017, at 5:15 AM, Douwe Gelling via cfe-dev <
cfe-dev at lists.llvm.org> wrote:
> > >
> > > Hi all,
> > >
> > > When taking the typeinfo of a pointer to incomplete type, clang++
emits type info for the pointer in the resulting binary, but makes that
typeinfo non-external (on osx at least). Is this intended? I'd have
expected it to be weak external so that when the type is defined in other
compilation units, the typeinfo isn't defined multiple times.
> > >
> > > sample code:
> > >
> > > #include <typeinfo>
> > > #include <iostream>
> > > class SomeClass;
> > >
> > > int main()
> > > {
> > >   std::cout << typeid(SomeClass*).name() << std::endl;
> > > }
> > >
> > > when running nm -m on the resulting binary, it contains:
> > >
> > > 0000000000000414 (__TEXT,__const) non-external __ZTSP9SomeClass
> > >
> > > _______________________________________________
> > > cfe-dev mailing list
> > > cfe-dev at lists.llvm.org
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
>
>

_______________________________________________
cfe-dev mailing list
cfe-dev at lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20170124/4485ef20/attachment.html>