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

[Vedant Kumar via cfe-dev]

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
> >
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>