[cfe-dev] Clang emits unreachable `internal linkage` constructor

[Mehdi Amini via cfe-dev]

> On Oct 19, 2016, at 6:53 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> 
> On Tue, Oct 18, 2016 at 8:37 AM, Duncan P. N. Exon Smith via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> 
> > On 2016-Oct-13, at 22:46, Mehdi Amini via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> >
> > Hi,
> >
> > This fairly simple (and valid) code does not link (on MacOS):
> >
> >
> > #include <iostream>
> > #include <string>
> > namespace {
> >       struct VA {
> >       };
> >       struct A : virtual public VA {
> >               A() {
> >                       static bool b = false;
> >                       std::string str;
> >               }
> >       };
> > }
> > int main(int argc, char* argv[]) {
> >       A::A a;
> > }
> >
> >
> >
> > The issue is that clang emits two constructors (the complete one and the base one) for struct A because it has a virtual base class.
> >
> > Because struct A is declared in an anonymous namespace, these constructors are internal linkage. Only one of them is actually called, the other is unreachable.
> >
> > The “always-inliner” does not visit unreachable internal function. However this constructor is calling into a function from libc++ that are marked “available_externally” and “always inline”: the basic_string constructor.
> >
> > The problem is that the linkage “available_externally" is a “lie”: there is an external template instantiation in the header, but it is marked “visibility hidden”. So the function cannot be linked to in the libc++ dylib. (I think it’d work with a static libc++).
> >
> > I see multiple ways to address such cases, possibly many of them can be implemented:
> >
> > - Don’t emit unreferenced internal_linkage function. (We should try to do this anyway right?)
> 
> This makes sense to me.  It seems to me like a bug that it's emitted.
> 
> Regardless, resolving it does not fix the problem here. For instance, the same issue would arise under -femit-all-decls, or if uses exist at the AST level but are optimized away after IR generation.

If uses are optimized away after IR generation, this is not a problem from an LLVM point of view: clang didn’t emit an unreachable internal function in the first place, LLVM will track that the uses were removed and delete the `newly unreachable` internal function.

Your point stands for -femit-all-decls though.

— 
Mehdi


> 
> > - Have the CGSCC run on the unreachable part of the call-graph.
> 
> This seems to pessimize the normal case.
> 
> Nonetheless, it's necessary if we want to provide the guarantee that the always_inline attribute is supposed to provide. (It's not a hint; per the GCC documentation it is an error if a function is annotated always_inline and is not inlined.)
> 
> If we don't want to provide that guarantee, so be it, but then libc++'s use of this attribute is simply wrong, since it's relying on that guarantee.
> 
> > - Run global-DCE at O0 (might be a good idea anyway if it speeds-up the build)
> 
> Would we be surprised by anything it deletes?  If it's only going to delete unreferenced static globals, it seems (1) harmless and (2) unnecessary unless there are frontend bugs.  But maybe it's
> 
> > - We should have the “available_externally” compatible with “hidden visibility”. Maybe we need a way to have to mark some method excluded from one  `external template` instantiation so that it would be ODR.
> 
> This seems useful.  I think individual functions should be able to opt-out of a class-wide `extern template` instantiation.
> 
> > - We could make the basic_string constructor and other method in the same situation internal linkage instead.
> 
> Could pointer equality fail on the function pointers of basic_string::basic_string?
> 
> >
> >
> > Thoughts?
> >
> > —
> > Mehdi
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