[PATCH] Fix linkage for vtables of class template specializations with explicit instantiation declarations

[Richard Smith]

Hi John,

If a class template specialization has an explicit instantiation
declaration and needs a VTT, we currently emit all the vtable data as
linkonce_odr. That causes link failures in some cases, because recent g++
versions put all the vtable-related data in a COMDAT, and if we emit some
of the symbols but not all of them, we can cause the COMDAT from the
explicit instantiation definition to not be chosen, resulting in missing
symbols.

This patch fixes that and removes some FIXMEs by switching us to
available_externally. However, testing that fix revealed another problem:
if (through inlining and constant propagation through the VTT) we end up
with a reference to a construction vtable in the generated code, we will
sometimes have no definition for that construction vtable. This is because
the Itanium ABI does not require a compiler to emit construction vtables
with any particular mangling or linkage, and indeed recent versions of
libstdc++.so make all the construction vtable symbols internal.

So this patch makes two changes:
1) Emit externally-available primary vtables and VTTs as
available_externally rather than linkonce_odr, and
2) Emit externally-available construction vtables as internal rather than
linkonce_odr.

I believe point (2) conforms to the ABI -- while we're required to use the
same primary vtable for all objects of the same most-derived type, the same
restriction does not appear to apply to construction vtables.

If duplicating the construction vtables concerns you (in the rare case
where we manage to propagate the address of one out of the VTT), I'd be
fine with never emitting externally-available VTTs instead -- but that will
lose us some optimization opportunities.

Thanks,
Richard
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