[llvm-dev] Issue with libc++abi Demangling for Sanitizers

[Leonard Chan via llvm-dev]

Hi all,

I ran into an issue where it looks like the libc++abi demangler isn't being
used at all by sanitizer runtimes despite libc++abi being statically linked
against. That is, if I were to make an executable that statically linked
against libc++abi.a (which has a strong definition for `__cxa_demangle`)
and libclang_rt.tsan.a (which has a weak reference to `__cxa_demangle`),
then my final executable will contain an undefined weak `__cxa_demangle`
symbol. This happens because the tsan runtime has no strong reference to
`__cxa_demangle`, and thus will not link in
libc++abi.a(cxx_demangle.cpp.o), the object file containing the actual
definition for __cxa_demangle.

I hit this while running compiler-rt tests and found that some tests which
require libc++abi (and actually link against libc++abi.a) do not actually
symbolize correctly and thus fail. We could work around this in tests by
adding `-u __cxa_demangle` to compiler-rt tests, but that seems like an
undesirable workaround that's only limited to tests, and would differ from
how users actually use sanitizers (unless it was added to the driver given
some sanitizer flags, but I'm not sure if we'd want that).

My question is: *what are ways libc++abi can correctly be statically linked
in with sanitizer runtimes (that is, get __cxa_demangle to properly
manifest)? *I'm assuming that it was intentional for (1) __cxa_demangle to
be weakly referenced because sanitizers should still be able to work
without libc++abi, and (2) libc++abi.a is meant to be statically linked in
with sanitizer runtimes so, if it was used, then it would only link parts
of libc++abi and not the bulk of it.

Thanks,
Leonard
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