Created attachment 186671 [details]
Small c++ program to demonstrate the problem
The attached small c++ program should print the demangled names of a couple symbols when it runs, but it does not:
# make tester && ./tester
c++ -O -pipe tester.cc -o tester
name '3Foo' = '3Foo' status=-2
name 'Z4mainE3Bar' = '(null)' status=-2
The current demangling code in libcxxrt is a snapshot of the libelftc demangler from 10-ish years ago. If you replace it with the current libelftc demangling code and make some other small changes you can get abi::__cxa_demangle() to the point where it returns the same slightly-less-wrong results as c++filt (see PR 222562).
Part of the problem with abi::__cxa_demangle() is that it is not correctly glued to the libelftc demangling code. The demangler wants to see a prefix on the symbol name, like what you see in nm(1) output. That is, typeid(bar).name() returns "Z4mainE3Bar" and the demangle code expects "_ZTSZ4mainE3Bar". (This is the small change I referred to above.)
One potential fix for this would be to examine llvm's libcxxabi and possibly import its demangler (or maybe the entire library). I gather this libcxxabi didn't exist when we created/imported libcxxrt.