[Bug 222563] libcxxrt abi::__cxa_demangle() does not successfully demangle names

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222563

            Bug ID: 222563
           Summary: libcxxrt abi::__cxa_demangle() does not successfully
                    demangle names
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: ian at FreeBSD.org

Created attachment 186671
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=186671&action=edit
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.

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