Hi

     Here is the patch to add backtrace generation on _GLIBCXX_DEBUG 
assertions thanks to libbacktrace.

     In addition to this integration I am also improving the generation 
of the assertion message thanks to the "%.*s" printf format, it avoids 
an intermediate buffer most of the time. I am also removing the "__" 
used for uglification to get a nicer output. I can propose it in a 
dedicated patch if you prefer.

     I am adding GLIBCXX_3.4.30 abi version to properly export the 2 new 
weak symbols. Let me know if it isn't necessary.

     libstdc++: [_GLIBCXX_DEBUG] Add backtrace generation thanks to 
libbacktrace

       Add _GLIBCXX_DEBUG_BACKTRACE macro to activate backtrace 
generation on
     _GLIBCXX_DEBUG assertions using libbacktrace.

             * config/abi/pre/gnu.ver: Add GLIBCXX_3.4.30 version and 
new exports.
             * include/debug/formatter.h [_GLIBCXX_DEBUG_BACKTRACE]:
             Include <backtrace-supported.h>.
             [_GLIBCXX_DEBUG_BACKTRACE && BACKTRACE_SUPPORTED]:
             Include <backtrace.h>.
             [(!_GLIBCXX_DEBUG_BACKTRACE || !BACKTRACE_SUPPORTED) &&
             _GLIBCXX_USE_C99_STDINT_TR1]: Include <stdint.h>.
             [_GLIBCXX_DEBUG_USE_LIBBACKTRACE]
             (__gnu_debug::__create_backtrace_state): New.
             [_GLIBCXX_DEBUG_USE_LIBBACKTRACE]
             (__gnu_debug::__render_backtrace): New.
[_GLIBCXX_DEBUG_USE_LIBBACKTRACE](_Error_formatter::_M_print_backtrace):
             New.
[_GLIBCXX_DEBUG_USE_LIBBACKTRACE](_Error_formatter::_M_backtrace_state):
             New.
             (_Error_formatter::_Error_formatter): Outline definition.
             * src/c++11/debug.cc: Include <cstring>.
             (_Print_func_t): New.
             (print_word): Use '%.*s' format in fprintf to render only 
expected
             number of chars.
             (print_raw(PrintContext&, const char*, ptrdiff_t)): New.
             (print_function(PrintContext&, const char*, 
_Print_func_t)): New.
             (print_type): Use latter.
             (print_string(PrintContext&, const char*, const 
_Parameter*, size_t)):
             Change signature to...
             (print_string(PrintContext&, const char*, ptrdiff_t, const 
_Parameter*,
             size_t)): ...this and adapt. Remove intermediate buffer to 
render input
             string.
             (print_string(PrintContext&, const char*, ptrdiff_t)): New.
             [_GLIBCXX_DEBUG_USE_LIBBACKTRACE]
             (print_backtrace(void*, uintptr_t, const char*, int, const 
char*)): New.
             (_Error_formatter::_M_error()): Adapt.
             [_GLIBCXX_DEBUG_USE_LIBBACKTRACE]
             (__gnu_debug::__create_backtrace_state): New, weak symbol.
             [_GLIBCXX_DEBUG_USE_LIBBACKTRACE]
             (__gnu_debug::__render_backtrace): New, weak symbol.
             * testsuite/util/testsuite_abi.cc: Add new symbol version.
             * doc/xml/manual/debug_mode.xml: Document 
_GLIBCXX_DEBUG_BACKTRACE.
             * doc/xml/manual/using.xml: Likewise.

Tested under Linux x86_64.

Ok to commit ?

François