Re: libbacktrace integration for _GLIBCXX_DEBUG mode

[François Dumont <frs dot dumont at gmail dot com>]

On 5/29/19 12:06 AM, Jonathan Wakely wrote:
> On 23/05/19 07:39 +0200, François Dumont wrote:
> > Hi
> >
> >     So here what I come up with.
> >
> >     _GLIBCXX_DEBUG_BACKTRACE controls the feature. If the user define 
>
> Thanks for making this opt-in.
>
> > it and there is a detectable issue with libbacktrace then I generate 
> > a compilation error. I want to avoid users defining it but having no 
> > backtrace in the end in the debug assertion.
>
> Why do you want to avoid that?
>
> This means users can't just define the macro in their makefiles
> unconditionally, they have to check if libbacktrace is installed and
> supported. That might mean having platform-specific conditionals in
> the makefile to only enable it sometimes.
>
> What harm does it do to just ignore the _GLIBCXX_DEBUG_BACKTRACE macro
> if backtraces can't be enabled? Or just use #warning instead of
> #error?
What I want to avoid is PR  saying that despite _GLIBCXX_DEBUG_BACKTRACE 
being defined there isn't any backtrace displayed in the assertion message.

Maybe I can still fail to compile if I can't include 
backtrace-supported.h to make clear that the -I... option is missing but 
ignore if !BACKTRACE_SUPPORTED. Would it be fine ?

> >     With this new setup I manage to run testsuite with it like that:
> >
> > export LD_LIBRARY_PATH=/home/fdt/dev/gcc/install/lib/
> > make CXXFLAGS='-D_GLIBCXX_DEBUG_BACKTRACE 
> > -I/home/fdt/dev/gcc/install/include -lbacktrace' check-debug
> >
> >     An example of result:
> >
> > /home/fdt/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/debug/vector:606: 
> >
> > In function:
> >     std::__debug::vector<_Tp, _Allocator>::iterator
> >     std::__debug::vector<_Tp, 
> > _Allocator>::insert(std::__debug::vector<_Tp,
> >     _Allocator>::const_iterator, _InputIterator, _InputIterator) [with
> >     _InputIterator = int*; <template-parameter-2-2> = void; _Tp = int;
> >     _Allocator = std::allocator<int>; std::__debug::vector<_Tp,
> >     _Allocator>::iterator =
> > __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<int*, std::
> >     vector<int> >, std::__debug::vector<int>,
> >     std::random_access_iterator_tag>; typename 
> > std::iterator_traits<typename
> >     std::vector<_Tp, _Alloc>::iterator>::iterator_category =
> >     std::random_access_iterator_tag; typename std::vector<_Tp,
> >     _Alloc>::iterator = __gnu_cxx::__normal_iterator<int*, 
> > std::vector<int>
> >     >; std::__debug::vector<_Tp, _Allocator>::const_iterator =
> > __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<const int*,
> >     std::vector<int> >, std::__debug::vector<int>,
> >     std::random_access_iterator_tag>; typename 
> > std::iterator_traits<typename
> >     std::vector<_Tp, _Alloc>::const_iterator>::iterator_category =
> >     std::random_access_iterator_tag; typename std::vector<_Tp,
> >     _Alloc>::const_iterator = __gnu_cxx::__normal_iterator<const 
> > int*, std::
> >     vector<int> >]
> >
> > Backtrace:
> >     0x402718 
> > __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<int*, 
> > std::vector<int> >> std::__debug::vector<int>::insert<int*, 
> > void>(__gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<int 
> > const*, std::vector<int> >>, int*, int*)
> > /home/fdt/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/debug/vector:606 
> >
> >     0x402718 test01()
> > /home/fdt/dev/gcc/git/libstdc++-v3/testsuite/23_containers/vector/debug/57779_neg.cc:29 
> >
> >     0x401428 main
> > /home/fdt/dev/gcc/git/libstdc++-v3/testsuite/23_containers/vector/debug/57779_neg.cc:34 
> >
> >
> > Error: attempt to insert with an iterator range [__first, __last) 
> > from this
> > container.
> >
> > Objects involved in the operation:
> >     iterator "__first" @ 0x0x7fff730b96b0 {
> >       type = int* (mutable iterator);
> >     }
> >     iterator "__last" @ 0x0x7fff730b96b8 {
> >       type = int* (mutable iterator);
> >     }
> >     sequence "this" @ 0x0x7fff730b9720 {
> >       type = std::__debug::vector<int>;
> >     }
>
> This is nice.

Yes, I forgot to say that I made an effort to clean a little the 
demangle names but I think you saw it already.

I was surprised to see that the content of the __PRETTY_FUNCTION__ macro 
is quite different from the demangling of the same symbol, too bad.

> > diff --git a/libstdc++-v3/doc/xml/manual/debug_mode.xml 
> > b/libstdc++-v3/doc/xml/manual/debug_mode.xml
> > index 570c17ba28a..27873151dae 100644
> > --- a/libstdc++-v3/doc/xml/manual/debug_mode.xml
> > +++ b/libstdc++-v3/doc/xml/manual/debug_mode.xml
> > @@ -104,9 +104,11 @@
> > <para>The following library components provide extra debugging
> >   capabilities in debug mode:</para>
> > <itemizedlist>
> > + <listitem><para><code>std::array</code> (no safe 
> > iterators)</para></listitem>
> > <listitem><para><code>std::basic_string</code> (no safe iterators and 
> > see note below)</para></listitem>
> > <listitem><para><code>std::bitset</code></para></listitem>
> > <listitem><para><code>std::deque</code></para></listitem>
> > + <listitem><para><code>std::forward_list</code></para></listitem>
> > <listitem><para><code>std::list</code></para></listitem>
> > <listitem><para><code>std::map</code></para></listitem>
> > <listitem><para><code>std::multimap</code></para></listitem>
> > @@ -160,6 +162,13 @@ which always works correctly.
> >   <code>GLIBCXX_DEBUG_MESSAGE_LENGTH</code> can be used to request a
> >   different length.</para>
> >
> > +<para>Starting with GCC 10 libstdc++ has integrated
>
> I think "integrated" is the wrong word, because you haven't made
> libbacktrace and libstdc++ into a single thing. You've just added the
> ability for libstdc++ to use libbacktrace.
>
> I suggest "libstdc++ is able to use"
>
> > +  <link xmlns:xlink="http://www.w3.org/1999/xlink";
> > + 
> > xlink:href="https://github.com/ianlancetaylor/libbacktrace";>libbacktrace</link>
> > +  to produce backtrace on error. Use 
> > <code>-D_GLIBCXX_DEBUG_BACKTRACE</code> to
>
> Should be "produce backtraces".
>
> > +  activate it. Note that if not properly installed or if 
> > libbacktrace is not
> > +  supported compilation will fail. You'll also have to use the
>
> Comma after "supported". Remove "the" at the end of the line.
>
>
> > +  <code>-lbacktrace</code> to build your application.</para>
> > </section>
>
> The new _GLIBCXX_DEBUG_BACKTRACE macro should be documented in
> doc/xml/manual/using.xml too. I think we also have a table in that
> chapter which lists relevant GCC options, so -lbacktrace could go in
> there too.


Ok, I'll consider your suggestions accordingly and complete this chapter.

> Enabling backtrace support doesn't alter the ABI of the debug mode
> containers, right? It only affects the size of the _Error_formatter
> type, which is not a member of any container or iterator

Indeed, no ABI breaking change.