libbacktrace integration for _GLIBCXX_DEBUG mode

François Dumont frs.dumont@gmail.com
Wed May 29 17:45:00 GMT 2019 

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.

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