Re: [PATCH] Add _GLIBCXX_DEBUG backtrace generation

["François Dumont" <frs.dumont@gmail.com>]

On 31/08/22 12:11, Jonathan Wakely wrote:
> On Wed, 31 Aug 2022 at 06:05, François Dumont <frs.dumont@gmail.com> wrote:
>> After a second thought here is an even cleaner version. No more function
>> rename, current pretty_print is fine.
>>
>>       libstdc++: [_GLIBCXX_DEBUG] Add backtrace generation on demand
>>
>>         Add _GLIBCXX_DEBUG_BACKTRACE macro to activate backtrace
>> generation on
>>       _GLIBCXX_DEBUG assertions. Prerequisite is to have configure the
>> lib with:
>>
>>       --enable-libstdcxx-backtrace=yes
>>
>>       libstdc++-v3/ChangeLog:
>>
>>               * include/debug/formatter.h
>>               [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_state): Declare.
>> [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_create_state): Declare.
>> [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full_callback): Define.
>> [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_error_callback): Define.
>> [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full_func): Define.
>>               [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full): Declare.
>> [_GLIBCXX_HAVE_STACKTRACE](_Error_formatter::_M_backtrace_state): New.
>> [_GLIBCXX_HAVE_STACKTRACE](_Error_formatter::_M_backtrace_full): New.
>>               * src/c++11/debug.cc
>> [_GLIBCXX_HAVE_STACKTRACE](print_backtrace): New.
>>               (_Error_formatter::_M_error()): Adapt.
>>               * src/libbacktrace/Makefile.am: Add backtrace.c.
>>               * src/libbacktrace/Makefile.in: Regenerate.
>>               * src/libbacktrace/backtrace-rename.h (backtrace_full): New.
>>               *
>> testsuite/23_containers/vector/debug/assign4_backtrace_neg.cc: New test.
>>               * doc/xml/manual/debug_mode.xml: Document
>> _GLIBCXX_DEBUG_BACKTRACE.
>>               * doc/xml/manual/using.xml: Likewise.
>> Ok to commit ?
> OK for trunk, thanks.
>
> The small change to print_raw in this patch makes me wonder whether
> that function is actually useful.
>
> It supports two modes, print with max precision, and print without.
> The only time we use it to print with max precision we pass a string
> of exactly the right length, so the precision is not needed (but the
> caller has to get the string length correct: if we increase _S_indent
> and do not increase the "    " literal passed to print_raw, the
> effects would be wrong).
>
> Wouldn't it be better to just use fprintf directly when we want to
> print without precision, and use a minimum field width instead of
> precision for indenting? i.e. ...
>
> --- a/libstdc++-v3/src/c++11/debug.cc
> +++ b/libstdc++-v3/src/c++11/debug.cc
> @@ -608,15 +608,6 @@ namespace
>      print_literal(PrintContext& ctx, const char(&word)[Length])
>      { print_word(ctx, word, Length - 1); }
>
> -  void
> -  print_raw(PrintContext& ctx, const char* str, ptrdiff_t nbc = -1)
> -  {
> -    if (nbc >= 0)
> -      ctx._M_column += fprintf(stderr, "%.*s", (int)nbc, str);
> -    else
> -      ctx._M_column += fprintf(stderr, "%s", str);
> -  }
> -
>    void
>    print_word(PrintContext& ctx, const char* word, ptrdiff_t nbc = -1)
>    {
> @@ -643,12 +634,9 @@ namespace
>         || (ctx._M_column + visual_length < ctx._M_max_length)
>         || (visual_length >= ctx._M_max_length && ctx._M_column == 1))
>        {
> -       // If this isn't the first line, indent
> +       // If this isn't the first line, indent.
>         if (ctx._M_column == 1 && !ctx._M_first_line)
> -         {
> -           const char spacing[PrintContext::_S_indent + 1] = "    ";
> -           print_raw(ctx, spacing, PrintContext::_S_indent);
> -         }
> +         ctx._M_column += fprintf(stderr, "%*c", PrintContext::_S_indent, ' ');
I did not know this syntax, it looks definitely better.
>
>         int written = fprintf(stderr, "%.*s", (int)length, word);
>
> @@ -1112,7 +1100,7 @@ namespace __gnu_debug
>      PrintContext ctx;
>      if (_M_file)
>        {
> -       print_raw(ctx, _M_file);
> +       ctx._M_column += fprintf(stderr, "%s", _M_file);
>         print_literal(ctx, ":");
>         go_to_next_line = true;
>        }
>
Do you take care or you prefer I do ?