[Bug c++/44783] implement -ftemplate-backtrace-limit=

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44783

--- Comment #11 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-02-14 16:26:10 UTC ---
(In reply to comment #10)
> By the way, for the actual dejagnu testcase, a few weeks ago I noticed that
> apparently testing for 'note:' is pretty weak, like if additional notes are
> emitted (additional in the sense that there is no corresponding dg-message to
> match) that doesn't lead to failure. Did you see anything like that?

Any number of notes will be matched by a single "note:" test, since the
testsuite does not detect duplicates. There is PR about this somewhere.

> Anyway, about the code, if I understand correctly your explanation, 'n_total >
> template_backtrace_limit + 1' would lead to the exact same default behavior we
> have now, but appears to misbehave a little when template_backtrace_limit == 1
> and n_total == 2 because we really want 1 note in that case, that is what the
> user really asked, right? Then probably what you wrote in the draft is fine,
> but I don't have a strong opinion, really, I only noticed something 'weird'
> going on with that condition and decided to ask.

So I used the testcase testsuite/g++.dg/cpp0x/noexcept15.C to play a bit. 

I noticed that my earlier version never prints "skipping 1 instantiation
contexts". Now I recall that this was done on purpose. It seems pointless to
print a line "skipping 1" instead of the actual instantiation context. Also, we
don't have to deal with singular/plural. ;-)

However, this means that in some cases, we would actually print one more than
the limit. If we just say that the limit is a limit not the exact number
printed, a workaround is to print one less in that case. So:

limit n_total printed skipped
5     5       5       0
5     7       5       2
5     6       4       2

I also added the option name to the message for convenience (clang also does
this if I recall correctly).


-  if (n_total >= 12) 
+  if (template_backtrace_limit &&
+      n_total > template_backtrace_limit) 
     {
-      int skip = n_total - 10;
-      for (n = 0; n < 5; n++)
+      int skip = n_total - template_backtrace_limit;
+      int head = template_backtrace_limit/ 2;
+      /* Avoid skipping just 1. If so, skip 2.  */
+      if (skip == 1)
+       {
+         skip = 2;
+         head = (template_backtrace_limit - 1) / 2;
+       }
+      
+      for (n = 0; n < head; n++)
        {
          gcc_assert (t != NULL);
          if (loc != t->locus)
            print_instantiation_partial_context_line (context, t, loc,
                                                      /*recursive_p=*/false);
          loc = t->locus;
          t = t->next;
        }
-      if (t != NULL && skip > 1)
+      if (t != NULL && skip > 0)
        {
          expanded_location xloc;
          xloc = expand_location (loc);
          if (context->show_column)
            pp_verbatim (context->printer,
-                        _("%s:%d:%d:   [ skipping %d instantiation contexts
]\n"),
+                        _("%s:%d:%d:   [ skipping %d instantiation contexts,
use -ftemplate-backtrace-limit=0 to disable ]\n"),
                         xloc.file, xloc.line, xloc.column, skip);
          else
            pp_verbatim (context->printer,
-                        _("%s:%d:   [ skipping %d instantiation contexts
]\n"),
+                        _("%s:%d:   [ skipping %d instantiation contexts, use
-ftemplate-backtrace-limit=0 to disable ]\n"),
                         xloc.file, xloc.line, skip);