Re: [PATCH] temporary fix for bogus language check

[Elena Zannoni <ezannoni@cygnus.com>]

Fred Fish writes:
 > In lookup_symbol(), gdb checks the current language and if it is C++,
 > trys to demangle the symbol name before looking it up.  This works
 > fine if we are both looking up a C++ symbol and the current language
 > is C++.  However it breaks down if we are trying to lookup a C++
 > symbol and the current language is not C++, such as when stopped
 > inside a C function (perhaps a C library call for example).
 > 
 > Consider decode_line_1 in linespec.c, which is called when you do
 > something like "br PCReader::work".  This function calls
 > find_methods(), which later calls lookup_symbol() with the mangled
 > name "work__8PCReader".  If the current language is C++,
 > lookup_symbol() demangles it to "PCReader::work(void)" and calls
 > lookup_symbol_aux with the demangled name, which succeeds.  If however
 > you try to set the same breakpoint when stopped at a C function
 > setting the breakpoint fails.
 > 
 > The following patch works around the problem by always attempting to
 > demangle the symbol to be looked up.  We could fix this problem by
 > eliminating the test of the current language, but then every gdb
 > session would incur the additional overhead of attempting to demangle
 > every symbol regardless of whether or not any C++ symbols were
 > present.  Another option is to set some global flag whenever symbols
 > for a C++ function are read in, and then do the current language test
 > unconditionally once we know that there might be C++ symbols
 > somewhere.  Yet another option is to add a parameter to lookup_symbol
 > that says whether to consider the possibility that the symbol to be
 > looked up is a C++ symbol, set that appropriately when calling
 > lookup_symbol, and use that value to decide whether or not to try
 > demangling the symbol.


Hmm, I can see the problem. We have the language information stored in
the symtabs, but we don't know where the symbol is yet, so that's
useless.  I am not sure that the idea of saving somewhere the language
information as you read symbols in would work here. It may be that you
are setting your very first breakpoint in a c++ function, and you
haven't read the symbols in yet. Is psymtab_to_symtab called before
lookup_symbol or not?

Actually I start to like the idea that the caller of lookup_symbol
should know whether or not the name should be demangled. After all
decode_line_1 has already figured out it is dealing with a c++ name,
why throw that info away? Or (thinking out loud) would it make sense
to never pass a mangled symbol to lookup_symbol (can the caller take
care of demangling it?).

Elena


 > 
 > Suggestions?
 > 
 > -Fred
 > 
 > Index: symtab.c
 > ===================================================================
 > RCS file: /cvs/src/src/gdb/symtab.c,v
 > retrieving revision 1.48
 > diff -u -r1.48 symtab.c
 > --- symtab.c	2001/11/13 16:42:50	1.48
 > +++ symtab.c	2001/11/25 18:49:46
 > @@ -528,8 +528,11 @@
 >        modified_name = (char *) name;
 >  
 >    /* If we are using C++ language, demangle the name before doing a lookup, so
 > -     we can always binary search. */
 > -  if (current_language->la_language == language_cplus)
 > +     we can always binary search.
 > +     NOTE: We need to always try to demangle since the current_language might
 > +     be something other than C++ at the point when we are trying to set a
 > +     breakpoint in C++ code.  -fnf */
 > +  if (1 || (current_language->la_language == language_cplus))
 >      {
 >        modified_name2 = cplus_demangle (modified_name, DMGL_ANSI | DMGL_PARAMS);
 >        if (modified_name2)