Re: Question about blockframe.c:inside_main_func()

[Andrew Cagney <cagney@gnu.org>]

> Hi all,
> 
> We're bringing up the currentish gdb sources here at Apple and I was debugging a problem with inside_main_func () [*] when I noticed that there seems to be a bit of extra computation that has snuck into the function during the changes since July.
> 
> Previously, inside_main_func() would find the "main" function in the "symfile_objfile", find its start and end addresses (if debug symbols were present I guess) and on subsequent invocations, use those cached addresses to determine if the addr in question is contained within the "main" function.
> 
> The current inside_main_func() will do
> 
> msymbol = lookup_minimal_symbol (main_name (), NULL, symfile_objfile);// every time
> 
>   if (msymbol != NULL             // once
>       && symfile_objfile->ei.main_func_lowpc == INVALID_ENTRY_LOWPC
>       && symfile_objfile->ei.main_func_highpc == INVALID_ENTRY_HIGHPC)
> 
>   if (msymbol != NULL && MSYMBOL_TYPE (msymbol) == mst_text)  // every time
>     {
>  [... lots of stuff ...]
>     }
> 
> I realize this is hardly a performance critical function, but it's still a long shot from the version that existed before July which would find the start/end addresses and then do
> 
>   if (symfile_objfile->ei.main_func_lowpc == INVALID_ENTRY_LOWPC &&  // once
>       symfile_objfile->ei.main_func_highpc == INVALID_ENTRY_HIGHPC)
>   [... lookup symbol ... ]
> 
>   return (symfile_objfile->ei.main_func_lowpc <= pc
>           && symfile_objfile->ei.main_func_highpc > pc);

> Is there some reason why this shortcut has been dropped?  Is there a reason not to add a conditional to the top to detect "main"'s bounds being detected and short-circuit the searching we're doing every time.

Per Joel's comments, I'd guess accident.

However, I think the entire function's contents are bogus.  It should 
look like:

	if (symtab_find_function_range_by_name (main_name (), &low_pc, &high_pc))
	  return pc in [low_pc, high_pc);
	else
	  return 0;

so that the logic is pushed back into the symbol table (an obvious thing 
for lookup_function_range_by_name to do is implement a look-aside cache).

This also lets us kill off main_func_lowpc and main_func_highpc (they 
need to be killed off anyway as PIE breaks the assumption that the 
values are constant across function invocations).

> Jason
> 
> [*] We have something called "ZeroLink" where the main executable -- the symfile_objfile -- is a tiny stub that demand-loads each object file (formatted like a shared library) as functions/global variables in those .o's are referenced.  So in our case, the symfile_objfile doesn't contain main at all; hence me looking into this function and scratching my head about why it's re-searching for this function every time...

you might want to look at PIE.

Andrew