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Re: Question about blockframe.c:inside_main_func()


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



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