Question about blockframe.c:inside_main_func()
Andrew Cagney
cagney@gnu.org
Thu Apr 29 15:09:00 GMT 2004
> 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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