From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 857 invoked by alias); 29 Apr 2004 15:09:29 -0000 Mailing-List: contact gdb-patches-help@sources.redhat.com; run by ezmlm Precedence: bulk List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sources.redhat.com Received: (qmail 850 invoked from network); 29 Apr 2004 15:09:28 -0000 Received: from unknown (HELO mx1.redhat.com) (66.187.233.31) by sources.redhat.com with SMTP; 29 Apr 2004 15:09:28 -0000 Received: from int-mx1.corp.redhat.com (int-mx1.corp.redhat.com [172.16.52.254]) by mx1.redhat.com (8.12.10/8.12.10) with ESMTP id i3TF9SKI026970 for ; Thu, 29 Apr 2004 11:09:28 -0400 Received: from localhost.redhat.com (to-dhcp51.toronto.redhat.com [172.16.14.151]) by int-mx1.corp.redhat.com (8.11.6/8.11.6) with ESMTP id i3TF9Rv11313; Thu, 29 Apr 2004 11:09:27 -0400 Received: from gnu.org (localhost [127.0.0.1]) by localhost.redhat.com (Postfix) with ESMTP id 0FC902B9D; Thu, 29 Apr 2004 11:09:29 -0400 (EDT) Message-ID: <40911AA8.9080106@gnu.org> Date: Thu, 29 Apr 2004 15:09:00 -0000 From: Andrew Cagney User-Agent: Mozilla/5.0 (X11; U; NetBSD macppc; en-GB; rv:1.4.1) Gecko/20040217 MIME-Version: 1.0 To: Jason Molenda , Joel Brobecker Cc: gdb-patches@sources.redhat.com Subject: Re: Question about blockframe.c:inside_main_func() References: <9B827536-9972-11D8-9EA7-000A9569836A@apple.com> In-Reply-To: <9B827536-9972-11D8-9EA7-000A9569836A@apple.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-SW-Source: 2004-04/txt/msg00673.txt.bz2 > 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