From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 24661 invoked by alias); 17 Mar 2003 16:38:51 -0000 Mailing-List: contact gdb-help@sources.redhat.com; run by ezmlm Precedence: bulk List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-owner@sources.redhat.com Received: (qmail 24605 invoked from network); 17 Mar 2003 16:38:50 -0000 Received: from unknown (HELO crack.them.org) (65.125.64.184) by sources.redhat.com with SMTP; 17 Mar 2003 16:38:50 -0000 Received: from nevyn.them.org ([66.93.61.169] ident=mail) by crack.them.org with asmtp (Exim 3.12 #1 (Debian)) id 18uzWo-00036M-00; Mon, 17 Mar 2003 12:40:09 -0600 Received: from drow by nevyn.them.org with local (Exim 3.36 #1 (Debian)) id 18uxdL-0001JN-00; Mon, 17 Mar 2003 11:38:43 -0500 Date: Mon, 17 Mar 2003 16:38:00 -0000 From: Daniel Jacobowitz To: Andrew Cagney Cc: gdb@sources.redhat.com Subject: Re: frame->unwind->this_base() Message-ID: <20030317163843.GA11494@nevyn.them.org> Mail-Followup-To: Andrew Cagney , gdb@sources.redhat.com References: <3E74F4F4.50003@redhat.com> <20030316221008.GA19037@nevyn.them.org> <3E75121F.4030405@redhat.com> <20030317001407.GA20827@nevyn.them.org> <3E75F64B.5040700@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3E75F64B.5040700@redhat.com> User-Agent: Mutt/1.5.1i X-SW-Source: 2003-03/txt/msg00256.txt.bz2 On Mon, Mar 17, 2003 at 11:22:35AM -0500, Andrew Cagney wrote: > > >>However, shouldn't the only thing needing the `virtual frame pointer' / > >>get_frame_base() be the code that needs a virtual base pointer when > >>computing the value of a local variable? > > > > > >Yes, and that's the only time that we search for the frame base. But > >what difference does it make? > > (gdb) info frame > > will display the correct value. What does "correct" mean though? > >At that point we have an offset that we > >know is relative to DW_AT_frame_base, but we don't know if it's > >relative to what the rest of GDB considers the frame base (since we > >never use DW_AT_frame_base to compute the frame base in the first > >place; and it's not clear to me that we should be). > > Where, apart from `info frame', and variable evaluation, is it correct > for GDB to use the frame base? I'm sorry, but I just don't understand what you're asking. We use the frame base all over. The current frame base (i.e. id.base) is produced by target specific code - often via prologue analysis; on x86-64 via CFI; etc. The prologue analyzer, CFI code, etc. use the frame base when finding saved registers, the saved PC value, et cetera. For instance i386 uses it to read the saved PC off of the stack. This defines what GDB expects to be the frame base. The compiler doesn't necessarily have the same idea of the frame base. It can pick a convenient location, which may be biased some number of words away from what GDB considers the frame base. There's any number of reasons this might happen. Depending on, for instance, the architecture's ABI and the available instructions for frame-relative loads. When using information computed by GDB we may need to use the current frame base. When using information provided by the compiler we probably need to use the compiler provided frame base. Are you saying that you want GDB always to use the DW_AT_frame_base? That's a bit of a leap from where we are today. -- Daniel Jacobowitz MontaVista Software Debian GNU/Linux Developer