From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 862 invoked by alias); 5 Dec 2008 02:46:20 -0000 Received: (qmail 811 invoked by uid 22791); 5 Dec 2008 02:46:19 -0000 X-Spam-Check-By: sourceware.org Received: from mail.codesourcery.com (HELO mail.codesourcery.com) (65.74.133.4) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Fri, 05 Dec 2008 02:45:24 +0000 Received: (qmail 14409 invoked from network); 5 Dec 2008 02:45:21 -0000 Received: from unknown (HELO orlando.local) (pedro@127.0.0.2) by mail.codesourcery.com with ESMTPA; 5 Dec 2008 02:45:21 -0000 From: Pedro Alves To: gdb-patches@sourceware.org Subject: Re: Get rid of stop_pc (was: [RFA] dummy frame handling cleanup, plus inferior fun call signal handling improvement) Date: Fri, 05 Dec 2008 02:46:00 -0000 User-Agent: KMail/1.9.10 Cc: "Ulrich Weigand" , Doug Evans References: <200812050018.mB50I05V031478@d12av02.megacenter.de.ibm.com> <200812050036.56899.pedro@codesourcery.com> <200812050115.30692.pedro@codesourcery.com> In-Reply-To: <200812050115.30692.pedro@codesourcery.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200812050245.20430.pedro@codesourcery.com> X-IsSubscribed: yes Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org X-SW-Source: 2008-12/txt/msg00089.txt.bz2 On Friday 05 December 2008 01:15:30, Pedro Alves wrote: > On Friday 05 December 2008 00:36:56, Pedro Alves wrote: > > On Friday 05 December 2008 00:18:00, Ulrich Weigand wrote: > > > Pedro Alves wrote: > > > > On Thursday 04 December 2008 22:32:12, Doug Evans wrote: > > > > > In the original code, is there a case when stop_pc != registers.pc? > > > > > > > > Here, > > > > > > > > > > > > (gdb) set $pc = 0xf00 > > > > (gdb) call func() > > > > > > Huh. But that case is in fact *broken*, because GDB will use stop_pc > > > incorrectly: for example, the check whether we are about to continue > > > at a breakpoint will look at stop_pc, but then continue at $pc. > > > > This one I believe was the original intention. The rationale being > > that you'd not want to hit a breakpoint again at stop_pc (0x1234), > > because there's where you stopped; but, you'd want to hit a a breakpoint > > at 0xf00, sort of like jump *$pc hits a breakpoint at $pc. > > > > Note, I'm not saying I agree with this. I did say that probably nobody > > would notice if we got rid of stop_pc. > > > > > It seems to me just about every current user of stop_pc *really* wants > > > to look at regcache_read_pc (get_current_regcache ()) ... > > Is using read_pc instead OK with you? It's what I had written already. > > > I've been sneaking the idea of getting rid of stop_pc for a while now: > > http://sourceware.org/ml/gdb-patches/2008-06/msg00450.html > > > > In fact, I have a months old patch here that completelly removes stop_pc. > > IIRC, there were no visible changes in the testsuite. Say the word, > > and I'll brush it up, regtest, submit it. > > Here it is, it still applied cleanly. It's smallish. Regtested on > x86-64-unknown-linux-gnu. > > My original motivation was to get rid of the ugly checks > in switch_to_thread, and to try to minimize the extra thread > switching and register reads in non-stop mode. > > I had held posting this when I wrote it, since I was not sure we'd not > miss stop_pc in some case. > I should say that I also considered going the other direction and adding a stop_pc per thread for use in `proceed', while still replacing most other references to stop_pc by read_pc. Say something like, in all-stop mode, the thread that hit the breakpoint would have stop_pc set to read_pc, and all the other threads would have it set to say (CORE_ADDR) -1. This was to consider the following case: Say where you're debugging an inferior with 2 threads, and two distinct breakpoints installed. thread 1 has just reported a hit on breakpoint 1; and thread 2 happens to have hit breakpoint 2 simultaneously, but, since events are serialized, linux-nat.c:cancel_breakpoint took action, so thread 2 got the PC rolled back. The user does: (gdb) b foo (gdb) b bar (gdb) continue (gdb) thread 2 (gdb) delete 1 proceed'ing now doesn't need to step over breakpoint 1 (prepare_to_proceed will do nothing), since breakpoint 1 is gone. (gdb) continue At this point, current GDB will step over breakpoint 2, although it was never reported as hit. proceed: if (addr == (CORE_ADDR) -1) { if (pc == stop_pc && breakpoint_here_p (pc) ^^^^^^^^^^^^^ If this was instead tp->stop_pc, and it was -1 at this point, because thread 2 was not the thread that reported the last event, breakpoint 2 would not be missed. -- Pedro Alves