Date: Wed, 31 Oct 2007 22:31:38 +0100 From: Kris Kennaway <kris@FreeBSD.org> To: "Stephan F. Yaraghchi" <stephan@yaraghchi.org> Cc: freebsd-current@freebsd.org Subject: Re: loader breaks with -O2 optimizations Message-ID: <4728F43A.1030500@FreeBSD.org> In-Reply-To: <25f52a3d0710310303w7138bf5ds92698f1b6af2f655@mail.gmail.com> References: <25f52a3d0710300732x425f0c45vde1a29361096c018@mail.gmail.com> <47284E4F.5030008@FreeBSD.org> <25f52a3d0710310303w7138bf5ds92698f1b6af2f655@mail.gmail.com>
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Stephan F. Yaraghchi wrote: > On 10/31/07, Kris Kennaway <kris@freebsd.org> wrote: >> Stephan F. Yaraghchi wrote: >>> After making world on a freshly installed 7.0-BETA1 >>> the system does not boot anymore due to a broken loader: >>> >>> >>> FreeBSD/i386 bootstrap loader, Revision 1.1 >>> (root@bigblue, Tue Oct 30 11:26:32 CET 2007) >>> Can't work out which disk we are booting from. >>> Guessed BIOS device 0xffffffff not found by probes defaulting to disk0: >>> >>> panic: free: guard1 fail@ 0x6ded4 from >>> /usr/src/sys/boot/i386/loader/../../common/module.c:959 >>> >>> --> Press a key on the console to reboot <-- >>> >>> >>> I found out that the following line in my make.conf causes >>> the problem: >>> >>> CFLAGS= -O2 -funroll-loops -pipe >>> >>> >>> After changing down to -O1 and making /usr/src/sys/boot again >>> the systems behaves properly at boot. >>> >>> Is this behavior intended? >> Maybe, what happens if you use just -O2 -pipe? -funroll-loops is not an >> appropriate thing to be using globally anyway, unless your intention is >> to randomly make some code slower. >> >> Kris >> > > Hi Kris, > > I tried all possible combinations of these switches -- only -O2 led to > the described > behaviour. Presumably you mean -O2 -funroll-loops, not -O2. Or are you saying the latter also breaks the loader? > Anyway, it's very interesting to hear that adding these optimizations > to make.conf > is not recommended, even that -funroll-loops is possibly slowing down > certain code. > > I'm sure many people use it since it's a common tuning tip found on the web. > I read about it in Dru Lavigne's "BSD Hacks" (O'Reilly)... Yes, unfortunately it's bogus advice. What does she say this option is good for? Kris
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