Date: Thu, 01 Nov 2007 10:23:41 +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: <47299B1D.7030507@FreeBSD.org> In-Reply-To: <25f52a3d0711010027t6b124227gfbddcada12b16e24@mail.gmail.com> References: <25f52a3d0710300732x425f0c45vde1a29361096c018@mail.gmail.com> <47284E4F.5030008@FreeBSD.org> <25f52a3d0710310303w7138bf5ds92698f1b6af2f655@mail.gmail.com> <4728F43A.1030500@FreeBSD.org> <25f52a3d0711010027t6b124227gfbddcada12b16e24@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: >>> 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 >> > > Hi Kris, > > you are right: It's the combination of both that causes loader to break. > > Oliver suggested the defaults (-O2 -fno-strict-aliasing -pipe) and it works. > > By the way: "BSD Hacks" is a collection of hacks compiled by Dru. > Hack #69 deals with tuning and is authored by Avleen Vig. > It says: "The -funroll-loops saves one CPU register that would otherwise > be tied up in tracking the iteration of the loop, but at the expense of making > a slightly larger binary." OK, so it's factually correct but just not good advice :) Kris
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