Date: Sun, 1 Jan 2012 14:25:19 -0800 From: Garrett Cooper <yanegomi@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: [patch] Cleaning up amd64 kernel optimization options Message-ID: <CAGH67wTGn2WV=Cw7KkJRE_g1AKLn3iBFM7vVwsH3Fy83T6MFEw@mail.gmail.com> In-Reply-To: <CAGH67wRca4cg9n3FEkcGebyxHxRW%2BgxhymtaTXdM7ePVA8bm5A@mail.gmail.com> References: <4EF34E52.2040905@FreeBSD.org> <20111223005932.GA65042@freebsd.org> <25FBBF23-CDFA-429E-966D-A90409D8F2BD@gmail.com> <201112230937.08971.jhb@freebsd.org> <CAGH67wRca4cg9n3FEkcGebyxHxRW%2BgxhymtaTXdM7ePVA8bm5A@mail.gmail.com>
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On Fri, Dec 23, 2011 at 12:34 PM, Garrett Cooper <yanegomi@gmail.com> wrote= : > On Fri, Dec 23, 2011 at 6:37 AM, John Baldwin <jhb@freebsd.org> wrote: >> On Thursday, December 22, 2011 9:51:47 pm Garrett Cooper wrote: >>> On Dec 22, 2011, at 4:59 PM, Alexander Best <arundel@freebsd.org> wrote= : >>> >>> > On Thu Dec 22 11, Benjamin Kaduk wrote: >>> >> On Thu, 22 Dec 2011, Alexander Best wrote: >>> >> >>> >>> On Thu Dec 22 11, Dimitry Andric wrote: >>> >>>> Hi, >>> >>>> >>> >>>> I would like to ask some feedback on the attached patch, which cle= ans up >>> >>>> the kernel optimization options for amd64. =A0This was touched upo= n >>> >>>> earlier by Alexander Best in freebsd-toolchain, here: >>> >>> >>> >>> i've been using such settings for a few months now and haven't noti= ced any >>> >>> problems. >>> >>> >>> >>> however bruce evans raised a good point (in a private mail). when y= ou >>> >>> compile a >>> >>> kernel without debugging enabled, -O2 is the default. if you experi= ence >>> >>> issues, >>> >>> and enable debugging, -O0 now becomes the default. in case the prob= lems >>> >>> with >>> >>> the kernel were caused by the -O2 option and aren't present with th= e -O0 >>> >>> option, the newly built kernel with debugging enabled will not help= you >>> >>> fix the >>> >>> problems. in that case you would need to set -O2 explicitly in CFLA= GS. his >>> >>> exact words were: >>> >>> >>> >>> " >>> >>> I don't like -O2 for anything. =A0However, if it is only a default,= it is >>> >>> not a problem provided it can be canceled easily. =A0And for debugg= ing, you >>> >>> want the default to be the same as without debugging, so that (by d= efault) >>> >>> you debug the same code that caused the problem. >>> >>> " >>> >>> >>> >>> however i don't think this is fixable. using -O0 for debuggable and >>> >>> non-debuggable kernels will cause too much of a slowdown. >>> >>> >>> >>> having -O2 as the default flag for non-debuggable kernels and -O2 -= g for >>> >>> debuggable kernels might cause situations, where debugging isn't po= ssible, >>> >>> where with -O0 -g it would have been. >>> >>> >>> >>> so i guess although bruces concerns are valid, they are impossible = to >>> >>> solve. >>> >> >>> >> Where does -O0 come in? =A0I only see talk of -O (i.e. -O1) versus -= O2. >>> > >>> > sorry. of course i meant -O: >>> > >>> > .if defined(DEBUG) >>> > _MINUS_O=3D =A0 =A0 =A0 -O >>> > CTFFLAGS+=3D =A0 =A0 =A0-g >>> > .else >>> > [..] >>> >>> Back in the 7.x days, I ran into some code that wasn't easily to debug = because the compiler optimized things out with -O2 by inlining and >> otherwise shifting around code, so setting breakpoints in gdb became dif= ficult. So from that point on I've gotten into the habit of doing -O >> explicitly in make.conf if DEBUG_FLAGS was specified. Just a thought.. >> >> I still leave -O2 in, but what I do is this: >> >> =A0make DEBUG_FLAGS=3D"-g -fno-inline" >> >> Just adding -fno-inline makes a world of difference. > > Sweet -- thanks for the tip ;). Just as a sidenote, this option doesn't work when compiling mii(4), re(4), etc [as modules at least]. Thanks, -Garrett
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