Date: Mon, 12 Sep 2011 07:43:01 -0400 From: John Baldwin <jhb@freebsd.org> To: Andriy Gapon <avg@freebsd.org> Cc: freebsd-fs@freebsd.org, FreeBSD-Current <freebsd-current@freebsd.org> Subject: Re: archaic/useless CFLAGS options for x86 boot blocks Message-ID: <201109120743.02181.jhb@freebsd.org> In-Reply-To: <4E6DB696.1080608@FreeBSD.org> References: <4E6DB696.1080608@FreeBSD.org>
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On Monday, September 12, 2011 3:36:54 am Andriy Gapon wrote: > > This email is in part inspired by the following problem: > http://article.gmane.org/gmane.os.freebsd.current/135292 > So "harmful" could also be added to the subject line. > > So here is my proposal. > > Part I. ZFS and GPT bootblocks. > > I believe that we do not need here any extra optimizations and happy dances that > seem to be carried over from boot2. > I think that just the -Os should be sufficient of optimization flags. Maybe > even that is not really required. > Rationale: > - these boot blocks are not as nearly space-constrained as boot2 > - using untypical flags increases chances of hitting compiler bugs, > especially for those compilers where we are stuck with > unsupported / locally-maintained versions or where a compiler is maturing yet > - assembly / machine code and debugging may become easier > > Additionally, the '/align/d' '/nop/d' filtering of the intermediate assembly > file seems to be not needed for zfsboot. > > Part II. The original boot2. > > My testing shows that -Os -fomit-frame-pointer are sufficient to produce a small > enough boot2 image (~300 bytes remain available with gcc, 51 bytes for clang). > -mrtd -mregparm=3 do not change size with gcc, but with clang they increase > _available_ space to 79 bytes. > > The '/align/d' '/nop/d' filtering seems to shave off only 7 bytes here. > Not suggesting anything, just an observation... > > Part III. History. > > It seems that all those optimization related options were introduced very long > time ago when the compiler(s) were quite different from what they are now. > So, some re-evaluation may be (long over)due. > For example, -fno-unit-at-a-time is definitely an anti-optmization option and it > was introduced to fight some gcc bugs way back in 2004 (r132870). Its merits > have never been re-evaluated after switch to gcc 4.2, it seems. > -fno-guess-branch-probability and -mno-align-long-strings are even less obvious > options (see e.g. r108149). > > > Finally, here is a diff: > http://people.freebsd.org/~avg/boot-cflags.diff > All the boot blocks are boot tested in qemu. > boot2 is also tested with -mrtd -mregparm removed. I suspect some of the recent changes to shave space down for Clang have made some of the optimization options no longer necessary. I think the patch is fine, and I'd even prefer to go ahead and drop the extra cruft (like removing nops and aligns as well as -mrtd and -mregparm) from the UFS boot2 as well. -- John Baldwin
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