Date: Fri, 11 May 2012 11:09:06 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Andriy Gapon <avg@freebsd.org> Cc: freebsd-stable@freebsd.org, John Baldwin <jhb@freebsd.org>, Alfred Bartsch <bartsch@dssgmbh.de> Subject: Re: i386 -march=xxx behavior [Was: FreeBSD 8 i386 gptboot corrupt - SOLVED] Message-ID: <20120511080906.GL2358@deviant.kiev.zoral.com.ua> In-Reply-To: <4FACB396.6060800@FreeBSD.org> References: <4FAA3912.3030801@dssgmbh.de> <4FAA4A11.808@FreeBSD.org> <4FAA5E70.7030508@dssgmbh.de> <4FACB396.6060800@FreeBSD.org>
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[-- Attachment #1 --] On Fri, May 11, 2012 at 09:37:10AM +0300, Andriy Gapon wrote: > on 09/05/2012 15:09 Alfred Bartsch said the following: > > Am 09.05.2012 12:42, schrieb Andriy Gapon: > >> on 09/05/2012 12:29 Alfred Bartsch said the following: > >>> This behavior is restricted to 32-bit servers (i386), all 64-bit > >>> servers (amd64) work without any problem, as expected. > >>> > >>> After some analyzing, it seems to me that the actual size of gptboot > >>> does matter (16723 bytes, >16kB). In amd64 environment (same source > >>> version) the actual size of /boot/gptboot is only 15443 bytes. > > > >> Weird. Both amd64 and i386 builds should produce the same binaries as > >> the boot code is built with -m32 -march=i386 on amd64. But I can > >> reproduce this, so it seems that the compilation is indeed done > >> differently. > > > >> Heh, it seems that it is -march=i386 flag that makes all the difference. > >> Maybe we should use this flag even when doing native i386 builds... > > > > > > after adding "-march=i386" to CFLAGS in Makefile everything looks ok > > (filesize: 15443, as you predicted), so I would opt for using this flag in > > the future. > > Here is a small investigation into the -march flag. Not sure if it is of any > practical significance, I just was curious. > > First, seems that neither of i386/i486/i586/i686 values for this flag nor > absence of it implies features like MMX, SSE, and so on. (Saying this because > of some assumptions about i686) > > For the base GCC specifying -march with the above values is equivalent to > specifying -mtune with the same values, when mtune is not explicitly set. > Using "i686" or omitting the flag is equivalent to -mtune=generic. > > Note that this happens despite a FreeBSD-specific change to (base) GCC that > makes i486 a default arch. Derivation of the tune value from the arch value, > if any, or defaulting it otherwise is done earlier than defaulting of the arch > value. > Specifically I am talking about the block that deals with ix86_tune_string > that precedes the block for ix86_arch_string. > > So it seems that at the moment our sys/boot code is effectively compiled with > -mtune=generic for i386 target (amd64 target has an explicit -march=i386 - I > wonder why not i486). > > I think that in terms of instructions repertoire the difference is only in > availability of cmpxchg, cmpxchg8b, and xadd instructions (ignoring the > "system" instructions that should not be generated by a compiler from C code). > And I guess that the sys/boot code is simple enough to not require these > instructions? > Otherwise, mtune seems to affect layout of the generated code and preference > for some instructions over others. > > Again, not sure what conclusions can be made... -march=i686 also turns on use of cmov*. [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAk+sySIACgkQC3+MBN1Mb4jxSgCdFSGMbh8P08s87b4LwDLT19wr KcEAoKcNxpqeL8szwWNSnn9X2tTRZx8R =07m4 -----END PGP SIGNATURE-----help
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