i386 -march=xxx behavior [Was: FreeBSD 8 i386 gptboot corrupt - SOLVED]

[Konstantin Belousov]

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*.
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