Re: Checking for SSE/MMX support

[Hugo Rivera <uair9%yahoo.com@localhost>]

 --- Martijn van Buul <martijnb%atlas.ipv6.stack.nl@localhost>
escribi=F3:

> It occurred to me that Hugo Rivera wrote in
> gmane.os.netbsd.devel.pkgsrc.wip.general:
> > # The line below it's what I'm talking about
> > #
> >     cpuinfof =3D os.popen('sysctl -n
> hw.instruction_sse')
> > #
> > #=09
>=20
> [snip]
>=20
> > That's the freebsd part that came by default in
> the
> > SConstruct file, I was trying to create a similar
> > section for netbsd, so, what am I doing wrong?
>=20
> The concept of determining whether to include
> SSE/MMX or not *at build time*
> is wrong altogether. It assumes that the machine
> used for *building* is
> the same machine as the one used at *runtime*. This
> is not the case per se.
>=20
> In fact, in an ideal world[1], the build host can be
> of a completely different
> architecture, not even capable of running the
> binaries it just generated. This
> is called cross-compilation, and it's considered a
> Good Thing (TM) within
> NetBSD. It allows me to build the NetBSD system for
> my old NeXTstation (which
> is way too slow for this) on my spiffy fast AMD64.
> Unfortunately, because of
> autoconf (Which makes the same assumption about
> "host =3D=3D target", and compiles
> small testprograms to be executed on the build host.
> Therefore, the build
> host needs to be able to execute target binaries,
> which is not the case
> in the amd64 vs. m68k example), I can't easily
> compile binary packages for my
> old slab on my AMD64. However, I'm still able to
> compile software for my
> Pentium laptop, as they're running the same
> architecture of NetBSD.
>=20
> Most binary packages are compiled this way, on
> speedy fast modern machines, so
> people with older hardware can just use the binary
> packages instead of building
> themselves. However, this would *break* the minute a
> package starts doing
> tests like the one you described above.=20
>=20
> On my spiffy fast AMD64, it would detect SSE and
> MMX, compile my package
> accordingly, and it would no longer work on my
> Pentium laptop. In fact, since
> it's being probed automatically, it's even
> *impossible* to build it in such
> a way that I could run it on my pentium, unless I
> compiled it on there - which
> is not what I want.
>=20
> So, therefore, there are two ways how to deal with
> this:
>=20
> 1) Do the detection at runtime. However, this
> probably means rather serious
>    modifications to the package, so it may not be a
> valid option.
> 2) Don't use detection, but make the usage of
> SSE/MMX optional. We have a
>    mechanism for this. It would allow people to use
> SSE or MMX, by means of
>    specifying PKG_OPTIONS.dangerdeep+=3D"sse" or
> PKG_OPTIONS.dangerdeep+=3D"mmx"
>    in /etc/mk.conf, but it would *default* to
> building a package which would
>    run on *all* systems - not just the system which
> happens to have the
>    same capabilities than the build host.
>=20
> Hope this helps to clear things up,
>=20
> Martijn.
>=20
> [1] e.g. a world without autoconf, or at the very
> least a world without crummy
>     configure scripts


Martijn:

Thanks a lot for your help, I forgot about
crosscompiling.

Hugo


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