[gentoo-amd64] Re: Re: Difference between multilib & no-multilib stages

[Duncan]

Samir Mishra posted <4448CA27.8020804@×××.ae>, excerpted below,  on Fri,
21 Apr 2006 16:03:51 +0400:

>> However, 32-bit is possible by adding the appropriate CFLAGS.
>
> I assume this can be achieved by switching the CHOST environment
> variable to the X86 relevant one. In this instance, if I change the
> CHOST variable, is it also necessary to add -m32 to the CFLAGS, or for
> that matter, make any changes to the C[XX]FLAGS?

I'm not sure about the effect of CHOST.  The docs mention -m32 (and -m64)
in CFLAGS, and that's the way they manage it with the glibc and grub
ebuilds.

Other changes to the CFLAGS would depend...  Some CFLAGS work on both but
the effect is somewhat different (-fomit-frame-pointer kills certain
debugging on x86, but not on amd64/x86_64). Some don't work on on both
(certain alignment flags), or don't work on all x86 sub-archs (SSE/MMX and
the like). However, if you stick to the recommended -O2 -pipe (and
possibly -fomit-frame-pointer if you don't care about the debugging on
x86), -m32/64 is the only one to worry about, and again, I'm not sure how
that works against CHOST.  (-O flags will change their behavior based on
arch, but in /general/, the idea is to only select the generally safe
optimizations for a platform and include those in the various levels of
-O, as appropriate.)

> Also, given that I have the multilib libraries installed, is there any
> way to ensure that only the 64-bit libraries are used when compiling, in
> effect disable all 32-bit libraries during the compile stage? I'm more
> interested in having things work, but I would like to keep track of all
> packages that use any of the 32-bit libraries and upgrade these to the
> full 64-bit versions when updates to the packages are available.

As I said, you don't really need to do that, because if it's a 32-bit
package, it can only use 32-bit libraries; if it's a 64-bit package, it
can only use 64-bit libraries.  The only exceptions in portage are well
known -- gcc, glibc, sandbox, grub, and the 32-bit bin/compatibility
packages that don't compile anything but simply install precompiled 32-bit
binaries for compatibility purposes.  gcc does both in ordered to be
multilib.  glibc actually compiles separate libraries for 32-bit and
64-bit -- it iterates thru the build process up to four times, once each
for nptl and linux threads (if so configured), for each of 32-bit and
64-bit (on a multilib profile).  sandbox likewise duplicates the build
process, building both a 32-bit lib and a 64-bit lib, so it can provide
the sandbox services to both bitnesses.  grub is 32-bit only, because the
hardware /must/ start in 32-bit mode and only switches to 64-bit mode
after the kernel loads.

Anything that you compile outside of portage, presumably, you know and
deliberately tell it which bitness to compile for.

Thus, the only cases where an unintended bitness libraries are used are
accidents, and in those cases, gcc will bail anyway, not completing the
build, because it simply /cannot/ link 32-bit and 64-bit together.

(The exception is where special thunking layers are designed in.  The
partially binary-only slaveryware that's NVidia's videodrivers are an
example of this.  However, again, this is going to be very rare as it's a
huge pain to do, so anything that does it you'll know about, because it
will be a feature of the product.)

Here are some interesting links you may have missed.

The handbook, which you already mentioned, but for completeness.  Note
that many folks  only read the installation section, and miss the many
good hints about actually working with and sysadmin-ing their Gentoo
system.  The Working with Gentoo and Working with Portage sections should
also be at least scanned, and the network section can be very helpful for
those setting it up for the first time as well:

http://www.gentoo.org/doc/en/handbook/handbook-amd64.xml

Gentoo AMD64 FAQ, which deals with some of this:

http://www.gentoo.org/doc/en/gentoo-amd64-faq.xml

The now deprecated Technotes.  Most of the stuff here is covered in the
FAQ, but here's a somewhat dated historical viewpoint.  Still well worth
the read, as they deal with some of these questions as well.

http://www.gentoo.org/proj/en/base/amd64/technotes/index.xml

The 32-bit chroot guide.  If you decide you'll be doing a lot of 32-bit
stuff, this is the way to go.

http://www.gentoo.org/proj/en/base/amd64/howtos/index.xml?part=1&chap=2

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman in
http://www.linuxdevcenter.com/pub/a/linux/2004/12/22/rms_interview.html


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