[gentoo-user] The CHOST variable

[Nils Holland]

Hi folks,

well, it's not that a certain thing I'm intending to do has pointed me
to it, but I've just noticed that something I've taken for granted for
years is something I probably fail to understand correctly. And as I'm
always eager to learn, I'm wondering if someone can point me in the
right direction.

I'm talking about the CHOST variable as defined in /etc/make.conf, and
what actually lies behind it.

Obviously, whatever is specified in make.conf as CHOST is passed to
./configure as --host when emerging packages. Ok, I know several
things about that: The contents of the CHOST variable is a "target
triplet" in the form of (for example) i686-pc-linux-gnu. According to
the autoconf info documentation, packages using the GNU build
environment use this triplet to properly configure the package at hand
for the specified target architecture. If no --host is given to
configure, it will try to determine the correct triplet for the
current system by the rules specified in "config.sub". Beside --host,
configure also accepts --build and --target, which specify the system
on which the package is being configured and built (--build) and the
the type of architecture for which a compiler being configured is
supposed to be able to generate code (--target, which - I believe -
only really seems to make sense when building a compiler like
GCC). These can also be influenced via make.conf, where they are
called CBUILD and CTARGET.

All of this is fine so far and relatively understandable. But a few
questions remain:

1) So a package using the GNU build system determines or is passed
(via --host aka. CHOST) a target triplet specifying the system on
which the resulting compiled code is supposed to run. What does the
package do with that information? Does it only use it to determine
what it has to compile (different / special code for different systems
/ architectures), or does this already have an influence on the
optimization of the resulting code for a certain (sub-)architecture?
Forthermore: Does configuring a package with, say,
--host=i686-pc-linux-gnu already automatically mean that said package
would not be able to run on (for example) an i486-pc-linux-gnu host?
Or can this only be said to be true if the package itself decides to
compile different / incompatible code for i686 and i486; in other
words: If the package itself does not make any distinction on the CPU
subtype, then the result would run "everywhere", as --host does not by
itself imply any such thing as --march=?

2) /etc/make.conf contains a note that one should not change the CHOST
lightly (not that I'm planning to) and links to a nice document
explaining how it can be done anyway (which, I have to admit, didn't
make me any wiser, however). The question is, out of curiosity, why
the CHOST should not be changed and what would happen if one did it
anyway. I willingly believe that it would lead to problems, but would
the actual cause of these problems actually be caused by the
configuration of the machine being mixed up (for example, by the GNU
build system / autoconf suddenly looking for a compiler or similiar
tools / libraries under a path or by a name involving, for example,
i486-pc-linux-gnu, which does not automatically exist of the
appropriate tools have not been installed accordingly. Or would
problems arise because code generated with the new CHOST does no
longer "fit" to code generated with the previous / old CHOST?

Any hints here would be welcome. Again, it's not that I need to know
in order to do something, but all of this stuff has always worked so
well every time I've built a package, and if feels kind of strange not
to know why / how it actually works. As neither the docs of autoconf,
binutils nor GCC could properly enlighten me, I thought I'd ask
here. ;-)

Greetings,
Nils


-- 
Nils Holland * Ti Systems, Wunstorf-Luthe (Germany)
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