On Fri, Feb 4, 2011 at 3:50 PM, Nils Holland <n...@tisys.org> wrote: > Alan McKinnon wrote: > >> Interestingly, Ubuntu has always built for basic arches, and they seem to >> get away with it. >> >> IIRC they are now on i586 but for the longest time used i386. No >> performance issues. You might want to investigate how they do >> their builds and see if you can use their tricks. > > The question is, I guess, if the target host, when of the same arch (i.e. > i[3456]86) does actually have any influence on the code that gets > generated in terms of performance or ability to run on other sub-arches. > This is what I really couldn't find out so far and would find highly > interesting to know. > > For example, why not just go (and stay) with CHOST="i386-pc-linux-gnu" and > on an i686 machine, set march or mcpu = i686 via CFLAGS if you want to > optimize for the particular subarch at hand? Why should it be necessary / > what would the (dis)advantages be of of such a setup vs. also having CHOST > set to "i686-pc-linux-gnu"? > > Concering the Gentoo doc about changing the CHOST that was mentioned: Yep, > I've read that. If I understood it correctly, problems when changing CHOST > mainly seem to arise because of the way GCC and related basic build utils > install themselves (using the host triplet as part of their path or > executable name), leaving wrong / messed up references when changing the > CHOST. > > For example, as I've said previously, the CHOST value gets passed to > ./configure as --host for each package that gets build. That would make > configure try to select a compiler called <CHOST>-gcc in order to compile > the package, i.e. when CHOST is i486-pc-linux-gnu, a compiler called > i486-pc-linux-gnu-gcc would be used. Include file directories for glibc > and / or glibc itself sems to be selected by a similiar mechanism. All > right, no problem, so far this only determines how things are called and > where they are located, but are there any other "real" effects of all this > stuff? > > The Gentoo CHOST document that was mentioned says something about nptl not > being available on i386. If true, and if the host variable generally > influences the availability of features, things would slowly start to make > sens as to why the CHOST might be important. On the other hand, I've read > through some documentation of the GNU C library (glibc) and didn't even > find anything about nptl not being available on i386, not to mention > anything else about different features on different subarches. > > You see, I'm probably not entirely "getting it" yet. ;-) > > Greetings, > Nils > > >
Hi, I am not a big guru there, but i have changed CHOST variable successfully few years ago. If I remember, the steps were like that: Change CHOST variable. Bootstrap system (like building from stage 1): # /usr/portage/scripts/bootstrap.sh # emerge -e system # emerge -e world Before gentoo has been providing daily stages, I was installing my systems from stage1. It was a nice learning curve :) -- mv
