[gentoo-amd64] Re: New install - basic question

[Duncan <1i5t5.duncan@cox.net>]

Paul Stear posted on Sat, 05 Feb 2011 15:45:08 +0000 as excerpted:

> I am currently using amd64 2 core processor but now have the chance
> to use  an intel quad 64 processor.
> This might be a silly question but is it best to do a compltly new
> install? If so which stage do I download from the following list:-
>  
>  alpha: iso stages 
>  amd64: iso stages 
>  arm: stages 
>  hppa: stages 
>  ia64: iso stages 
>  ppc/ppc64: iso stages 
>  s390/s390x: stages 
>  sh: stages 
>  sparc: iso stages 
>  x86: iso stages
> 
> Which chost and cflags should I use?

Intel em64t (as opposed to the Itanics/Itaniums) is amd64 by another name.
So use the amd64 stages if you want 64-bit, or the x86 stages if you want
legacy 32-bit.  (FWIW, ia64, Intel Arch 64, would be for the Itaniums.  
Sometimes people get that mixed up, but unless you know you have that 
hardware, it's pretty safe to assume it's amd64/em64t/x86_64, NOT ia64.)

Assuming amd64, whether you /have/ to start from fresh stages or can use 
your existing install, depends on how similar the hardware extensions are, 
and whether you used -march or -mtune on your previous install.  If you 
used -mtune, the old install should work as it will have used only 
instruction ordering for your target hardware, with generic instructions.
If you used -march, it will have used targeted instructions as well, and 
may or may not work.  You can try it if you like as going older to newer 
it's likely to work, but really, starting with a new CPU is a good excuse 
to start fresh, eliminating any possible cruft left over from the old 
system, so I'd recommend starting from new stages even if the old ones 
would work, unless you're lazy and simply want to avoid that extra 
compiling.

If you want to target the new hardware and are using a new enough gcc (I 
think the stage supplied one is new enough, now), you can simply use 
-march=native to take advantage of its instructions.  If you'd prefer to 
have the compiled packages be installable on either machine (say if you 
run FEATURES=buildpkg to get the binpkgs to do so), but tuned for the new 
one, use -mtune=native on the new one instead, possibly with -march set 
for the common subset of both instruction sets, tho you'd of course have 
to set that subset specifically, which means researching what it is.

CHOST will normally be the same, x86_64-pc-linux-gnu , unless you're doing 
something strange or using legacy 32-bit x86 instead of amd64.

-- 
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