[lfs-support] Ensuring a generic x86_64 build

Ken Moffat zarniwhoop at ntlworld.com
Mon Sep 18 19:10:40 PDT 2017


On Mon, Sep 18, 2017 at 06:02:49PM -0500, Bruce Dubbs wrote:
> 
> AFAIK, this is it only package in LFS, but it may not be for packages in
> BLFS.  There is at least one package in multimedia where you need to disable
> some optimizations manually. For example gst-libav requires yasm. Indeed,
> any package that uses yasm/nasm is a candidate for problems as those are
> required for processor instructions not available in C or C++.
> 
I suspect that many multimedia packages in BLFS (and outside it)
will do their best to use as many optimizations as possible - that
is based on old experience on ppc where the optimizations differed
between what apple called a G3 and a G4 (750 and 7455, I think).
Generally, multimedia packages seem to turn everything up to 11 -
on x86_64 most of this is probably common to current CPUs.

The original post mentioned -mtune=skylake : looking at 'info gcc' it
was not obvious to me whether that would be set to what is fastest on
skylake, or alternatively to using things that might not be available
on older CPUs (basically, the -march | -mtune variation: I forget
which is which, and after looking at the maze of different very
similar CPU names for recent AMD I was not minded to research this ;)

But if people need to detune packages, I'm sure that binary
distributions (particularly fedora, debian-unstable) might sometimes
show examples.  And maybe gentoo from the source distros.  Of course,
fedora tends to ignore many AV packages (licensing/patent fears).
Or, just build it on the good machine, ship the full install
(including headers) to the lesser machine, and rebuild there if it
breaks.

Final thought: if you really need the optimizations, you probably
need a more potent CPU :-(  I used a K6-2 when I first came here,
and I went through all the likely CFLAGS, but none made a noticeable
improvement.

ĸen
-- 
Truth, in front of her huge walk-in wardrobe, selected black leather
boots with stiletto heels for such a barefaced truth.
                                     - Unseen Academicals


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