RFC: pentium optimized debian

[Joey Hess <joey@kitenet.net>]

	There was some talk on the list a few months ago about whether a
pentium optimized version of debian makes sense. Some points against are
that: 

  - pentium optimized binaries tend to be bigger
  - only some packages will benefit from pentium optimization

	Some points in favor are that:

  - pentium optimized binaries of things that are CPU bound tend to be at
    least 10% faster by my benchmarks. Stampede linux claims 10 - 30% speed
    improvement.
  - PR value.

	I propose a compromise: instead of splitting off a whole new
pentium architecture, which would double building time, etc, we just set up
a small collection of pentium optimized .debs. This could be organized on
the ftp site as a directory tree with many symlinks back to i386 packages,
and a few pentium optimized debs.

  	I've already hacked together a system[1] to enable building of
pentium optimized packages with egcc, and have built most of the base 
system, plus a few extras like libc6, and xaos. I've been using these
packages for a few days, and have discovered no problems. If anyone would
like to try it, you can download them from 
http://kitenet.net/~joey/pentium/OK/ (28.8 line, sorry!)

	The main change I had to make was a modification to dpkg to make it
support a new tag, Optimization:, in binary package files. This tag simply 
tells what version of an architecture a package was built optimized for, and
is optional. If the tag exists, its value is used in determining the package
name (so we can have xaos_3.0-3_pentium.deb for example). And of course, you 
can see it when querying dpkg for info about a package.

Implementation:

	There needs to be a way to signal the build process that the
system is surrently building a package with a given level of optimization. 
This could be accomplished in a variety of ways, like a new flag to 
dpkg-buildpackage, an environment variable, a config file, etc.
This is the detail I'm most uncertian about. It could easily be implmented
as a new flag in dpkg-buildpackage and dpkg-genchanges. In my
implementation, I decided to use an environment variable, since it is also
used by my pentium-builder[1] package to force egcc to use -mpentium 
optimization. Plus, it's easy to implment. So if DEBIAN_OPTIMIZATION is set,
the value of that environment variable populates the Optimization: tag. This
means that building a pentium optimized package is as easy as
"DEBIAN_OPTIMIZATION=pentium dpkg-buildpackage -rfakeroot".

	I had to modify dpkg-gencontrol to recognize this new variable and
use it when generating the control file and debian/files.

	dpkg-name and dpkg-deb --build also needed modification, so that
they look for the Optimization: tag and if it is present, use it instead
of the architecture when determining what filename to name the .deb as.

	I made a small modification to dpkg-scanpackages, so it includes the
Optimization tag in the Packages file.

	I've uploaded dpkg version 1.4.0.26.1 to experimental with these 
changes in it.

  	I have left out at least one thing - the .changes files will need to 
have some way to specifiy to dinstall that the uploaded files are for a 
given subarch. I don't know enough about dinstall to know how this would 
work. It might be sufficient to let dinstall figure out which packages go 
where by looking at their filenames.

-- 
see shy jo


[1] I've created a package called pentium-builder, now in experimental, that
    diverts gcc, cc, g++, etc, with shell scripts that call egcs with the
    flag "-m$DEBIAN_OPTIMIZATION" if DEBIAN_OPTIMIZATION is set. It's a hack,
    but it works.
    


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