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sse{2,3,4.2}, altivec, neon, ...

Hi ladies and the ugly kind!

i386:[~]$ chromium
.----[ xmessage ]
The hardware on this system lacks support for the sse2 instruction set.
The upstream chromium project no longer supports this configuration.
For more information, please read and posibly provide input to their
bug tracking system at http://crbug.com/348761.
i386:[~]$ PCSX2
Illegal instruction

Two observations:
* losing at runtime, be it an error message or outright SIGILL, is ɴᴏᴛ ɴɪᴄᴇ.
* I had to use qemu -cpu to reproduce this.  I tried raiding junk piles
  of nearby friends and businesses, failing to find a single pre-SSE3
  machine[1].  Likewise, my 2009 phone has NEON too.

Thus, there's rather little point in spending too much time trying to get
ancient non-baseline extensions non-mandatory on some packages.  No one is
going to run scientific number-crunching on a Pentium2.

There are also some packages where such support would be nice (like
libhighwayhash, a good general-purpose hash -- no reason to exclude crappy
gear), yet upstream refuses to do the porting and you don't have the tuits

Dropping baseline support is giving up, but let's at least surrender nicely.

Thus, here's a proposed solution: in unstable, there's now a bunch of
packages that do such checking in preinst, and thus refuse (overridably) to
even install such software.

Currently this includes:
* sse2: i386 (inc. hurd-i386, kfreebsd-i386)
* sse3: i386, amd64, x32 (〃, kfreebsd-amd64)
* sse4.2: i386, amd64, x32 (〃)
* neon: armhf
* altivec: powerpc
(amd64 always has sse2, arm64 neon, ppc64el altivec)
Package names are $(ISA)-support, thus sse2-support if you need SSE2.

To use: make your package:
Depends: sse2-support [any-i386]

Lemme know if anything is amiss; in particular, adding a new extension is a
two-line change.


[1]. You can eBay one, but you can get a working PDP-11 that way too.
⢀⣴⠾⠻⢶⣦⠀ What Would Jesus Do, MUD/MMORPG edition:
⣾⠁⢰⠒⠀⣿⡁ • multiplay with an admin char to benefit your mortal
⢿⡄⠘⠷⠚⠋⠀ • abuse item cloning bugs (the five fishes + two breads affair)
⠈⠳⣄⠀⠀⠀⠀ • use glitches to walk on water