Bug#775235: Bug#776911: gnome-shell: fails to start on i386 when [Mesa was] built with llvm-3.5

[Michael Gilbert <mgilbert@debian.org>]

control: severity -1 wishlist
control: forwarded -1 https://freedesktop.org/patch/34445
control: retitle -1 mesa: use llvm's getCPUTargetFeatures() instead of
getHostCPUName()

It turns out that mesa uses llvm's getHostCPUName(), and qemu i386 by
default (correctly) reports itself as pentium2, which is assumed
non-sse2. gnome-shell uses a mesa feature that requires sse2, so it
fails at startup on qemu i386.

On llvm trunk they've implemented getCPUTargetFeatures(), which can be
used to check for specific cpu features like sse2, and that is the
long term solution to the problem:
https://llvm.org/bugs/show_bug.cgi?id=23201

In the meantime, the problem can be worked around by using a
non-default qemu cpu options.  Some examples:

$ qemu -cpu pentium3 test.img
$ kvm -cpu host test.img

Since there workarounds are straightforward I am reducing the severity.

Best wishes,
Mike