#41 Autotuning randomly freezes on Radeon R9 270

[Bruce Merry]

This is happening with 1.4.0 with the Catalyst 14.12 drivers on Ubuntu 14.04, while tuning the scan kernel for ushort3. Some investigations show that
- it always seems to happen when tuning for the larger problem problem
- it always seems to happen in the timing pass, not the warmup pass (which is identical!)
- it happens on a different parameter set each time (I haven't checked for a common parameter value)
- it doesn't seem to happen on any other types (uint3, ulong3 seem unaffected)
- it is not a kernel cache problem: disabling the cache has no effect
- hacking the kernel defines to use ushort4 when ushort3 is required seems (?) to make the problem go away
- I have not been able to reproduce the problem in a standalone program.

Killing the frozen program causes the X server to hang, which strongly suggests a driver bug.