Autotuning randomly freezes on Radeon R9 270

[bmerry]

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.

Reported by: bmerry

Original Ticket: clogs/tickets/41