Running the attached test using Amber (https://github.com/google/amber):

amber -d unreachable-barrier-in-loops.amber

should lead to the test passing.

Instead, I find that my system immediately locks up.  I believe it is a whole system freeze, rather than just a display freeze.

Build: Mesa 19.2.0-devel (git-a110a8090d) (Debug)
Device: Radeon RX Vega

I can reproduce the failure with your Mesa HEAD, however current Mesa master branch (git-4379dcc12d) passes the test. Please re-test.

Operating System: Ubuntu 18.04
Linux kernel: 5.0.0-23-generic
Mesa: 19.3.0-devel (git-4379dcc12d)
GPU: AMD RADV POLARIS10 (LLVM 8.0.1)

(In reply to Alastair Donaldson from comment #1)
> Running the attached test using Amber (https://github.com/google/amber):
> 
> amber -d unreachable-barrier-in-loops.amber
> 
> should lead to the test passing.
> 
> Instead, I find that my system immediately locks up.  I believe it is a
> whole system freeze, rather than just a display freeze.
> 

Regarding this, it's probably a GPU hang in your computer. I have enabled amdgpu.gpu_recovery=1 functionality to avoid it.

> Build: Mesa 19.2.0-devel (git-a110a8090d) (Debug)
> Device: Radeon RX Vega

I can confirm that the test passes for me on master, so I guess we should close this issue; I'll open another one if it turns out the issue still persists and can be exposed by a different test.

Thanks for the advice regarding GPU recovery.