[Bug 105670] [regression][hang] Trine1EE hangs GPU after loading screen on Mesa3D-17.3 and later

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=105670

--- Comment #2 from Gert Wollny <gw.fossdev at gmail.com> ---
I can confirm this on Radeon 6870 HD. 

I was able to track the issue to the series beginning with  

   ab23b759f24 glsl: don't drop instructions from unreachable 
               terminators continue branch

The first time around I had a few difficulties get the correct patch, it seems
to be either 646621c66da9 or 7a7fb90af75. In any case, yes it is related to
loop unrolling.

The original TGSI goes like 

BGNLOOP
  ISGE TEMP[13].x, TEMP[8].xxxx, IMM[7].yyyy
  UIF TEMP[13].xxxx
     BRK
  ENDIF
  ...
ENDLOOP

and with the series in question applied it is 

BGNLOOP
  ISLT TEMP[13].x, TEMP[8].xxxx, IMM[7].yyyy
  UIF TEMP[13].xxxx
  ...
  ENDI
ENDLOOP

This UIF should have an else path with the BRK to resemble the original code. 
(There are more BRK statements in the LOOP but they are the same in both
versions.

Regarding bisecting this, after a failure one usually must reboot the system,
otherwise the graphics card is in a bad state. But given the nature of the bug
one should also be able to reproduce the endless loop with
LIBGL_ALWAYS_SOFTWARE=1 thereby not clobbering the hardware.

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