[Bug 91719] [SNB, HSW, BYT] dEQP regressions associated with using NIR for vertex shaders

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91719

Jason Ekstrand <jason at jlekstrand.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mattst88 at gmail.com

--- Comment #1 from Jason Ekstrand <jason at jlekstrand.net> ---
After a bit more analysis, this bug appears to be a coalescing bug.  Thanks to
Kristian for spotting the broken swizzle!  What happens is that NIR gives us

vec1 ssa_9 = imul ssa_7, ssa_8
vec4 ssa_10 = vec4 ssa_2, ssa_2.y, ssa_9, ssa_2.z

in SSA which then becomes

r7 = imul r5, r6
r8.xyw = imov ssa_2.xyz
r8.z = imov r7.x

When we go out of SSA.  This then turns into

   9: mul acc0:D, vgrf5.xyzw:D, u0.xyzw:D
  10: mach null:D, vgrf5.xyzw:D, u0.xyzw:D
  11: mov vgrf7.0.x:D, acc0.xxxx:D
  12: mov vgrf8.0.xyw:D, vgrf13.xyxz:D
  13: mov vgrf8.0.z:D, vgrf7.xyxw:D

which register coalesce happily turns into

   9: mul acc0:D, vgrf5.xyzw:D, u0.xyzw:D
  10: mach null:D, vgrf5.xyzw:D, u0.xyzw:D
  11: mov vgrf8.0.z:D, acc0.xxxx:D

Note that we completely miss the fact that line 13 above moves the x channel to
the z channel and we just coalesce to mask out all but z.  The result is that
mul/mach write to .x but we read from .z.  If I run the test with register
coalescing turned off, it passes.

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