[Bug 111241] Shadertoy shader causing hang

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111241

--- Comment #6 from Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer at amd.com> ---
I've created a different shadertoy showing the problem:
https://www.shadertoy.com/view/Wt2SW1 (but this one doesn't hang the GPU).

The shader for "Buffer A" is:

  0: MOV TEMP[0], SV[0]
  1: MAD TEMP[0].y, SV[0], CONST[0][2].xxxx, CONST[0][2].yyyy
  2: MOV OUT[0], IMM[0].xxxx
  3: USEQ TEMP[1].x, CONST[0][1].xxxx, IMM[1].xxxx
  4: UIF TEMP[1].xxxx
  5:   ADD TEMP[2], TEMP[2], -TEMP[2]
  6: ELSE
  [...]
 13: MOV OUT[0], TEMP[2]
 14: END

TEMP[2] is used before being assigned a value, so I suppose that's what allows
LLVM to turn line 5 in:

   v_mov_b32_e32 v3, 0x7fc00000
   v_mov_b32_e32 v2, 0x7fc00000
   v_mov_b32_e32 v1, 0x7fc00000
   v_mov_b32_e32 v0, 0x7fc00000

(ie: output is NaN)

A possible way to fix this is to transform "dst = x - x" operations in "dst =
0" which is what nir does in its nir_opt_algebraic pass.

I've open a MR to fix/discuss this issue:
https://gitlab.freedesktop.org/mesa/mesa/merge_requests/1681

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