[Bug 93594] Flickering Shadows in The Talos Principle

[bugzilla-daemon@freedesktop.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=93594

--- Comment #8 from Marek Olšák <maraeo@gmail.com> ---
I've captured one of the problematic draw calls, which used this fragment
shader:

FRAG
DCL IN[0], GENERIC[9], PERSPECTIVE
DCL OUT[0], COLOR
DCL SAMP[0]
DCL SVIEW[0], 2D, FLOAT
DCL CONST[0..13]
DCL CONST[15]
DCL TEMP[0..3], LOCAL
IMM[0] FLT32 {    1.0000,     0.0000,     0.2500,     4.0000}
  0: MOV TEMP[0].xy, IN[0].xyyy
  1: TEX TEMP[0].w, TEMP[0], SAMP[0], 2D
  2: MUL TEMP[0].x, TEMP[0].wwww, IN[0].wwww
  3: MAD TEMP[1].x, TEMP[0].xxxx, CONST[1].xxxx, CONST[1].yyyy
  4: MOV_SAT TEMP[1].x, TEMP[1].xxxx
  5: MOV TEMP[0].w, TEMP[1].xxxx
  6: FSLT TEMP[1].x, TEMP[1].xxxx, CONST[0].xxxx
  7: AND TEMP[1].x, TEMP[1].xxxx, IMM[0].xxxx
  8: KILL_IF -TEMP[1].xxxx
  9: MAD TEMP[0].x, IN[0].zzzz, CONST[0].zzzz, CONST[0].yyyy
 10: MOV_SAT TEMP[1].x, TEMP[0].xxxx
 11: DDX TEMP[2].x, TEMP[1].xxxx
 12: ABS TEMP[2].x, TEMP[2].xxxx
 13: MUL TEMP[3], CONST[15].xxxx, TEMP[1].xxxx
 14: DDY TEMP[3].x, TEMP[3]
 15: ABS TEMP[3].x, TEMP[3].xxxx
 16: ADD TEMP[2].x, TEMP[2].xxxx, TEMP[3].xxxx
 17: MAD TEMP[0].x, TEMP[2].xxxx, IMM[0].zzzz, TEMP[1].xxxx
 18: MUL TEMP[1].x, TEMP[0].xxxx, TEMP[0].xxxx
 19: ADD TEMP[1].x, TEMP[0].xxxx, -TEMP[1].xxxx
 20: MUL TEMP[1].x, IMM[0].wwww, TEMP[1].xxxx
 21: ADD TEMP[1].x, IMM[0].xxxx, -TEMP[1].xxxx
 22: MOV TEMP[0].y, TEMP[1].xxxx
 23: MOV TEMP[0].z, IMM[0].yyyy
 24: MOV OUT[0], TEMP[0]
 25: END

If KILL_IF masks out some but not all invocations in a 2x2 quad, the subsequent
DDX and DDY opcodes can result in undefined values, resulting in garbage on the
output.

The simple solution is to move KILL_IF to the end of the shader. I've verified
that it works, but it's inefficient. The best solution would be to:
- save the resulting EXEC mask after KILL_IF
- use S_WQM on the exec mask to get a whole-quad mask
- execute DDX and DDY
- restore the EXEC mask (this must be done after both DDX & DDY but before PS
exports)

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