Comment # 8
on bug 93594
from Marek Olšák
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)