[Mesa-dev] [PATCH 2/3] i965: Don't record a WAW dependency between implicit MRF operations.

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> Meta-instructions that implicitly write then read/consume a MRF value
> don't pose write-after-write conflicts with each other, since they're
> actually:
>
> - Write value 1, then consume it.
> - Write value 2, then consume it.

OK, here's the case I'm thinking of:

1: MOV m2, temp1
2: send m2 no_implied_write
3: send m2 implied_write_m2
4: send m2 implied_write_m2
5: MOV m2, temp2
6: send m2 no_implied_write

Before this series we had deps:
2 -> 1 (RAW)
3 -> 2 (WAW)
4 -> 3 (WAW)
5 -> 4 (WAW)
6 -> 5 (RAW)

I'm concerned that now you only have deps:
2 -> 1 (RAW)
3 -> 2 (WAW)
5 -> 4 (WAW)
5 -> 2 (RAW)
6 -> 5 (RAW)

and that now a possible ordering would be:

1: MOV m2, value
2: send m2 no_implied_write
4: send m2 implied_write_m2
5: MOV m2, value
3: send m2 implied_write_m2
6: send m2 no_implied_write

and "6: send" will be broken.

I could definitely have missed something -- this stuff is super twisty.
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