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

[Kenneth Graunke]

On 02/13/2013 06:31 PM, Eric Anholt wrote:
> 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.

I double checked your work and got an extra 3 -> 1 (WAW) dependency, but 
that doesn't matter.  The reordering you mentioned is indeed possible. 
Thanks.

Consider patches 1 & 2 NAK'd...I'm working on a new series...