[Mesa-dev] [PATCH 02/15] i965/fs: Don't kill ACP entries due to their generating instruction.

[Paul Berry]

On 18 August 2013 17:06, Kenneth Graunke <kenneth at whitecape.org> wrote:

> On 08/15/2013 10:06 AM, Paul Berry wrote:
>
>> On 12 August 2013 13:11, Kenneth Graunke <kenneth at whitecape.org
>> <mailto:kenneth at whitecape.org>**> wrote:
>>
>>     fs_copy_prop_dataflow::setup_**kills() walks through each basic
>> block's
>>     instructions, looking for instructions which overwrite registers used
>>     in ACP entries.
>>
>>     This would be fine, except that it didn't exclude the instructions
>>     which generated the ACP entries in the first place.  This meant that
>>     every copy was killed by its own generating instruction, making the
>>     dataflow analysis useless.
>>
>>
>> I don't think this is actually a problem, because the bd[b].kills bits
>> are only used to prevent liveness information from being propagated from
>> bd[b].livein to bd[b].liveout, and the fs_copy_prop_dataflow constructor
>> initializes bd[b].liveout with the set of ACP's that are generated from
>> block b.
>>
>
> It doesn't by the end of the series, but I think that's moot...


Fair point.


>
>
>  So the only situation in which your patch would allow liveness
>> information to be propagated from bd[b].livein to bd[b].liveout is for
>> bits in bd[b].liveout that are already set.
>>
>> Or am I misunderstanding things?
>>
>
> I think you're right that this is unnecessary.  The user of kill is:
>
>    bd[b].liveout[i] =
>       bd[b].copy[i] | (bd[b].livein[i] & ~bd[b].kill[i]);
>
> So even though copies are marked as killed, they'll be reintroduced as
> liveout due to the union with the COPY set.  With the rest of the bugs in
> the algorithm fixed, dropping this patch makes no difference.
>
> I'm happy to drop it.  Would you prefer that?
>

Yeah, I think I have a minor preference for dropping it.  But I won't press
the issue.
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