[Mesa-dev] [PATCH 06/12] i965/fs: Add a sampler program key for whether the texture is 16x MSAA

[Neil Roberts]

Ben Widawsky <ben at bwidawsk.net> writes:

> On Thu, Sep 17, 2015 at 05:00:08PM +0100, Neil Roberts wrote:
>> When 16x MSAA is used for sampling with texelFetch the compiler needs
>> to use a different instruction which passes more arguments for the MCS
>> data. Previously on skl+ it was unconditionally using this new
>> instruction. However since 16x MSAA is probably going to be pretty
>> rare, it is probably worthwhile to avoid using this instruction for
>> the other sample counts. In order to do that this patch adds a new
>> member to brw_sampler_prog_key_data to track when a sampler refers to
>> a buffer with 16 samples.
>> 
>> Note that this isn't done for the vec4 backend because it wouldn't
>> change how many registers it uses.
>
> Hmm. I was of the other opinion actually, I'd prefer to always use the
> new instruction to limit the variability between tests. I suppose if
> we're often passing the LOD parameter that is problematic, but we
> don't do that either, right?

Using the ld2dms_w instruction on SIMD8 or SIMD16 requires sending one
extra parameter to the sampler regardless of whether the LOD is sent.
This requires at least one extra instruction to fill it in. Also, if the
r parameter is sent then on SIMD16 the message length will flow over the
maximum of 11 registers and the send message will have to be split into
two with an exec size of 8. I think this is quite a big overheard so I'm
strongly in favour of avoiding this where possible seeing as 16x MSAA is
probably going to be quite rare. All of the piglit tests dealing with
MSAA seem to test the whole range of sample counts so I don't think
there's too much worry of this code path not getting tested.

Shader db reports this:

total instructions in shared programs: 7651410 -> 7651318 (-0.00%)
instructions in affected programs:     4906 -> 4814 (-1.88%)
helped:                                52
HURT:                                  0
GAINED:                                0
LOST:                                  0

Regards,
- Neil