On Wed, Sep 23, 2015 at 06:08:33PM +0100, Neil Roberts wrote: > Ben Widawsky <b...@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.
Hmm. As I read it, it sounded like you didn't have to send LOD it's implied to be 0 if you don't send it. If I am wrong about that, then I agree with you completely. > 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 _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
