Bug 90158 - Make shader precompile guess MSAA layout properly
Summary: Make shader precompile guess MSAA layout properly
Alias: None
Product: Mesa
Classification: Unclassified
Component: Drivers/DRI/i965 (show other bugs)
Version: git
Hardware: Other All
: medium enhancement
Assignee: Intel 3D Bugs Mailing List
QA Contact: Intel 3D Bugs Mailing List
Depends on:
Blocks: i965-perf
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Reported: 2015-04-24 06:34 UTC by Kenneth Graunke
Modified: 2019-09-25 18:53 UTC (History)
0 users

See Also:
i915 platform:
i915 features:


Description Kenneth Graunke 2015-04-24 06:34:16 UTC
Currently, brw_fs_precompile doesn't set compressed_multisample_layout_mask, leaving it at the default value of 0 (non-CMS).

This is dumb.  Virtually all multisampled color buffers on Gen7+ use the CMS layout, not UMS or IMS.  So, basically every shader that uses a sampler2DMS type (including most of WebGL) will require a recompile.

A better guess would be:
- 1 for sampler2DMS on Gen7+ (0 on Gen6)
- 0 for sampler2DMSShadow
- 0 for non-MS sampler types.

This should eliminate almost all recompiles due to compressed_multisample_layout_mask.

To accomplish this, we may want to create a bitfield similar to SamplersUsed or ShadowSamplers...SamplerIsMSAA or such.  Then the precompile could easily check it.  Alternatively, we could stop using bitfields and walk a list of NIR uniforms.  It should be pretty simple.
Comment 1 Kenneth Graunke 2015-05-09 00:11:34 UTC
Ilia pointed out that there is no such thing as a sampler2DMSShadow.

So you can happily ignore that :)
Comment 2 Kenneth Graunke 2016-11-05 05:50:35 UTC
Assuming it works, here's a better idea: kill the field altogether.  Just treat all surfaces like CMS.

According to the docs:
"If ld_mcs is issued on a surface with MCS disabled, this message returns zeros in all channels."

I'm pretty sure it'll work for UMS and maybe even IMS.  It's a bit of extra work in the shader for the non-CMS case, but most things are CMS anyway.
Comment 3 GitLab Migration User 2019-09-25 18:53:50 UTC
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