[Bug 103939] 4% performance drop in GLB 2.7 Egypt from "Use a 565 format for GL_RGB and GL_UNSIGNED_SHORT_5_6_5 textures"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=103939

            Bug ID: 103939
           Summary: 4% performance drop in GLB 2.7 Egypt from "Use a 565
                    format for GL_RGB and GL_UNSIGNED_SHORT_5_6_5
                    textures"
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: intel-3d-bugs at lists.freedesktop.org
          Reporter: eero.t.tamminen at intel.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

Following commit was bisected (on SKL GT2) to be cause for a performance drop
in GLB 2.7 Egypt onscreen and offscreen tests:
-------------------------------------------------------------
commit 18bdf73556440ca2eaac2d22e1e3ebe1be7b6c1d
Author:     Kenneth Graunke <kenneth at whitecape.org>
AuthorDate: Wed Oct 4 01:08:37 2017 -0700
Commit:     Kenneth Graunke <kenneth at whitecape.org>
CommitDate: Thu Oct 5 14:30:47 2017 -0700

    mesa: Use a 565 format for GL_RGB and GL_UNSIGNED_SHORT_5_6_5 textures.

    Found while trying to optimize an application.

    Not observed to help performance on i965, but should at least reduce
    the memory usage of such textures a bit.

    Reviewed-by: Eric Anholt <eric at anholt.net>
    Tested-by: Eero Tamminen <eero.t.tamminen at intel.com>
-------------------------------------------------------------

Egypt performance drop is clearly visible on following platforms:
* 4% on BSW N3050 & SKL GT2 & KBL GT2
* 2% on BXT J4205 & HSW GT2 & SKL GT4e
* 2% on BYT N2820 (only offscreen version)

(BSW & BYT have 1-channel memory, others 2-channel.)

On some other platforms the performance also increases, as was expected:
* 3% on BDW GT3
* 1% on BDW GT2 & SNB GT2 (only offscreen version)


(Sorry for filing this so late. I had been waiting to see whether blorp rework
/ texture RBC would affect this, but forgot it.  That didn't affect Egypt.)

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