[Bug 100913] 6% performance drop in GpuTest v0.7 Volplosion with "remove GLSL IR optimisation loop"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=100913

            Bug ID: 100913
           Summary: 6% performance drop in GpuTest v0.7 Volplosion with
                    "remove GLSL IR optimisation loop"
           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

Patch series culminating in this commit changed performance in several tests:
-------------------------------------------------------
commit ad55b1a7701ad51234af3b9fc30f4c54d2546b86
Author:     Timothy Arceri <timothy.arceri at collabora.com>
AuthorDate: Wed Jan 18 10:28:22 2017 +1100
Commit:     Timothy Arceri <tarceri at itsqueeze.com>
CommitDate: Mon Apr 24 12:08:14 2017 +1000

    i965: remove GLSL IR optimisation loop

    IVB is running into some spilling issues in piglit with the
    loop removed. However those tests are not really reflective
    of a real world use case, also fp64 is brand new to IVB
    so we leave the spilling issues to be resolved at a later
    time.
-------------------------------------------------------

On most of the platforms GpuTest v0.7 PixMark Volplosion test performance
dropped by ~6%, on SKL GT4e, a bit more.

(There were few performance improvement around same time, but they were small
enough that I couldn't bisect them to this commit, besides the SynMark shader
compilation speed test which improved by ~9%.)

Volplosion test is fairly pure ALU test.  While it has one copy shader, none of
the other shaders do any texture accesses, including the heavy main fragment
shader, i.e. texture access scheduling doesn't affect it.

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