[Bug 98409] 4.5% perf drop in CSDor with "nir: Optimize integer division and modulus with 1"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=98409

            Bug ID: 98409
           Summary: 4.5% perf drop in CSDor with "nir: Optimize integer
                    division and modulus with 1"
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: idr at freedesktop.org
          Reporter: eero.t.tamminen at intel.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

Following commit drops SynMark2 CSDof test performance on all platforms
supporting compute shaders:

commit 4d35683d91e3d61bf14b76d801bf6ae17237e162
Author: Ian Romanick <ian.d.romanick at intel.com>
Date:   Wed Oct 19 08:53:10 2016 -0700

    nir: Optimize integer division and modulus with 1

    The previous power-of-two rules didn't catch idiv (because i965 doesn't
    set lower_idiv) and imod cases.  The udiv and umod cases should have
    been caught, but I included them for orthogonality.

    This fixes silly code observed from compute shaders with local_size_[xy]
    = 1.

Commit seems clear optimization, so I assume this regression is some kind of
bad interaction in the optimization passes.

On SKL GT2 drop is 4.5% and more on on GT4(e).  INTEL_DEBUG=perf reports a lot
of register spilling and tells about inefficient fallback code for CS variable
indexing with this test.   No other tests besides CSDof were affected, but it's
the only test in our set that is register spilling currently.

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