[Bug 109609] Up to 20% perf drop in Sacha Willems' Vulkan Triangle demo

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=109609

            Bug ID: 109609
           Summary: Up to 20% perf drop in Sacha Willems' Vulkan Triangle
                    demo
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: minor
          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

Summary
-------

Between following Mesa commits:
  - 2018-12-13 17:49:48 9ebc00f32e: i965: Enable nir_opt_idiv_const for 32 and
64-bit integers
  - 2018-12-14 17:40:27 5c454661c6: i965/gen9: Add workarounds for object
preemption

There's 10-20% perf drop in Sacha Willems' Vulkan Triangle demo.

Setup
-----

- SKL (i5) or CFL GT2 (desktop) device
- Ubuntu 18.04 w/ Unity
- drm-tip git kernel v4.20 or newer
- X server with modifier support enabled (newer than v1.20.0)
- Mesa git version


Test-case
---------

 ./triangle --fullscreen --benchmark --benchwarmup 3 --benchruntime 20


Notes
-----

* No other tests were impacted either
* Regression isn't visible at all on SKL GT3e / GT4e NUCs, nor on KBL GT2 NUC,
BXT or earlier GENs
* Triangle demo is run at FullHD fullscreen, with Vsync disabled i.e. NOT using
mailbox mode as that seems to get Vsynched although Vsync is supposed to be
disabled (see patch attached to bug 109216)

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