[Mesa-dev] [Bug 96518] [llvmpipe] piglit linestipple regression

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=96518

GitLab Migration User  changed:

   What|Removed |Added

 Status|NEW |RESOLVED
 Resolution|--- |MOVED

--- Comment #3 from GitLab Migration User  ---
-- GitLab Migration Automatic Message --

This bug has been migrated to freedesktop.org's GitLab instance and has been
closed from further activity.

You can subscribe and participate further through the new bug through this link
to our GitLab instance: https://gitlab.freedesktop.org/mesa/mesa/issues/241.

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[Mesa-dev] [Bug 96518] [llvmpipe] piglit linestipple regression

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=96518

Timothy Arceri  changed:

   What|Removed |Added

  Component|Mesa core   |Drivers/Gallium/llvmpipe

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[Mesa-dev] [Bug 96518] [llvmpipe] piglit linestipple regression

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=96518

Vinson Lee  changed:

   What|Removed |Added

 Blocks||98471


Referenced Bugs:

https://bugs.freedesktop.org/show_bug.cgi?id=98471
[Bug 98471] [TRACKER] Mesa 13.0 release tracker
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[Mesa-dev] [Bug 96518] [llvmpipe] piglit linestipple regression

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=96518

Vinson Lee  changed:

   What|Removed |Added

Version|git |13.0

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[Mesa-dev] [Bug 96518] [llvmpipe] piglit linestipple regression

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=96518

Jose Fonseca  changed:

   What|Removed |Added

 CC||bri...@vmware.com

--- Comment #2 from Jose Fonseca  ---
Althought generally FMA is more precise, it's possible that it give less
precise results.  Especially when dealing with things like whole integers, it's
quite possible that the lower precision of a non-fused MAD ends up "snapping"
things to the right values.  Whereas FMA preserves any intermediate rounding
error in the final result.


>From my POV, the issue is either:

- llvmpipe/draw stipple implementation needs better rounding (eg.g,
RoundToNearest as opposed to Truncate)

- the linestipple piglit test needs to be updated to not rely on precise
rounding of the stipple.


Either way I doubt the solution is to avoid FMA.

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[Mesa-dev] [Bug 96518] [llvmpipe] piglit linestipple regression

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=96518

--- Comment #1 from Roland Scheidegger  ---
I suppose I missed that when I did a piglit run...
This is fairly interesting, it seems to be due to inaccurate float math.
The stipple code gets for position x0,x1 10.5, 34.5 leading to a length 24.
However, with the MADs used for vertex transform replaced by fmas, I get
position 10.52, 34.50 instead. Hence length 23.98. And subsequently
patterns getting different (I think it makes no difference on the first
segment, but the second due to the stipple->counter += length line).

Not sure though why the results would be seemingly less accurate with fma. (And
not sure there shouldn't be some different behavior wrt rounding in the line
stipple code?)

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[Mesa-dev] [Bug 96518] [llvmpipe] piglit linestipple regression

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=96518

Bug ID: 96518
   Summary: [llvmpipe] piglit linestipple regression
   Product: Mesa
   Version: git
  Hardware: x86-64 (AMD64)
OS: Linux (All)
Status: NEW
  Keywords: bisected, regression
  Severity: normal
  Priority: medium
 Component: Mesa core
  Assignee: mesa-dev@lists.freedesktop.org
  Reporter: v...@freedesktop.org
QA Contact: mesa-dev@lists.freedesktop.org
CC: jfons...@vmware.com, srol...@vmware.com

mesa: 4825264f75c83576f251290547f121f066b46a70 (master 12.1.0-devel)

$ ./bin/linestipple -auto
Testing Baseline:
PIGLIT: {"subtest": {"Baseline" : "pass"}}
Testing Restarting lines within a single Begin-End block:
PIGLIT: {"subtest": {"Restarting lines within a single Begin-End block" :
"pass"}}
Testing Line strip:
Probe color at (34,20)
  Expected: 0.00 0.00 0.00
  Observed: 1.00 1.00 0.00
PIGLIT: {"subtest": {"Line strip" : "fail"}}
Testing Line loop:
PIGLIT: {"subtest": {"Line loop" : "pass"}}
Testing Factor 2x:
PIGLIT: {"subtest": {"Factor 2x" : "pass"}}
Testing Factor 3x:
PIGLIT: {"subtest": {"Factor 3x" : "pass"}}
PIGLIT: {"result": "fail" }


320d1191c61a0a82444605c12e5c4b2ee0b241eb is the first bad commit
commit 320d1191c61a0a82444605c12e5c4b2ee0b241eb
Author: Jose Fonseca 
Date:   Mon Apr 4 00:05:33 2016 +0100

gallivm: Use llvm.fmuladd.*.

Reviewed-by: Roland Scheidegger 

:04 04 31dca10c80a2741d596cb6d81cd6015e1312dc2e
e24948d177adab4b2fc35cd186d8dbffec3f3a0d M  src
bisect run success

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