Bug 77456 - i965/fs generates slow code for vector comparisons
Summary: i965/fs generates slow code for vector comparisons
Status: NEW
Alias: None
Product: Mesa
Classification: Unclassified
Component: glsl-compiler (show other bugs)
Version: unspecified
Hardware: Other All
: medium normal
Assignee: Ian Romanick
QA Contact: Intel 3D Bugs Mailing List
URL:
Whiteboard:
Keywords:
Depends on:
Blocks: i965-perf
  Show dependency treegraph
 
Reported: 2014-04-14 23:50 UTC by Matt Turner
Modified: 2014-04-28 09:38 UTC (History)
2 users (show)

See Also:
i915 platform:
i915 features:


Attachments
t.shader_test (349 bytes, text/plain)
2014-04-14 23:50 UTC, Matt Turner
Details

Note You need to log in before you can comment on or make changes to this bug.
Description Matt Turner 2014-04-14 23:50:16 UTC
Created attachment 97371 [details]
t.shader_test

The fragment shader runs in scalar mode, so to do vec4 comparisons we generate multiple compares and join them together using and or ors, depending on the comparison.

INTEL_DEBUG=fs,no16 bin/shader_runner t.shader_test -auto

generates:

      cmp.e.f0(8)     g3<1>D          g2.3<0,1,0>F    g2.7<0,1,0>F
      cmp.e.f0(8)     g4<1>D          g2.2<0,1,0>F    g2.6<0,1,0>F
      cmp.e.f0(8)     g5<1>D          g2.1<0,1,0>F    g2.5<0,1,0>F
      cmp.e.f0(8)     g6<1>D          g2<0,1,0>F      g2.4<0,1,0>F
      and(8)          g7<1>D          g5<8,8,1>D      g6<8,8,1>D
      and(8)          g8<1>D          g4<8,8,1>D      g7<8,8,1>D
      and(8)          g9<1>D          g3<8,8,1>D      g8<8,8,1>D
      and.ne.f0(8)    null            g9<8,8,1>D      1D        
...
(+f0) sel ...

We could have just predicated all but the first cmp instruction and skipped the and instructions completely:

      cmp.e.f0(8)     g3<1>D          g2.3<0,1,0>F    g2.7<0,1,0>F
(+f0) cmp.e.f0(8)     g4<1>D          g2.2<0,1,0>F    g2.6<0,1,0>F
(+f0) cmp.e.f0(8)     g5<1>D          g2.1<0,1,0>F    g2.5<0,1,0>F
(+f0) cmp.e.f0(8)     g6<1>D          g2<0,1,0>F      g2.4<0,1,0>F
...
(+f0) sel ...

I think a similar thing can be done for !=, where the join operation is or.


Use of freedesktop.org services, including Bugzilla, is subject to our Code of Conduct. How we collect and use information is described in our Privacy Policy.