[Bug 77456] New: i965/fs generates slow code for vector comparisons

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=77456

          Priority: medium
            Bug ID: 77456
          Assignee: idr at freedesktop.org
           Summary: i965/fs generates slow code for vector comparisons
        QA Contact: intel-3d-bugs at lists.freedesktop.org
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: mattst88 at gmail.com
          Hardware: Other
            Status: NEW
           Version: unspecified
         Component: glsl-compiler
           Product: Mesa

Created attachment 97371
  --> https://bugs.freedesktop.org/attachment.cgi?id=97371&action=edit
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.

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