[Mesa-dev] i965: Uniform loading via samplers with discard

[Cody Northrop]

On Tue, Jun 24, 2014 at 9:17 AM, Ian Romanick <idr at freedesktop.org> wrote:

> Send patches with git-send-email and no other method.  Save this message
> and apply it to origin/master using git-am to see why.
>
> Hopefully Matt or Ken can review this...
>
> On 06/12/2014 12:35 PM, Cody Northrop wrote:
> > Commit 17c7ead7 exposed a bug in how uniform loading happens in the
> > presence of discard.  It manifested itself in an application as randomly
> > incorrect pixels on the borders of conditional areas.
> >
> > We believe it is due to how discards jump to the end of the shader
> > incorrectly for some channels.  The current implementation checks each
> > 2x2 subspan to preserve derivatives.  When uniform loading via samplers
> > was turned on, it uses a full execution mask, as stated in
> > lower_uniform_pull_constant_loads(), and only populates four channels of
> > the destination (see generate_uniform_pull_constant_load_gen7()).  We
> > think that is happening incorrectly when the first subspan has been
> > jumped over.
> >
> > A possible fix is to only jump to the end of the shader if all relevant
> > channels are disabled, i.e. all 8 or 16, depending on dispatch.  This
> > preserves the original speedup noted in commit beafced2.  There are
> > other more heavyweight fixes (i.e. don't use sampler for uniforms if
> > discard in use), but this seems like a good fix.  I've appended it below
> > as a patch.  It changes the shader accordingly:
> >
> > before    : 0x000000d0: (-f0.1.any4h) halt(8) 17 2
> > null                            { align1 WE_all 1Q };
> > after(8)  : 0x000000d0: (-f0.1.any8h) halt(8) 17 2
> > null                            { align1 WE_all 1Q };
> > after(16) : 0x00000250: (-f0.1.any16h) halt(16) 17 2
> > null                            { align1 WE_all 1H };
>
> All of this information should go in the commit message.
>

Will do.


>
> > I attached a test case to the bugzilla entry below.
>
> Was the test also sent to the piglit mailing list for inclusion in the
> the test suite?
>

No, it wasn't a full featured test.  I'll invest the time in creating a
better example.


>
> > Thanks for any review or feedback.  Curious if anyone sees a better fix.
> >
> > -C
> >
> >
> > From 871671c4ab8ff00e85b434865e8855fc356efa8f Mon Sep 17 00:00:00 2001
> > From: Cody Northrop <cody at lunarg.com <mailto:cody at lunarg.com>>
> > Date: Thu, 12 Jun 2014 09:07:18 -0600
> > Subject: [PATCH] i965/fs: Update discard jump to preserve uniform loads
> via
> >  sampler.
> >
> > The series that implemented this optimization was done before
> > the changes to use samplers for uniform loads.  Uniform sampler
> > loads use special execution masks and only populate four
> > channels, so we can't jump over those or corruption ensues.
> > Use a more conservative jump mask which only jumps to the end
> > if all relevant channels are disabled.
> >
> > No change was observed in GLbenchmark 2.7, so the optimization
> > is preserved.
> >
> > Signed-off-by: Cody Northrop <cody at lunarg.com <mailto:cody at lunarg.com>>
> > Reviewed-by: Mike Stroyan <mike at lunarg.com <mailto:mike at lunarg.com>>
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79948
> > ---
> >  src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> > b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> > index 8858852..fe05715 100644
> > --- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> > +++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> > @@ -1907,7 +1907,15 @@ fs_visitor::visit(ir_discard *ir)
> >         */
> >        fs_inst *discard_jump = emit(FS_OPCODE_DISCARD_JUMP);
> >        discard_jump->flag_subreg = 1;
> > -      discard_jump->predicate = BRW_PREDICATE_ALIGN1_ANY4H;
> > +
> > +      /* Uniforms are now loaded using samplers with a routine that has
> > +       * its own execution mask, so we can only jump if all relevant
> > +       * channels are dead.  This is more conservative than the previous
> > +       * four channel checking, but still preserves speedups.
> > +       */
> > +      discard_jump->predicate = (8 == dispatch_width)
> > +                                ? BRW_PREDICATE_ALIGN1_ANY8H
> > +                                : BRW_PREDICATE_ALIGN1_ANY16H;
> >        discard_jump->predicate_inverse = true;
> >     }
> >  }
> > --
> > 1.8.3.2
> >
> > _______________________________________________
> > mesa-dev mailing list
> > mesa-dev at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
>


-- 
 Cody Northrop
 Graphics Software Engineer
 LunarG, Inc.- 3D Driver Innovations
 Email: cody at lunarg.com
 Website: http://www.lunarg.com
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