[Mesa-dev] [PATCH] i965: Rewrite ir_tex to ir_txl with lod 0 for vertex shaders

[Kenneth Graunke]

On Wednesday, April 15, 2015 02:47:01 PM Ian Romanick wrote:
> On 04/15/2015 12:41 PM, Kristian Høgsberg wrote:
> > The ir_tex opcode turns into a sample or sample_c message, which will try to
> > compute derivatives to determine the lod. This produces garbage for
> > non-fragment shaders where the sample coordinates don't correspond to
> > subspans.
> > 
> > We fix this by rewriting the opcode from ir_tex to ir_txl and setting the
> > lod to 0.
> 
> This seems logical, but... why the heck hasn't this been a problem
> before?  Is this some weirdness of just BSW?  Should we perhaps only do
> this on BSW?

In the vec4 backend, we've always made ir_tex use the SAMPLE_LOD message
with lod set to 0.0 - so Gen4-7.5 have always done this workaround.

When Kristian added SIMD8 VS support (Oct 2014, Gen8+), we started using
SAMPLE (by virtue of not changing the FS behavior).  We honestly have no
idea why it appears to work on Broadwell - it sure seems like it ought
to be broken there.  Maybe we've just been lucky.

There's also a message header bit for "Force LOD to Zero", which the
docs say is mandatory if you're going to use the "SAMPLE" message in
SIMD4x2 stages (i.e. not fragment).  We've wondered whether headerless
messages in non-fragment stages magically get that right.  That could
make the issue less noticable...

Using SAMPLE_LOD is easier than setting the message header bit, and
seems just as effective.  I think we should do it on Broadwell too.

Thanks for fixing this, Kristian!

Cc: "10.5" <mesa-stable at lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>

> 
> > https://bugs.freedesktop.org/show_bug.cgi?id=89457
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89457
> 
> That allows the bin/bugzilla_mesa.sh script to find it for generating
> release notes.
> 
> > Signed-off-by: Kristian Høgsberg <krh at bitplanet.net>
> > ---
> >  src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> > index 0049b2d..4e99366 100644
> > --- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> > +++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> > @@ -1839,6 +1839,15 @@ fs_visitor::emit_texture_gen7(ir_texture_opcode op, fs_reg dst,
> >        offset_value.file != BAD_FILE && offset_value.file != IMM;
> >     bool coordinate_done = false;
> >  
> > +   /* The sampler can only meaningfully compute LOD for fragment shader
> > +    * messages. For all other stages, we change the opcode to ir_txl and
> > +    * hardcode the LOD to 0.
> > +    */
> > +   if (stage != MESA_SHADER_FRAGMENT && op == ir_tex) {
> > +      op = ir_txl;
> > +      lod = fs_reg(0.0f);
> > +   }
> > +
> >     /* Set up the LOD info */
> >     switch (op) {
> >     case ir_tex:
> > 
> 
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> 
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