[Mesa-dev] [PATCH v2 14/16] i965: Add fs_visitor::run_vs() to generate scalar vertex shader code

[Kenneth Graunke]

On Thursday, November 13, 2014 04:28:20 PM Kristian Høgsberg wrote:
> This patch uses the previous refactoring to add a new run_vs() method
> that generates vertex shader code using the scalar visitor and
> optimizer.
> 
> Signed-off-by: Kristian Høgsberg <krh at bitplanet.net>
> ---
>  src/mesa/drivers/dri/i965/brw_fs.cpp         |  99 ++++++++-
>  src/mesa/drivers/dri/i965/brw_fs.h           |  21 +-
>  src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 303 
++++++++++++++++++++++++++-
>  3 files changed, 412 insertions(+), 11 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp 
b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index 4dce0a2..8007977 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -1828,6 +1828,56 @@ fs_visitor::assign_urb_setup()
>        urb_start + prog_data->num_varying_inputs * 2;
>  }
>  
> +void
> +fs_visitor::assign_vs_urb_setup()
> +{
> +   brw_vs_prog_data *vs_prog_data = (brw_vs_prog_data *) prog_data;
> +   int grf, count, slot, channel, attr;
> +
> +   assert(stage == MESA_SHADER_VERTEX);
> +   count = _mesa_bitcount_64(vs_prog_data->inputs_read);
> +   if (vs_prog_data->uses_vertexid || vs_prog_data->uses_instanceid)
> +      count++;
> +
> +   /* Each attribute is 4 regs. */
> +   this->first_non_payload_grf =
> +      payload.num_regs + prog_data->curb_read_length + count * 4;
> +
> +   unsigned vue_entries =
> +      MAX2(count, vs_prog_data->base.vue_map.num_slots);
> +
> +   vs_prog_data->base.urb_entry_size = ALIGN(vue_entries, 4) / 4;
> +   vs_prog_data->base.urb_read_length = (count + 1) / 2;
> +
> +   assert(vs_prog_data->base.urb_read_length <= 15);

What enforces this assertion?  It obviously needs to be true, since
the "Vertex URB Entry Read Length" field in 3DSTATE_VS has a maximum
value of 15 in SIMD8 mode.

But this looks like the exact same calculation done in
vec4_vs_visitor::setup_attributes, and the limit there is [1, 63].

The documentation indicates that the API may allow more vertex attributes
than can practically be supported by pushing VUEs in as payload registers:

"The EU limit of 128 GRFs imposes a maximum limit of 30 elements per vertex
 pushed into the payload, though the practical limit may be lower. If input
 vertices exceed the practical limit, software must decide between resorting
 to pulling elements during thread execution or dropping back to SIMD4x2
 dispatch."

I don't see any code in this series for falling back, nor for pulling VUEs
out of the URB (and avoiding overwriting them before they're loaded), so
I expect this would simply assert fail on a vertex shader with too many
inputs.

--Ken
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