[Mesa-dev] [PATCH 09/12] i965/fs: Add support for gl_ViewportIndex input

[Ian Romanick]

On 01/24/2014 10:51 PM, Chris Forbes wrote:
> Same idea as gl_Layer -- this is delivered in part of R0.0.

NAK.

The spec says:

    "... the fragment stage will read the same value written by
    the geometry stage, even if that value is out of range."

If the geometry shader passes 0xDEADBEEF, the fragment shader is
supposed to read 0xDEADBEEF.  That won't fit in the 4-bits available in
R0.0.  That's why I didn't implement this when I did the
GL_ARB_viewport_array work. :)

I think I want to provide an Intel extension that provides the value the
hardware will actually use.  I'm thinking take the existing ARB spec and
replace that one sentence with

    "If the value written by the geometry stage is out of range, the
    value read by the fragment stage is undefined."

We would also replace the next sentence with:

    "If a fragment shader contains a static access to gl_ViewportIndex,
    it will NOT count against the implementation defined limit for the
    maximum number of inputs to the fragment stage."

There are probably a couple other little edits too.

I'm also concerned about interactions with this extension and SSO.
Since we have to assign a real slot for gl_ViewportIndex, a separable
geometry shader that writes it will always have to write to the shadow
copy.  If the separable fragment shader doesn't read it, the varying
layouts may not match. :(

> Signed-off-by: Chris Forbes <chrisf at ijw.co.nz>
> ---
>  src/mesa/drivers/dri/i965/brw_fs.cpp         | 22 ++++++++++++++++++++++
>  src/mesa/drivers/dri/i965/brw_fs.h           |  1 +
>  src/mesa/drivers/dri/i965/brw_fs_visitor.cpp |  2 ++
>  3 files changed, 25 insertions(+)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index 17d5237..e32133b 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -1294,6 +1294,28 @@ fs_visitor::emit_layer_setup(ir_variable *ir)
>     return reg;
>  }
>  
> +fs_reg *
> +fs_visitor::emit_viewport_index_setup(ir_variable *ir)
> +{
> +   /* The value for gl_ViewportIndex is provided in bits 30:27 of R0.0. */
> +
> +   /* These bits are actually present on all Gen4+ h/w, but until GS is enabled
> +    * on earlier platforms we don't expect to get here on anything earlier
> +    * than Gen7.
> +    */
> +   assert(brw->gen >= 7);
> +
> +   this->current_annotation = "gl_ViewportIndex";
> +   fs_reg *reg = new(this->mem_ctx) fs_reg(this, ir->type);
> +   fs_reg temp = fs_reg(this, glsl_type::int_type);
> +   emit(BRW_OPCODE_SHR, temp,
> +         fs_reg(retype(brw_vec1_grf(0, 0), BRW_REGISTER_TYPE_D)),
> +         fs_reg(brw_imm_d(27)));
> +   emit(BRW_OPCODE_AND, *reg, temp,
> +         fs_reg(brw_imm_d(0x0f)));
> +   return reg;
> +}
> +
>  fs_reg
>  fs_visitor::fix_math_operand(fs_reg src)
>  {
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.h b/src/mesa/drivers/dri/i965/brw_fs.h
> index e04c341..e47fff4 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.h
> +++ b/src/mesa/drivers/dri/i965/brw_fs.h
> @@ -346,6 +346,7 @@ public:
>     fs_reg *emit_sampleid_setup(ir_variable *ir);
>     fs_reg *emit_samplemaskin_setup(ir_variable *ir);
>     fs_reg *emit_layer_setup(ir_variable *ir);
> +   fs_reg *emit_viewport_index_setup(ir_variable *ir);
>     fs_reg *emit_general_interpolation(ir_variable *ir);
>     void emit_interpolation_setup_gen4();
>     void emit_interpolation_setup_gen6();
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> index e949f4b..8864cf2 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
> @@ -63,6 +63,8 @@ fs_visitor::visit(ir_variable *ir)
>  	 reg = emit_frontfacing_interpolation(ir);
>        } else if (!strcmp(ir->name, "gl_Layer")) {
>           reg = emit_layer_setup(ir);
> +      } else if (!strcmp(ir->name, "gl_ViewportIndex")) {
> +         reg = emit_viewport_index_setup(ir);
>        } else {
>  	 reg = emit_general_interpolation(ir);
>        }
>