[Mesa-dev] [PATCH 39/78] i965/nir/vec4: Add swizzle utility method for vector ops

[Jason Ekstrand]

On Fri, Jun 26, 2015 at 1:06 AM, Eduardo Lima Mitev <elima at igalia.com> wrote:
> From: Antia Puentes <apuentes at igalia.com>
>
> For operations that have a predefined operand size > 0, defined in
> glsl/nir/nir_opcodes.c, NIR returns a swizzle containing zeros in the
> components from outside the source vector. However, the driver
> expects those components to have a swizzle value equal to the swizzle
> of the component in (number of vector elements - 1). For example,
>
>    for a vec2 operation with an identity swizzle (.xy):
>       - (NIR -> XYXX, driver ->XYYYY)
>
>    for a vec3 operation with swizzle (.zxy)
>       - (NIR-> ZXYX, driver -> ZXYY)

Why is this needed.  Was there some bug you ran into regarding this or
are you just trying to make the generated code match?  If the
component isn't used, then it seems to me like the swizzle shouldn't
matter.  NIR may give you bogus swizzles outside of the enabled
channels but it may do that regardless.  I'm also not seeing how
composing swizzles fixes anything.

Please explain
--Jason

> This method handles that.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89580
> ---
>  src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> index cf74693..6beb95d 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> @@ -734,6 +734,28 @@ brw_conditional_for_nir_comparison(nir_op op)
>     }
>  }
>
> +static src_reg
> +fix_swizzle_for_input_fixed_size(nir_op op, src_reg src)
> +{
> +   /* For operations that have a predefined operand size > 0, defined in
> +    * glsl/nir/nir_opcodes.c, NIR returns a swizzle containing zeros in the
> +    * components from outside the source vector. However, the driver
> +    * expects those components to have a swizzle value equal to the swizzle
> +    * of the component in (number of vector elements - 1). For example,
> +    * for a vec2 operation with an identity swizzle (.xy):
> +    *         - (NIR -> XYXX, driver ->XYYYY)
> +    * for a vec3 operation with swizzle (.zxy)
> +    *         - (NIR-> ZXYX, driver -> ZXYY)
> +    */
> +   unsigned size = nir_op_infos[op].input_sizes[0];
> +   assert(size > 0);
> +
> +   src.swizzle = brw_compose_swizzle(brw_swizzle_for_size(size),
> +                                     src.swizzle);
> +
> +   return src;
> +}
> +
>  void
>  vec4_visitor::nir_emit_alu(nir_alu_instr *instr)
>  {
> --
> 2.1.4
>
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