[Mesa-dev] [PATCH 1/2] Revert "i965/vec4: Change vec4_visitor::emit_lrp to use MAC for gen<6"

[Kenneth Graunke]

On 05/22/2014 09:55 AM, Matt Turner wrote:
> This reverts commit 2dfbbeca50b95ccdd714d9baa4411c779f6a20d9 with the
> comment about MAC and implicit accumulator removed.
> 
> Why this code didn't work in all circumstances is unknown and without a
> working Ironlake simulator (which uses a different AUB format) we'll
> probably never know, short of a lot of experimentation, and spending a
> bunch of time to try to optimize a few instructions on Ironlake is not
> time well spent.
> 
> Moreover, for mix(vec4, vec4, vec4) using the accumulator introduces a
> dependence between the otherwise independent per-component calculations.
> Not using the accumulator, even if it means an extra instruction per
> component might be preferable. We don't know, we don't have data, and
> we don't have the necessary register on Ironlake for shader_time to tell
> us.
> 
> Cc: "10.2" <mesa-stable at lists.freedesktop.org>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77703
> ---
>  src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> index 7bad81c..54a40dd 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
> @@ -1171,13 +1171,17 @@ vec4_visitor::emit_lrp(const dst_reg &dst,
>        /* Earlier generations don't support three source operations, so we
>         * need to emit x*(1-a) + y*a.
>         */
> -      dst_reg one_minus_a   = dst_reg(this, glsl_type::vec4_type);
> -      one_minus_a.writemask = dst.writemask;
> -
> +      dst_reg y_times_a           = dst_reg(this, glsl_type::vec4_type);
> +      dst_reg one_minus_a         = dst_reg(this, glsl_type::vec4_type);
> +      dst_reg x_times_one_minus_a = dst_reg(this, glsl_type::vec4_type);
> +      y_times_a.writemask           = dst.writemask;
> +      one_minus_a.writemask         = dst.writemask;
> +      x_times_one_minus_a.writemask = dst.writemask;
> +
> +      emit(MUL(y_times_a, y, a));
>        emit(ADD(one_minus_a, negate(a), src_reg(1.0f)));
> -      vec4_instruction *mul = emit(MUL(dst_null_f(), y, a));
> -      mul->writes_accumulator = true;
> -      emit(MAC(dst, x, src_reg(one_minus_a)));
> +      emit(MUL(x_times_one_minus_a, x, src_reg(one_minus_a)));
> +      emit(ADD(dst, src_reg(x_times_one_minus_a), src_reg(y_times_a)));
>     }
>  }
>  
> 

Both reverts are:
Acked-by: Kenneth Graunke <kenneth at whitecape.org>

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