[Mesa-dev] [PATCH] i965: Clean up FBH code.

[Ian Romanick]

On 10/29/2015 05:52 PM, Matt Turner wrote:
> It did a bunch of unnecessary stuff, emitting an extra MOV included.
> ---
>  src/mesa/drivers/dri/i965/brw_fs_nir.cpp   |  7 +++----
>  src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 18 +++++-------------
>  2 files changed, 8 insertions(+), 17 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> index 9c1f95c..b596614 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> @@ -906,12 +906,11 @@ fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr)
>         * from the LSB side. If FBH didn't return an error (0xFFFFFFFF), then
>         * subtract the result from 31 to convert the MSB count into an LSB count.
>         */
> -
>        bld.CMP(bld.null_reg_d(), result, fs_reg(-1), BRW_CONDITIONAL_NZ);
> -      fs_reg neg_result(result);
> -      neg_result.negate = true;
> -      inst = bld.ADD(result, neg_result, fs_reg(31));
> +
> +      inst = bld.ADD(result, result, fs_reg(31));
>        inst->predicate = BRW_PREDICATE_NORMAL;
> +      inst->src[0].negate = true;

This seems like a separate cleanup?  If you choose to split it into a
separate patch, that patch is

Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>

>        break;
>     }
>  
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> index 33cc02e..5463f3e 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> @@ -1291,26 +1291,18 @@ vec4_visitor::nir_emit_alu(nir_alu_instr *instr)
>  
>     case nir_op_ufind_msb:
>     case nir_op_ifind_msb: {
> -      src_reg temp = src_reg(this, glsl_type::uint_type);
> -
> -      inst = emit(FBH(dst_reg(temp), op[0]));
> -      inst->dst.writemask = WRITEMASK_XYZW;
> +      emit(FBH(retype(dst, BRW_REGISTER_TYPE_UD), op[0]));
>  
>        /* FBH counts from the MSB side, while GLSL's findMSB() wants the count
>         * from the LSB side. If FBH didn't return an error (0xFFFFFFFF), then
>         * subtract the result from 31 to convert the MSB count into an LSB count.
>         */
> +      src_reg src(dst);
> +      emit(CMP(dst_null_d(), src, src_reg(-1), BRW_CONDITIONAL_NZ));
>  
> -      /* FBH only supports UD type for dst, so use a MOV to convert UD to D. */
> -      temp.swizzle = BRW_SWIZZLE_NOOP;
> -      emit(MOV(dst, temp));
> -
> -      src_reg src_tmp = src_reg(dst);
> -      emit(CMP(dst_null_d(), src_tmp, src_reg(-1), BRW_CONDITIONAL_NZ));
> -
> -      src_tmp.negate = true;
> -      inst = emit(ADD(dst, src_tmp, src_reg(31)));
> +      inst = emit(ADD(dst, src, src_reg(31)));
>        inst->predicate = BRW_PREDICATE_NORMAL;
> +      inst->src[0].negate = true;
>        break;

Did retype() not exist when support for ir_unop_find_msb was added?  The
original code seems... like a weird way to do it.

Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>

>     }
>  
>