[Mesa-dev] [PATCH 09/13] nir/lower_double_ops: lower round_even()

[Jason Ekstrand]

Matt,
Care to take this one?  I'm not that familiar with roundeven.
--Jason

On Tue, Apr 12, 2016 at 1:05 AM, Samuel Iglesias Gonsálvez <
siglesias at igalia.com> wrote:

> From: Iago Toral Quiroga <itoral at igalia.com>
>
> At least i965 hardware does not have native support for round_even() on
> doubles.
> ---
>  src/compiler/nir/nir.h                  |  3 +-
>  src/compiler/nir/nir_lower_double_ops.c | 58
> +++++++++++++++++++++++++++++++++
>  2 files changed, 60 insertions(+), 1 deletion(-)
>
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index 4a57e74..0d17ce0 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2289,7 +2289,8 @@ typedef enum {
>     nir_lower_dtrunc = (1 << 3),
>     nir_lower_dfloor = (1 << 4),
>     nir_lower_dceil = (1 << 5),
> -   nir_lower_dfract = (1 << 6)
> +   nir_lower_dfract = (1 << 6),
> +   nir_lower_dround_even = (1 << 7)
>  } nir_lower_doubles_options;
>
>  void nir_lower_doubles(nir_shader *shader, nir_lower_doubles_options
> options);
> diff --git a/src/compiler/nir/nir_lower_double_ops.c
> b/src/compiler/nir/nir_lower_double_ops.c
> index 0f42c21..3d98075 100644
> --- a/src/compiler/nir/nir_lower_double_ops.c
> +++ b/src/compiler/nir/nir_lower_double_ops.c
> @@ -423,6 +423,55 @@ lower_fract(nir_builder *b, nir_ssa_def *src)
>     return nir_fsub(b, src, nir_ffloor(b, src));
>  }
>
> +static nir_ssa_def *
> +lower_round_even(nir_builder *b, nir_ssa_def *src)
> +{
> +   /* If fract(src) == 0.5, then we will have to decide the rounding
> direction.
> +    * We will do this by computing the mod(abs(src), 2) and testing if it
> +    * is < 1 or not.
> +    *
> +    * We compute mod(abs(src), 2) as:
> +    * abs(src) - 2.0 * floor(abs(src) / 2.0)
> +    */
> +   nir_ssa_def *two = nir_imm_double(b, 2.0);
> +   nir_ssa_def *abs_src = nir_fabs(b, src);
> +   nir_ssa_def *mod =
> +      nir_fsub(b,
> +               abs_src,
> +               nir_fmul(b,
> +                        two,
> +                        nir_ffloor(b,
> +                                   nir_fmul(b,
> +                                            abs_src,
> +                                            nir_imm_double(b, 0.5)))));
> +
> +   /*
> +    * If fract(src) != 0.5, then we round as floor(src + 0.5)
> +    *
> +    * If fract(src) == 0.5, then we have to check the modulo:
> +    *
> +    *   if it is < 1 we need a trunc operation so we get:
> +    *      0.5 -> 0,   -0.5 -> -0
> +    *      2.5 -> 2,   -2.5 -> -2
> +    *
> +    *   otherwise we need to check if src >= 0, in which case we need to
> round
> +    *   upwards, or not, in which case we need to round downwards so we
> get:
> +    *      1.5 -> 2,   -1.5 -> -2
> +    *      3.5 -> 4,   -3.5 -> -4
> +    */
> +   nir_ssa_def *fract = nir_ffract(b, src);
> +   return nir_bcsel(b,
> +                    nir_fne(b, fract, nir_imm_double(b, 0.5)),
> +                    nir_ffloor(b, nir_fadd(b, src, nir_imm_double(b,
> 0.5))),
> +                    nir_bcsel(b,
> +                              nir_flt(b, mod, nir_imm_double(b, 1.0)),
> +                              nir_ftrunc(b, src),
> +                              nir_bcsel(b,
> +                                        nir_fge(b, src, nir_imm_double(b,
> 0.0)),
> +                                        nir_fadd(b, src,
> nir_imm_double(b, 0.5)),
> +                                        nir_fsub(b, src,
> nir_imm_double(b, 0.5)))));
> +}
> +
>  static void
>  lower_doubles_instr(nir_alu_instr *instr, nir_lower_doubles_options
> options)
>  {
> @@ -466,6 +515,11 @@ lower_doubles_instr(nir_alu_instr *instr,
> nir_lower_doubles_options options)
>           return;
>        break;
>
> +   case nir_op_fround_even:
> +      if (!(options & nir_lower_dround_even))
> +         return;
> +      break;
> +
>     default:
>        return;
>     }
> @@ -501,6 +555,10 @@ lower_doubles_instr(nir_alu_instr *instr,
> nir_lower_doubles_options options)
>     case nir_op_ffract:
>        result = lower_fract(&bld, src);
>        break;
> +   case nir_op_fround_even:
> +      result = lower_round_even(&bld, src);
> +      break;
> +
>     default:
>        unreachable("unhandled opcode");
>     }
> --
> 2.5.0
>
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