[Mesa-dev] [PATCH v2] nir/lower_double_ops: lower trunc()

[Jason Ekstrand]

Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>

Did the v2 fix anything other than newly added tests?
--Jason

On Thu, Apr 21, 2016 at 10:13 PM, 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 truncating doubles.
>
> v2:
>   - Simplified the implementation significantly.
>   - Fixed the else branch, that was not doing what we wanted.
> ---
>  src/compiler/nir/nir.h                  |  1 +
>  src/compiler/nir/nir_lower_double_ops.c | 60
> +++++++++++++++++++++++++++++++++
>  2 files changed, 61 insertions(+)
>
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index 9bfb24a..ab8ae99 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2330,6 +2330,7 @@ typedef enum {
>     nir_lower_drcp = (1 << 0),
>     nir_lower_dsqrt = (1 << 1),
>     nir_lower_drsq = (1 << 2),
> +   nir_lower_dtrunc = (1 << 3),
>  } 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 e22e822..e8ae884 100644
> --- a/src/compiler/nir/nir_lower_double_ops.c
> +++ b/src/compiler/nir/nir_lower_double_ops.c
> @@ -299,6 +299,58 @@ lower_sqrt_rsq(nir_builder *b, nir_ssa_def *src, bool
> sqrt)
>      return res;
>  }
>
> +static nir_ssa_def *
> +lower_trunc(nir_builder *b, nir_ssa_def *src)
> +{
> +   nir_ssa_def *unbiased_exp = nir_isub(b, get_exponent(b, src),
> +                                        nir_imm_int(b, 1023));
> +
> +   nir_ssa_def *frac_bits = nir_isub(b, nir_imm_int(b, 52), unbiased_exp);
> +
> +   /*
> +    * Decide the operation to apply depending on the unbiased exponent:
> +    *
> +    * if (unbiased_exp < 0)
> +    *    return 0
> +    * else if (unbiased_exp > 52)
> +    *    return src
> +    * else
> +    *    return src & (~0 << frac_bits)
> +    *
> +    * Notice that the else branch is a 64-bit integer operation that we
> need
> +    * to implement in terms of 32-bit integer arithmetics (at least until
> we
> +    * support 64-bit integer arithmetics).
> +    */
> +
> +   /* Compute "~0 << frac_bits" in terms of hi/lo 32-bit integer math */
> +   nir_ssa_def *mask_lo =
> +      nir_bcsel(b,
> +                nir_ige(b, frac_bits, nir_imm_int(b, 32)),
> +                nir_imm_int(b, 0),
> +                nir_ishl(b, nir_imm_int(b, ~0), frac_bits));
> +
> +   nir_ssa_def *mask_hi =
> +      nir_bcsel(b,
> +                nir_ilt(b, frac_bits, nir_imm_int(b, 33)),
> +                nir_imm_int(b, ~0),
> +                nir_ishl(b,
> +                         nir_imm_int(b, ~0),
> +                         nir_isub(b, frac_bits, nir_imm_int(b, 32))));
> +
> +   nir_ssa_def *src_lo = nir_unpack_double_2x32_split_x(b, src);
> +   nir_ssa_def *src_hi = nir_unpack_double_2x32_split_y(b, src);
> +
> +   return
> +      nir_bcsel(b,
> +                nir_ilt(b, unbiased_exp, nir_imm_int(b, 0)),
> +                nir_imm_double(b, 0.0),
> +                nir_bcsel(b, nir_ige(b, unbiased_exp, nir_imm_int(b, 53)),
> +                          src,
> +                          nir_pack_double_2x32_split(b,
> +                                                     nir_iand(b, mask_lo,
> src_lo),
> +                                                     nir_iand(b, mask_hi,
> src_hi))));
> +}
> +
>  static void
>  lower_doubles_instr(nir_alu_instr *instr, nir_lower_doubles_options
> options)
>  {
> @@ -322,6 +374,11 @@ lower_doubles_instr(nir_alu_instr *instr,
> nir_lower_doubles_options options)
>           return;
>        break;
>
> +   case nir_op_ftrunc:
> +      if (!(options & nir_lower_dtrunc))
> +         return;
> +      break;
> +
>     default:
>        return;
>     }
> @@ -345,6 +402,9 @@ lower_doubles_instr(nir_alu_instr *instr,
> nir_lower_doubles_options options)
>     case nir_op_frsq:
>        result = lower_sqrt_rsq(&bld, src, false);
>        break;
> +   case nir_op_ftrunc:
> +      result = lower_trunc(&bld, src);
> +      break;
>     default:
>        unreachable("unhandled opcode");
>     }
> --
> 2.5.0
>
>
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