[Mesa-dev] [PATCH 04/59] compiler/spirv: implement 16-bit atan

[Jason Ekstrand]

On Tue, Dec 4, 2018 at 1:17 AM Iago Toral Quiroga <itoral at igalia.com> wrote:

> ---
>  src/compiler/spirv/vtn_glsl450.c | 36 +++++++++++++++++++++-----------
>  1 file changed, 24 insertions(+), 12 deletions(-)
>
> diff --git a/src/compiler/spirv/vtn_glsl450.c
> b/src/compiler/spirv/vtn_glsl450.c
> index 4345c9c61a3..9cda80c5137 100644
> --- a/src/compiler/spirv/vtn_glsl450.c
> +++ b/src/compiler/spirv/vtn_glsl450.c
> @@ -238,8 +238,10 @@ build_fsum(nir_builder *b, nir_ssa_def **xs, int
> terms)
>  static nir_ssa_def *
>  build_atan(nir_builder *b, nir_ssa_def *y_over_x)
>  {
> +   const uint32_t bit_size = y_over_x->bit_size;
> +
>     nir_ssa_def *abs_y_over_x = nir_fabs(b, y_over_x);
> -   nir_ssa_def *one = nir_imm_float(b, 1.0f);
> +   nir_ssa_def *one = nir_imm_floatN_t(b, 1.0f, bit_size);
>
>     /*
>      * range-reduction, first step:
> @@ -265,25 +267,35 @@ build_atan(nir_builder *b, nir_ssa_def *y_over_x)
>     nir_ssa_def *x_9  = nir_fmul(b, x_7, x_2);
>     nir_ssa_def *x_11 = nir_fmul(b, x_9, x_2);
>
> +   const float coef[] = {
> +       0.9999793128310355f,
> +      -0.3326756418091246f,
> +       0.1938924977115610f,
> +      -0.1173503194786851f,
> +       0.0536813784310406f,
> +      -0.0121323213173444f,
> +   };
> +
>     nir_ssa_def *polynomial_terms[] = {
> -      nir_fmul(b, x,    nir_imm_float(b,  0.9999793128310355f)),
> -      nir_fmul(b, x_3,  nir_imm_float(b, -0.3326756418091246f)),
> -      nir_fmul(b, x_5,  nir_imm_float(b,  0.1938924977115610f)),
> -      nir_fmul(b, x_7,  nir_imm_float(b, -0.1173503194786851f)),
> -      nir_fmul(b, x_9,  nir_imm_float(b,  0.0536813784310406f)),
> -      nir_fmul(b, x_11, nir_imm_float(b, -0.0121323213173444f)),
> +      nir_fmul(b, x,    nir_imm_floatN_t(b, coef[0], bit_size)),
> +      nir_fmul(b, x_3,  nir_imm_floatN_t(b, coef[1], bit_size)),
> +      nir_fmul(b, x_5,  nir_imm_floatN_t(b, coef[2], bit_size)),
> +      nir_fmul(b, x_7,  nir_imm_floatN_t(b, coef[3], bit_size)),
> +      nir_fmul(b, x_9,  nir_imm_floatN_t(b, coef[4], bit_size)),
> +      nir_fmul(b, x_11, nir_imm_floatN_t(b, coef[5], bit_size)),
>

Any reason why you split the coefficients out into their own array?  Just
to avoid line wrapping?

In a recent commit, I added nir_iadd_imm and nir_imul_imm helpers for
multiplying by or adding an immediate.  It might be worth doing the same
thing for multiplying and add by floats for these kinds of computations.


>     };
>
>     nir_ssa_def *tmp =
>        build_fsum(b, polynomial_terms, ARRAY_SIZE(polynomial_terms));
>
>     /* range-reduction fixup */
> +   nir_ssa_def *minus_2 = nir_imm_floatN_t(b, -2.0f, bit_size);
> +   nir_ssa_def *m_pi_2 = nir_imm_floatN_t(b, M_PI_2f, bit_size);
> +   nir_ssa_def *b2f = nir_b2f(b, nir_flt(b, one, abs_y_over_x));
> +   b2f->bit_size = bit_size; /* do we prefer b2f<bitsize> opcodes? */
>

I have patches which will fix this at least somewhat.  For right now, what
you did there is fine.


>     tmp = nir_fadd(b, tmp,
> -                  nir_fmul(b,
> -                           nir_b2f(b, nir_flt(b, one, abs_y_over_x)),
> -                           nir_fadd(b, nir_fmul(b, tmp,
> -                                                nir_imm_float(b, -2.0f)),
> -                                       nir_imm_float(b, M_PI_2f))));
> +                  nir_fmul(b, b2f,
> +                              nir_fadd(b, nir_fmul(b, tmp, minus_2),
> m_pi_2)));
>
>     /* sign fixup */
>     return nir_fmul(b, tmp, nir_fsign(b, y_over_x));
> --
> 2.17.1
>
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