I've sent a nit or two but the first 11 patches are
Reviewed-by: Jason Ekstrand <ja...@jlekstrand.net>
I'd be very happy for you to push them ahead of the rest of the series so
they don't end up in a v3 unless someone else requests significant changes.
On Wed, Dec 19, 2018 at 5:51 AM Iago Toral Quiroga <ito...@igalia.com>
wrote:
> The 16-bit polynomial execution doesn't meet Khronos precision
> requirements.
> Also, the half-float denorm range starts at 2^(-14) and with asin taking
> input
> values in the range [0, 1], polynomial approximations can lead to flushing
> relatively easy.
>
> An alternative is to use the atan2 formula to compute asin, which is the
> reference taken by Khronos to determine precision requirements, but that
> ends up generating too many additional instructions when compared to the
> polynomial approximation. Specifically, for the Intel case, doing this
> adds +41 instructions to the program for each asin/acos call, which looks
> like an undesirable trade off.
>
> So for now we take the easy way out and fallback to using the 32-bit
> polynomial approximation, which is better (faster) than the 16-bit atan2
> implementation and gives us better precision that matches Khronos
> requirements.
>
> v2:
> - Fallback to 32-bit using recursion (Jason).
> ---
> src/compiler/spirv/vtn_glsl450.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/src/compiler/spirv/vtn_glsl450.c
> b/src/compiler/spirv/vtn_glsl450.c
> index 2540331b6cc..0a641077513 100644
> --- a/src/compiler/spirv/vtn_glsl450.c
> +++ b/src/compiler/spirv/vtn_glsl450.c
> @@ -202,6 +202,20 @@ build_log(nir_builder *b, nir_ssa_def *x)
> static nir_ssa_def *
> build_asin(nir_builder *b, nir_ssa_def *x, float p0, float p1)
> {
> + if (x->bit_size == 16) {
> + /* The polynomial approximation isn't precise enough to meet
> half-float
> + * precision requirements. Alternatively, we could implement this
> using
> + * the formula:
> + *
> + * asin(x) = atan2(x, sqrt(1 - x*x))
> + *
> + * But that is very expensive, so instead we just do the polynomial
> + * approximation in 32-bit math and then we convert the result back
> to
> + * 16-bit.
> + */
> + return nir_f2f16(b, build_asin(b, nir_f2f32(b, x), p0, p1));
> + }
> +
> nir_ssa_def *one = nir_imm_floatN_t(b, 1.0f, x->bit_size);
> nir_ssa_def *abs_x = nir_fabs(b, x);
>
> --
> 2.17.1
>
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