On Fri, Dec 9, 2016 at 6:41 PM, Jason Ekstrand <ja...@jlekstrand.net> wrote:
> The new implementation is more correct because it clamps the incoming value
> to 10 to avoid floating-point overflow. It also uses a much reduced
> version of the formula which only requires 1 exp() rather than 2. This
> fixes all of the dEQP-VK.glsl.builtin.precision.tanh.* tests.
> ---
> src/compiler/spirv/vtn_glsl450.c | 23 ++++++++++++++---------
> 1 file changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/src/compiler/spirv/vtn_glsl450.c
> b/src/compiler/spirv/vtn_glsl450.c
> index cb0570d..f0c9544 100644
> --- a/src/compiler/spirv/vtn_glsl450.c
> +++ b/src/compiler/spirv/vtn_glsl450.c
> @@ -565,16 +565,21 @@ handle_glsl450_alu(struct vtn_builder *b, enum
> GLSLstd450 entrypoint,
> build_exp(nb, nir_fneg(nb, src[0]))));
> return;
>
> - case GLSLstd450Tanh:
> - /* (0.5 * (e^x - e^(-x))) / (0.5 * (e^x + e^(-x))) */
> - val->ssa->def =
> - nir_fdiv(nb, nir_fmul(nb, nir_imm_float(nb, 0.5f),
> - nir_fsub(nb, build_exp(nb, src[0]),
> - build_exp(nb, nir_fneg(nb,
> src[0])))),
> - nir_fmul(nb, nir_imm_float(nb, 0.5f),
> - nir_fadd(nb, build_exp(nb, src[0]),
> - build_exp(nb, nir_fneg(nb,
> src[0])))));
> + case GLSLstd450Tanh: {
> + /* tanh(x) := (0.5 * (e^x - e^(-x))) / (0.5 * (e^x + e^(-x)))
> + *
> + * With a little algebra this reduces to (e^2x - 1) / (e^2x + 1)
Seems this should be "(e^(2x) - 1) / (e^(2x) + 1)" (parens around 2x).
> + *
> + * We clamp x to (-inf, +10] to avoid precision problems. When x > 10,
> + * e^2x is so much larger than 1.0 that 1.0 gets flushed to zero in the
> + * computation e^2x +- 1 so it can be ignored.
> + */
> + nir_ssa_def *x = nir_fmin(nb, src[0], nir_imm_float(nb, 10));
> + nir_ssa_def *exp2x = build_exp(nb, nir_fmul(nb, x, nir_imm_float(nb,
> 2)));
> + val->ssa->def = nir_fdiv(nb, nir_fsub(nb, exp2x, nir_imm_float(nb, 1)),
> + nir_fadd(nb, exp2x, nir_imm_float(nb,
> 1)));
> return;
> + }
>
> case GLSLstd450Asinh:
> val->ssa->def = nir_fmul(nb, nir_fsign(nb, src[0]),
Otherwise, looks good!
Reviewed-by: Erik Faye-Lund <kusmab...@gmail.com>
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