[PATCH 3/4] aarch64: Optimise AdvSIMD log2

[Adhemerval Zanella Netto]

On 19/11/25 18:40, James Chesterman wrote:
> Optimise AdvSIMD log2 by vectorising the special case.
> For subnormal input values, use the same scaling technique as
> described in the single precision equivalent.
> Then check for inf, nan and x<=0.
> ---
> OK for master? If so, please commit for me as I don't have commit rights.
> Thanks,
> James

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>


>  sysdeps/aarch64/fpu/log2_advsimd.c | 94 +++++++++++++++++++++---------
>  1 file changed, 65 insertions(+), 29 deletions(-)
> 
> diff --git a/sysdeps/aarch64/fpu/log2_advsimd.c b/sysdeps/aarch64/fpu/log2_advsimd.c
> index 1130e47bf5..65d8670e28 100644
> --- a/sysdeps/aarch64/fpu/log2_advsimd.c
> +++ b/sysdeps/aarch64/fpu/log2_advsimd.c
> @@ -22,9 +22,11 @@
>  static const struct data
>  {
>    uint64x2_t off, sign_exp_mask, offset_lower_bound;
> -  uint32x4_t special_bound;
> +  uint64x2_t special_bound;
> +  uint32x4_t special_bound_u32;
>    float64x2_t c0, c2;
>    double c1, c3, invln2, c4;
> +  float64x2_t pinf, minf, nan;
>  } data = {
>    /* Each coefficient was generated to approximate log(r) for |r| < 0x1.fp-9
>       and N = 128, then scaled by log2(e) in extended precision and rounded back
> @@ -41,7 +43,12 @@ static const struct data
>       optimised register use subnormals are detected after offset has been
>       subtracted, so lower bound - offset (which wraps around).  */
>    .offset_lower_bound = V2 (0x0010000000000000 - 0x3fe6900900000000),
> -  .special_bound = V4 (0x7fe00000), /* asuint64(inf) - asuint64(0x1p-1022).  */
> +  .special_bound = V2 (0x7ffe000000000000),
> +  .special_bound_u32
> +  = V4 (0x7fe00000), /* asuint64(inf) - asuint64(0x1p-1022).  */
> +  .pinf = V2 (INFINITY),
> +  .minf = V2 (-INFINITY),
> +  .nan = V2 (NAN),
>  };
>  
>  #define N (1 << V_LOG2_TABLE_BITS)
> @@ -68,29 +75,9 @@ lookup (uint64x2_t i)
>    return e;
>  }
>  
> -static float64x2_t VPCS_ATTR NOINLINE
> -special_case (float64x2_t hi, uint64x2_t u_off, float64x2_t y, float64x2_t r2,
> -	      uint32x2_t special, const struct data *d)
> +static inline float64x2_t VPCS_ATTR
> +inline_log2 (uint64x2_t u, uint64x2_t u_off, const struct data *d)
>  {
> -  float64x2_t x = vreinterpretq_f64_u64 (vaddq_u64 (u_off, d->off));
> -  return v_call_f64 (log2, x, vfmaq_f64 (hi, y, r2), vmovl_u32 (special));
> -}
> -
> -/* Double-precision vector log2 routine. Implements the same algorithm as
> -   vector log10, with coefficients and table entries scaled in extended
> -   precision. The maximum observed error is 2.58 ULP:
> -   _ZGVnN2v_log2(0x1.0b556b093869bp+0) got 0x1.fffb34198d9dap-5
> -				      want 0x1.fffb34198d9ddp-5.  */
> -float64x2_t VPCS_ATTR V_NAME_D1 (log2) (float64x2_t x)
> -{
> -  const struct data *d = ptr_barrier (&data);
> -
> -  /* To avoid having to mov x out of the way, keep u after offset has been
> -     applied, and recover x by adding the offset back in the special-case
> -     handler.  */
> -  uint64x2_t u = vreinterpretq_u64_f64 (x);
> -  uint64x2_t u_off = vsubq_u64 (u, d->off);
> -
>    /* x = 2^k z; where z is in range [Off,2*Off) and exact.
>       The range is split into N subintervals.
>       The ith subinterval contains z and c is near its center.  */
> @@ -100,9 +87,6 @@ float64x2_t VPCS_ATTR V_NAME_D1 (log2) (float64x2_t x)
>  
>    struct entry e = lookup (u_off);
>  
> -  uint32x2_t special = vcge_u32 (vsubhn_u64 (u_off, d->offset_lower_bound),
> -				 vget_low_u32 (d->special_bound));
> -
>    /* log2(x) = log1p(z/c-1)/log(2) + log2(c) + k.  */
>    float64x2_t r = vfmaq_f64 (v_f64 (-1.0), z, e.invc);
>    float64x2_t kd = vcvtq_f64_s64 (k);
> @@ -118,7 +102,59 @@ float64x2_t VPCS_ATTR V_NAME_D1 (log2) (float64x2_t x)
>    y = vfmaq_laneq_f64 (y, r2, invln2_and_c4, 1);
>    y = vfmaq_f64 (p, r2, y);
>  
> -  if (__glibc_unlikely (v_any_u32h (special)))
> -    return special_case (hi, u_off, y, r2, special, d);
>    return vfmaq_f64 (hi, y, r2);
>  }
> +
> +static inline float64x2_t VPCS_ATTR
> +special_case (uint64x2_t u_off, const struct data *d)
> +{
> +  float64x2_t x = vreinterpretq_f64_u64 (vaddq_u64 (u_off, d->off));
> +  /* If x is special, compute 2log(sqrt(x)), else compute log(x).
> +     x might be subnormal, and sqrting it makes it larger.
> +     And the above two expressions are equivalent.  */
> +  uint64x2_t special
> +      = vcgeq_u64 (vsubq_u64 (u_off, d->offset_lower_bound), d->special_bound);
> +  float64x2_t x_sqrt = vbslq_f64 (special, vsqrtq_f64 (x), x);
> +
> +  u_off = vsubq_u64 (vreinterpretq_u64_f64 (x_sqrt), d->off);
> +
> +  /* Don't pass u into this, it isn't using x_sqrt.  */
> +  float64x2_t y = inline_log2 (vreinterpretq_u64_f64 (x_sqrt), u_off, d);
> +
> +  y = vbslq_f64 (special, vmulq_f64 (y, v_f64 (2.0f)), y);
> +
> +  /* Is true for +/- inf, +/- nan as well as all negative numbers.  */
> +  uint64x2_t is_infnan
> +      = vcgeq_u64 (vreinterpretq_u64_f64 (x), vreinterpretq_u64_f64 (d->pinf));
> +  uint64x2_t infnan_or_zero = vorrq_u64 (is_infnan, vceqzq_f64 (x));
> +
> +  y = vbslq_f64 (infnan_or_zero, d->nan, y);
> +  uint64x2_t ret_pinf = vceqq_f64 (x, d->pinf);
> +  uint64x2_t ret_minf = vceqzq_f64 (x);
> +  y = vbslq_f64 (ret_pinf, d->pinf, y);
> +  y = vbslq_f64 (ret_minf, d->minf, y);
> +  return y;
> +}
> +
> +/* Double-precision vector log2 routine. Implements the same algorithm as
> +   vector log10, with coefficients and table entries scaled in extended
> +   precision. The maximum observed error is 2.09 + 0.5 ULP:
> +   _ZGVnN2v_log2(0x1.0b556b093869bp+0) got 0x1.fffb34198d9dap-5
> +				      want 0x1.fffb34198d9ddp-5.  */
> +float64x2_t VPCS_ATTR V_NAME_D1 (log2) (float64x2_t x)
> +{
> +  const struct data *d = ptr_barrier (&data);
> +
> +  /* To avoid having to mov x out of the way, keep u after offset has been
> +     applied, and recover x by adding the offset back in the special-case
> +     handler.  */
> +  uint64x2_t u = vreinterpretq_u64_f64 (x);
> +  uint64x2_t u_off = vsubq_u64 (u, d->off);
> +
> +  uint32x2_t special_u32 = vcge_u32 (vsubhn_u64 (u_off, d->offset_lower_bound),
> +				     vget_low_u32 (d->special_bound_u32));
> +
> +  if (__glibc_unlikely (v_any_u32h (special_u32)))
> +    return special_case (u_off, d);
> +  return inline_log2 (u, u_off, d);
> +}