[PATCH 3/4] aarch64: Optimise AdvSIMD log2f

[Adhemerval Zanella Netto]

On 19/11/25 11:11, James Chesterman wrote:
> Optimise AdvSIMD log2f by vectorising the special case.
> Use scaling technique on subnormal values, then check for inf and
> nan values.
> The scaling technique used will sqrt the input then multiply the
> output by 2 because:
> log(sqrt(x)) = 1/2 log(x), so log(x) = 2log(sqrt(x))
> ---
> 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/log2f_advsimd.c | 97 +++++++++++++++++++----------
>  1 file changed, 65 insertions(+), 32 deletions(-)
> 
> diff --git a/sysdeps/aarch64/fpu/log2f_advsimd.c b/sysdeps/aarch64/fpu/log2f_advsimd.c
> index 28f1857fe4..ed44e6533f 100644
> --- a/sysdeps/aarch64/fpu/log2f_advsimd.c
> +++ b/sysdeps/aarch64/fpu/log2f_advsimd.c
> @@ -23,9 +23,11 @@ static const struct data
>  {
>    float32x4_t c0, c2, c4, c6, c8;
>    uint32x4_t off, offset_lower_bound;
> -  uint16x8_t special_bound;
> +  uint32x4_t special_bound;
> +  uint16x8_t special_bound_u16;
>    uint32x4_t mantissa_mask;
>    float c1, c3, c5, c7;
> +  float32x4_t pinf, minf, nan;
>  } data = {
>    /* Coefficients generated using Remez algorithm approximate
>       log2(1+r)/r for r in [ -1/3, 1/3 ].
> @@ -42,24 +44,72 @@ static const struct data
>    /* Lower bound is the smallest positive normal float 0x00800000. For
>       optimised register use subnormals are detected after offset has been
>       subtracted, so lower bound is 0x0080000 - offset (which wraps around).  */
> +  .off = V4 (0x3f2aaaab), /* 0.666667.  */
>    .offset_lower_bound = V4 (0x00800000 - 0x3f2aaaab),
> -  .special_bound = V8 (0x7f00), /* top16(asuint32(inf) - 0x00800000).  */
> -  .off = V4 (0x3f2aaaab),	/* 0.666667.  */
> +  .special_bound = V4 (0x7f000000), /* asuint32(inf) - 0x00800000.  */
> +  .special_bound_u16 = V8 (0x7f00),
>    .mantissa_mask = V4 (0x007fffff),
> +  .pinf = V4 (INFINITY),
> +  .minf = V4 (-INFINITY),
> +  .nan = V4 (NAN),
>  };
>  
> +static inline float32x4_t VPCS_ATTR
> +inline_log2f (uint32x4_t u_off, float32x4_t n, const struct data *d)
> +{
> +  uint32x4_t u = vaddq_u32 (vandq_u32 (u_off, d->mantissa_mask), d->off);
> +  float32x4_t r = vsubq_f32 (vreinterpretq_f32_u32 (u), v_f32 (1.0f));
> +
> +  /* y = log2(1+r) + n.  */
> +  float32x4_t r2 = vmulq_f32 (r, r);
> +
> +  float32x4_t c1357 = vld1q_f32 (&d->c1);
> +  float32x4_t c01 = vfmaq_laneq_f32 (d->c0, r, c1357, 0);
> +  float32x4_t c23 = vfmaq_laneq_f32 (d->c2, r, c1357, 1);
> +  float32x4_t c45 = vfmaq_laneq_f32 (d->c4, r, c1357, 2);
> +  float32x4_t c67 = vfmaq_laneq_f32 (d->c6, r, c1357, 3);
> +  float32x4_t p68 = vfmaq_f32 (c67, r2, d->c8);
> +  float32x4_t p48 = vfmaq_f32 (c45, r2, p68);
> +  float32x4_t p28 = vfmaq_f32 (c23, r2, p48);
> +  float32x4_t p = vfmaq_f32 (c01, r2, p28);
> +
> +  return vfmaq_f32 (n, p, r);
> +}
> +
>  static float32x4_t VPCS_ATTR NOINLINE
> -special_case (float32x4_t n, uint32x4_t u_off, float32x4_t p, float32x4_t r,
> -	      uint16x4_t cmp, const struct data *d)
> +special_case (uint32x4_t u_off, const struct data *d)
>  {
> -  /* Fall back to scalar code.  */
> -  return v_call_f32 (log2f, vreinterpretq_f32_u32 (vaddq_u32 (u_off, d->off)),
> -		     vfmaq_f32 (n, p, r), vmovl_u16 (cmp));
> +  float32x4_t x = vreinterpretq_f32_u32 (vaddq_u32 (u_off, d->off));
> +  uint32x4_t special
> +      = vcgeq_u32 (vsubq_u32 (u_off, d->offset_lower_bound), d->special_bound);
> +  float32x4_t x_sqrt = vbslq_f32 (special, vsqrtq_f32 (x), x);
> +
> +  u_off = vsubq_u32 (vreinterpretq_u32_f32 (x_sqrt), d->off);
> +  float32x4_t n = vcvtq_f32_s32 (
> +      vshrq_n_s32 (vreinterpretq_s32_u32 (u_off), 23)); /* signextend.  */
> +
> +  float32x4_t y = inline_log2f (u_off, n, d);
> +
> +  /* Scale down by multiplying output by two.
> +	  Because log(x) = 2log(sqrt(x)).  */
> +  y = vbslq_f32 (special, vmulq_f32 (y, v_f32 (2.0f)), y);
> +
> +  /* Is true for +/- inf, +/- nan as well as all negative numbers.  */
> +  uint32x4_t is_infnan
> +      = vcgeq_u32 (vreinterpretq_u32_f32 (x), vreinterpretq_u32_f32 (d->pinf));
> +  uint32x4_t infnan_or_zero = vorrq_u32 (is_infnan, vceqzq_f32 (x));
> +
> +  y = vbslq_f32 (infnan_or_zero, d->nan, y);
> +  uint32x4_t ret_pinf = vceqq_f32 (x, d->pinf);
> +  uint32x4_t ret_minf = vceqzq_f32 (x);
> +  y = vbslq_f32 (ret_pinf, d->pinf, y);
> +  y = vbslq_f32 (ret_minf, d->minf, y);
> +  return y;
>  }
>  
>  /* Fast implementation for single precision AdvSIMD log2,
>     relies on same argument reduction as AdvSIMD logf.
> -   Maximum error: 2.48 ULPs
> +   Maximum error: 1.99 + 0.5
>     _ZGVnN4v_log2f(0x1.558174p+0) got 0x1.a9be84p-2
>  				want 0x1.a9be8p-2.  */
>  float32x4_t VPCS_ATTR NOINLINE V_NAME_F1 (log2) (float32x4_t x)
> @@ -69,35 +119,18 @@ float32x4_t VPCS_ATTR NOINLINE V_NAME_F1 (log2) (float32x4_t x)
>    /* 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.  */
> -  uint32x4_t u_off = vreinterpretq_u32_f32 (x);
> +  uint32x4_t u_off = vsubq_u32 (vreinterpretq_u32_f32 (x), d->off);
>  
>    /* x = 2^n * (1+r), where 2/3 < 1+r < 4/3.  */
> -  u_off = vsubq_u32 (u_off, d->off);
>    float32x4_t n = vcvtq_f32_s32 (
>        vshrq_n_s32 (vreinterpretq_s32_u32 (u_off), 23)); /* signextend.  */
>  
> -  uint16x4_t special = vcge_u16 (vsubhn_u32 (u_off, d->offset_lower_bound),
> -				 vget_low_u16 (d->special_bound));
> -
> -  uint32x4_t u = vaddq_u32 (vandq_u32 (u_off, d->mantissa_mask), d->off);
> -  float32x4_t r = vsubq_f32 (vreinterpretq_f32_u32 (u), v_f32 (1.0f));
> -
> -  /* y = log2(1+r) + n.  */
> -  float32x4_t r2 = vmulq_f32 (r, r);
> -
> -  float32x4_t c1357 = vld1q_f32 (&d->c1);
> -  float32x4_t c01 = vfmaq_laneq_f32 (d->c0, r, c1357, 0);
> -  float32x4_t c23 = vfmaq_laneq_f32 (d->c2, r, c1357, 1);
> -  float32x4_t c45 = vfmaq_laneq_f32 (d->c4, r, c1357, 2);
> -  float32x4_t c67 = vfmaq_laneq_f32 (d->c6, r, c1357, 3);
> -  float32x4_t p68 = vfmaq_f32 (c67, r2, d->c8);
> -  float32x4_t p48 = vfmaq_f32 (c45, r2, p68);
> -  float32x4_t p28 = vfmaq_f32 (c23, r2, p48);
> -  float32x4_t p = vfmaq_f32 (c01, r2, p28);
> +  uint16x4_t special_u16 = vcge_u16 (vsubhn_u32 (u_off, d->offset_lower_bound),
> +				     vget_low_u16 (d->special_bound_u16));
>  
> -  if (__glibc_unlikely (v_any_u16h (special)))
> -    return special_case (n, u_off, p, r, special, d);
> -  return vfmaq_f32 (n, p, r);
> +  if (__glibc_unlikely (v_any_u16h (special_u16)))
> +    return special_case (u_off, d);
> +  return inline_log2f (u_off, n, d);
>  }
>  
>  libmvec_hidden_def (V_NAME_F1 (log2))