math/v_exp2f.c

/*
 * Single-precision vector 2^x function.
 *
 * Copyright (c) 2019, Arm Limited.
 * SPDX-License-Identifier: MIT
 */

#include "mathlib.h"
#include "v_math.h"
#if V_SUPPORTED

static const float Poly[] = {
  /* maxerr: 1.962 ulp.  */
  0x1.59977ap-10f,
  0x1.3ce9e4p-7f,
  0x1.c6bd32p-5f,
  0x1.ebf9bcp-3f,
  0x1.62e422p-1f,
};
#define C0 v_f32 (Poly[0])
#define C1 v_f32 (Poly[1])
#define C2 v_f32 (Poly[2])
#define C3 v_f32 (Poly[3])
#define C4 v_f32 (Poly[4])

#define Shift v_f32 (0x1.8p23f)

VPCS_ATTR
static v_f32_t
specialcase (v_f32_t poly, v_f32_t n, v_u32_t e, v_f32_t absn, v_u32_t cmp1, v_f32_t scale)
{
  /* 2^n may overflow, break it up into s1*s2.  */
  v_u32_t b = v_cond_u32 (n <= v_f32 (0.0f)) & v_u32 (0x82000000);
  v_f32_t s1 = v_as_f32_u32 (v_u32 (0x7f000000) + b);
  v_f32_t s2 = v_as_f32_u32 (e - b);
  v_u32_t cmp2 = v_cond_u32 (absn > v_f32 (192.0f));
  v_u32_t r2 = v_as_u32_f32 (s1 * s1);
  v_u32_t r1 = v_as_u32_f32 (v_fma_f32 (poly, s2, s2) * s1);
  /* Similar to r1 but avoids double rounding in the subnormal range.  */
  v_u32_t r0 = v_as_u32_f32 (v_fma_f32 (poly, scale, scale));
  return v_as_f32_u32 ((cmp2 & r2) | (~cmp2 & cmp1 & r1) | (~cmp1 & r0));
}

VPCS_ATTR
v_f32_t
V_NAME(exp2f) (v_f32_t x)
{
  v_f32_t n, r, r2, scale, p, q, poly, absn;
  v_u32_t cmp, e;

  /* exp2(x) = 2^n (1 + poly(r)), with 1 + poly(r) in [1/sqrt(2),sqrt(2)]
     x = n + r, with r in [-1/2, 1/2].  */
#if 0
  v_f32_t z;
  z = x + Shift;
  n = z - Shift;
  r = x - n;
  e = v_as_u32_f32 (z) << 23;
#else
  n = v_round_f32 (x);
  r = x - n;
  e = v_as_u32_s32 (v_round_s32 (x)) << 23;
#endif
  scale = v_as_f32_u32 (e + v_u32 (0x3f800000));
  absn = v_abs_f32 (n);
  cmp = v_cond_u32 (absn > v_f32 (126.0f));
  r2 = r * r;
  p = v_fma_f32 (C0, r, C1);
  q = v_fma_f32 (C2, r, C3);
  q = v_fma_f32 (p, r2, q);
  p = C4 * r;
  poly = v_fma_f32 (q, r2, p);
  if (unlikely (v_any_u32 (cmp)))
    return specialcase (poly, n, e, absn, cmp, scale);
  return v_fma_f32 (poly, scale, scale);
}
VPCS_ALIAS
#endif