DLLEXPORT long double
cbrtl(long double x)
{
union IEEEl2bits u, v;
long double r, s, t, w;
double dr, dt, dx;
float ft, fx;
u_int32_t hx;
u_int16_t expsign;
int k;
u.e = x;
expsign = u.xbits.expsign;
k = expsign & 0x7fff;
/*
* If x = +-Inf, then cbrt(x) = +-Inf.
* If x = NaN, then cbrt(x) = NaN.
*/
if (k == BIAS + LDBL_MAX_EXP)
return (x + x);
#ifdef __i386__
fp_prec_t oprec;
oprec = fpgetprec();
if (oprec != FP_PE)
fpsetprec(FP_PE);
#endif
if (k == 0) {
/* If x = +-0, then cbrt(x) = +-0. */
if ((u.bits.manh | u.bits.manl) == 0) {
#ifdef __i386__
if (oprec != FP_PE)
fpsetprec(oprec);
#endif
return (x);
}
/* Adjust subnormal numbers. */
u.e *= 0x1.0p514;
k = u.bits.exp;
k -= BIAS + 514;
} else
k -= BIAS;
u.xbits.expsign = BIAS;
v.e = 1;
x = u.e;
switch (k % 3) {
case 1:
case -2:
x = 2*x;
k--;
break;
case 2:
case -1:
x = 4*x;
k -= 2;
break;
}
v.xbits.expsign = (expsign & 0x8000) | (BIAS + k / 3);
/*
* The following is the guts of s_cbrtf, with the handling of
* special values removed and extra care for accuracy not taken,
* but with most of the extra accuracy not discarded.
*/
/* ~5-bit estimate: */
fx = x;
GET_FLOAT_WORD(hx, fx);
SET_FLOAT_WORD(ft, ((hx & 0x7fffffff) / 3 + B1));
/* ~16-bit estimate: */
dx = x;
dt = ft;
dr = dt * dt * dt;
dt = dt * (dx + dx + dr) / (dx + dr + dr);
/* ~47-bit estimate: */
dr = dt * dt * dt;
dt = dt * (dx + dx + dr) / (dx + dr + dr);
#if LDBL_MANT_DIG == 64
/*
* dt is cbrtl(x) to ~47 bits (after x has been reduced to 1 <= x < 8).
* Round it away from zero to 32 bits (32 so that t*t is exact, and
* away from zero for technical reasons).
*/
volatile double vd2 = 0x1.0p32;
volatile double vd1 = 0x1.0p-31;
#define vd ((long double)vd2 + vd1)
t = dt + vd - 0x1.0p32;
#elif LDBL_MANT_DIG == 113
/*
* Round dt away from zero to 47 bits. Since we don't trust the 47,
* add 2 47-bit ulps instead of 1 to round up. Rounding is slow and
* might be avoidable in this case, since on most machines dt will
* have been evaluated in 53-bit precision and the technical reasons
* for rounding up might not apply to either case in cbrtl() since
* dt is much more accurate than needed.
*/
t = dt + 0x2.0p-46 + 0x1.0p60L - 0x1.0p60;
#else
#error "Unsupported long double format"
#endif
/*
* Final step Newton iteration to 64 or 113 bits with
* error < 0.667 ulps
*/
s=t*t; /* t*t is exact */
r=x/s; /* error <= 0.5 ulps; |r| < |t| */
w=t+t; /* t+t is exact */
r=(r-t)/(w+r); /* r-t is exact; w+r ~= 3*t */
t=t+t*r; /* error <= 0.5 + 0.5/3 + epsilon */
t *= v.e;
#ifdef __i386__
if (oprec != FP_PE)
fpsetprec(oprec);
#endif
return (t);
}
/*
* exp2l(x): compute the base 2 exponential of x
*
* Accuracy: Peak error < 0.511 ulp.
*
* Method: (equally-spaced tables)
*
* Reduce x:
* x = 2**k + y, for integer k and |y| <= 1/2.
* Thus we have exp2l(x) = 2**k * exp2(y).
*
* Reduce y:
* y = i/TBLSIZE + z for integer i near y * TBLSIZE.
* Thus we have exp2(y) = exp2(i/TBLSIZE) * exp2(z),
* with |z| <= 2**-(TBLBITS+1).
*
* We compute exp2(i/TBLSIZE) via table lookup and exp2(z) via a
* degree-6 minimax polynomial with maximum error under 2**-69.
* The table entries each have 104 bits of accuracy, encoded as
* a pair of double precision values.
*/
long double
exp2l(long double x)
{
union IEEEl2bits u, v;
long double r, z;
long double twopk = 0, twopkp10000 = 0;
uint32_t hx, ix, i0;
int k;
/* Filter out exceptional cases. */
u.e = x;
hx = u.xbits.expsign;
ix = hx & EXPMASK;
if (ix >= BIAS + 14) { /* |x| >= 16384 or x is NaN */
if (ix == BIAS + LDBL_MAX_EXP) {
if (u.xbits.man != 1ULL << 63 || (hx & 0x8000) == 0)
return (x + x); /* x is +Inf or NaN */
else
return (0.0); /* x is -Inf */
}
if (x >= 16384)
return (huge * huge); /* overflow */
if (x <= -16446)
return (twom10000 * twom10000); /* underflow */
} else if (ix <= BIAS - 66) { /* |x| < 0x1p-66 */
return (1.0 + x);
}
#ifdef __i386__
/*
* The default precision on i386 is 53 bits, so long doubles are
* broken. Call exp2() to get an accurate (double precision) result.
*/
if (fpgetprec() != FP_PE)
return (exp2(x));
#endif
/*
* Reduce x, computing z, i0, and k. The low bits of x + redux
* contain the 16-bit integer part of the exponent (k) followed by
* TBLBITS fractional bits (i0). We use bit tricks to extract these
* as integers, then set z to the remainder.
*
* Example: Suppose x is 0xabc.123456p0 and TBLBITS is 8.
* Then the low-order word of x + redux is 0x000abc12,
* We split this into k = 0xabc and i0 = 0x12 (adjusted to
* index into the table), then we compute z = 0x0.003456p0.
*
* XXX If the exponent is negative, the computation of k depends on
* '>>' doing sign extension.
*/
u.e = x + redux;
i0 = u.bits.manl + TBLSIZE / 2;
k = (int)i0 >> TBLBITS;
i0 = (i0 & (TBLSIZE - 1)) << 1;
u.e -= redux;
z = x - u.e;
v.xbits.man = 1ULL << 63;
if (k >= LDBL_MIN_EXP) {
v.xbits.expsign = LDBL_MAX_EXP - 1 + k;
twopk = v.e;
} else {
v.xbits.expsign = LDBL_MAX_EXP - 1 + k + 10000;
twopkp10000 = v.e;
}
/* Compute r = exp2l(y) = exp2lt[i0] * p(z). */
long double t_hi = tbl[i0];
long double t_lo = tbl[i0 + 1];
/* XXX This gives > 1 ulp errors outside of FE_TONEAREST mode */
r = t_lo + (t_hi + t_lo) * z * (P1 + z * (P2 + z * (P3 + z * (P4
+ z * (P5 + z * P6))))) + t_hi;
/* Scale by 2**k. */
if (k >= LDBL_MIN_EXP) {
if (k == LDBL_MAX_EXP)
return (r * 2.0 * 0x1p16383L);
return (r * twopk);
} else {
return (r * twopkp10000 * twom10000);
}
}
