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/* origin: FreeBSD /usr/src/lib/msun/src/e_powf.c */
/*
* Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
*/
/*
* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
*
* Developed at SunPro, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/
#include "libm.h"
static const float
bp[] = {1.0, 1.5,},
dp_h[] = { 0.0, 5.84960938e-01,}, /* 0x3f15c000 */
dp_l[] = { 0.0, 1.56322085e-06,}, /* 0x35d1cfdc */
zero = 0.0,
one = 1.0,
two = 2.0,
two24 = 16777216.0, /* 0x4b800000 */
huge = 1.0e30,
tiny = 1.0e-30,
/* poly coefs for (3/2)*(log(x)-2s-2/3*s**3 */
L1 = 6.0000002384e-01, /* 0x3f19999a */
L2 = 4.2857143283e-01, /* 0x3edb6db7 */
L3 = 3.3333334327e-01, /* 0x3eaaaaab */
L4 = 2.7272811532e-01, /* 0x3e8ba305 */
L5 = 2.3066075146e-01, /* 0x3e6c3255 */
L6 = 2.0697501302e-01, /* 0x3e53f142 */
P1 = 1.6666667163e-01, /* 0x3e2aaaab */
P2 = -2.7777778450e-03, /* 0xbb360b61 */
P3 = 6.6137559770e-05, /* 0x388ab355 */
P4 = -1.6533901999e-06, /* 0xb5ddea0e */
P5 = 4.1381369442e-08, /* 0x3331bb4c */
lg2 = 6.9314718246e-01, /* 0x3f317218 */
lg2_h = 6.93145752e-01, /* 0x3f317200 */
lg2_l = 1.42860654e-06, /* 0x35bfbe8c */
ovt = 4.2995665694e-08, /* -(128-log2(ovfl+.5ulp)) */
cp = 9.6179670095e-01, /* 0x3f76384f =2/(3ln2) */
cp_h = 9.6191406250e-01, /* 0x3f764000 =12b cp */
cp_l = -1.1736857402e-04, /* 0xb8f623c6 =tail of cp_h */
ivln2 = 1.4426950216e+00, /* 0x3fb8aa3b =1/ln2 */
ivln2_h = 1.4426879883e+00, /* 0x3fb8aa00 =16b 1/ln2*/
ivln2_l = 7.0526075433e-06; /* 0x36eca570 =1/ln2 tail*/
float powf(float x, float y)
{
float z,ax,z_h,z_l,p_h,p_l;
float y1,t1,t2,r,s,sn,t,u,v,w;
int32_t i,j,k,yisint,n;
int32_t hx,hy,ix,iy,is;
GET_FLOAT_WORD(hx, x);
GET_FLOAT_WORD(hy, y);
ix = hx & 0x7fffffff;
iy = hy & 0x7fffffff;
/* y == zero: x**0 = 1 */
if (iy == 0)
return one;
/* x == 1: 1**y = 1, even if y is NaN */
if (hx == 0x3f800000)
return one;
/* y != zero: result is NaN if either arg is NaN */
if (ix > 0x7f800000 || iy > 0x7f800000)
return (x+0.0F) + (y+0.0F);
/* determine if y is an odd int when x < 0
* yisint = 0 ... y is not an integer
* yisint = 1 ... y is an odd int
* yisint = 2 ... y is an even int
*/
yisint = 0;
if (hx < 0) {
if (iy >= 0x4b800000)
yisint = 2; /* even integer y */
else if (iy >= 0x3f800000) {
k = (iy>>23) - 0x7f; /* exponent */
j = iy>>(23-k);
if ((j<<(23-k)) == iy)
yisint = 2 - (j & 1);
}
}
/* special value of y */
if (iy == 0x7f800000) { /* y is +-inf */
if (ix == 0x3f800000) /* (-1)**+-inf is 1 */
return one;
else if (ix > 0x3f800000) /* (|x|>1)**+-inf = inf,0 */
return hy >= 0 ? y : zero;
else /* (|x|<1)**+-inf = 0,inf */
return hy < 0 ? -y : zero;
}
if (iy == 0x3f800000) { /* y is +-1 */
if (hy < 0)
return one/x;
return x;
}
if (hy == 0x40000000) /* y is 2 */
return x*x;
if (hy == 0x3f000000) { /* y is 0.5 */
if (hx >= 0) /* x >= +0 */
return sqrtf(x);
}
ax = fabsf(x);
/* special value of x */
if (ix == 0x7f800000 || ix == 0 || ix == 0x3f800000) { /* x is +-0,+-inf,+-1 */
z = ax;
if (hy < 0) /* z = (1/|x|) */
z = one/z;
if (hx < 0) {
if (((ix-0x3f800000)|yisint) == 0) {
z = (z-z)/(z-z); /* (-1)**non-int is NaN */
} else if (yisint == 1)
z = -z; /* (x<0)**odd = -(|x|**odd) */
}
return z;
}
n = ((uint32_t)hx>>31) - 1;
/* (x<0)**(non-int) is NaN */
if ((n|yisint) == 0)
return (x-x)/(x-x);
sn = one; /* s (sign of result -ve**odd) = -1 else = 1 */
if ((n|(yisint-1)) == 0) /* (-ve)**(odd int) */
sn = -one;
/* |y| is huge */
if (iy > 0x4d000000) { /* if |y| > 2**27 */
/* over/underflow if x is not close to one */
if (ix < 0x3f7ffff8)
return hy < 0 ? sn*huge*huge : sn*tiny*tiny;
if (ix > 0x3f800007)
return hy > 0 ? sn*huge*huge : sn*tiny*tiny;
/* now |1-x| is tiny <= 2**-20, suffice to compute
log(x) by x-x^2/2+x^3/3-x^4/4 */
t = ax - 1; /* t has 20 trailing zeros */
w = (t*t)*(0.5f - t*(0.333333333333f - t*0.25f));
u = ivln2_h*t; /* ivln2_h has 16 sig. bits */
v = t*ivln2_l - w*ivln2;
t1 = u + v;
GET_FLOAT_WORD(is, t1);
SET_FLOAT_WORD(t1, is & 0xfffff000);
t2 = v - (t1-u);
} else {
float s2,s_h,s_l,t_h,t_l;
n = 0;
/* take care subnormal number */
if (ix < 0x00800000) {
ax *= two24;
n -= 24;
GET_FLOAT_WORD(ix, ax);
}
n += ((ix)>>23) - 0x7f;
j = ix & 0x007fffff;
/* determine interval */
ix = j | 0x3f800000; /* normalize ix */
if (j <= 0x1cc471) /* |x|<sqrt(3/2) */
k = 0;
else if (j < 0x5db3d7) /* |x|<sqrt(3) */
k = 1;
else {
k = 0;
n += 1;
ix -= 0x00800000;
}
SET_FLOAT_WORD(ax, ix);
/* compute s = s_h+s_l = (x-1)/(x+1) or (x-1.5)/(x+1.5) */
u = ax - bp[k]; /* bp[0]=1.0, bp[1]=1.5 */
v = one/(ax+bp[k]);
s = u*v;
s_h = s;
GET_FLOAT_WORD(is, s_h);
SET_FLOAT_WORD(s_h, is & 0xfffff000);
/* t_h=ax+bp[k] High */
is = ((ix>>1) & 0xfffff000) | 0x20000000;
SET_FLOAT_WORD(t_h, is + 0x00400000 + (k<<21));
t_l = ax - (t_h - bp[k]);
s_l = v*((u - s_h*t_h) - s_h*t_l);
/* compute log(ax) */
s2 = s*s;
r = s2*s2*(L1+s2*(L2+s2*(L3+s2*(L4+s2*(L5+s2*L6)))));
r += s_l*(s_h+s);
s2 = s_h*s_h;
t_h = 3.0f + s2 + r;
GET_FLOAT_WORD(is, t_h);
SET_FLOAT_WORD(t_h, is & 0xfffff000);
t_l = r - ((t_h - 3.0f) - s2);
/* u+v = s*(1+...) */
u = s_h*t_h;
v = s_l*t_h + t_l*s;
/* 2/(3log2)*(s+...) */
p_h = u + v;
GET_FLOAT_WORD(is, p_h);
SET_FLOAT_WORD(p_h, is & 0xfffff000);
p_l = v - (p_h - u);
z_h = cp_h*p_h; /* cp_h+cp_l = 2/(3*log2) */
z_l = cp_l*p_h + p_l*cp+dp_l[k];
/* log2(ax) = (s+..)*2/(3*log2) = n + dp_h + z_h + z_l */
t = (float)n;
t1 = (((z_h + z_l) + dp_h[k]) + t);
GET_FLOAT_WORD(is, t1);
SET_FLOAT_WORD(t1, is & 0xfffff000);
t2 = z_l - (((t1 - t) - dp_h[k]) - z_h);
}
/* split up y into y1+y2 and compute (y1+y2)*(t1+t2) */
GET_FLOAT_WORD(is, y);
SET_FLOAT_WORD(y1, is & 0xfffff000);
p_l = (y-y1)*t1 + y*t2;
p_h = y1*t1;
z = p_l + p_h;
GET_FLOAT_WORD(j, z);
if (j > 0x43000000) /* if z > 128 */
return sn*huge*huge; /* overflow */
else if (j == 0x43000000) { /* if z == 128 */
if (p_l + ovt > z - p_h)
return sn*huge*huge; /* overflow */
} else if ((j&0x7fffffff) > 0x43160000) /* z < -150 */ // FIXME: check should be (uint32_t)j > 0xc3160000
return sn*tiny*tiny; /* underflow */
else if (j == 0xc3160000) { /* z == -150 */
if (p_l <= z-p_h)
return sn*tiny*tiny; /* underflow */
}
/*
* compute 2**(p_h+p_l)
*/
i = j & 0x7fffffff;
k = (i>>23) - 0x7f;
n = 0;
if (i > 0x3f000000) { /* if |z| > 0.5, set n = [z+0.5] */
n = j + (0x00800000>>(k+1));
k = ((n&0x7fffffff)>>23) - 0x7f; /* new k for n */
SET_FLOAT_WORD(t, n & ~(0x007fffff>>k));
n = ((n&0x007fffff)|0x00800000)>>(23-k);
if (j < 0)
n = -n;
p_h -= t;
}
t = p_l + p_h;
GET_FLOAT_WORD(is, t);
SET_FLOAT_WORD(t, is & 0xffff8000);
u = t*lg2_h;
v = (p_l-(t-p_h))*lg2 + t*lg2_l;
z = u + v;
w = v - (z - u);
t = z*z;
t1 = z - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
r = (z*t1)/(t1-two) - (w+z*w);
z = one - (r - z);
GET_FLOAT_WORD(j, z);
j += n<<23;
if ((j>>23) <= 0) /* subnormal output */
z = scalbnf(z, n);
else
SET_FLOAT_WORD(z, j);
return sn*z;
}
|
