-/* origin: FreeBSD /usr/src/lib/msun/src/e_exp.c */
/*
- * ====================================================
- * Copyright (C) 2004 by Sun Microsystems, Inc. All rights reserved.
+ * Double-precision e^x function.
*
- * Permission to use, copy, modify, and distribute this
- * software is freely granted, provided that this notice
- * is preserved.
- * ====================================================
- */
-/* exp(x)
- * Returns the exponential of x.
- *
- * Method
- * 1. Argument reduction:
- * Reduce x to an r so that |r| <= 0.5*ln2 ~ 0.34658.
- * Given x, find r and integer k such that
- *
- * x = k*ln2 + r, |r| <= 0.5*ln2.
- *
- * Here r will be represented as r = hi-lo for better
- * accuracy.
- *
- * 2. Approximation of exp(r) by a special rational function on
- * the interval [0,0.34658]:
- * Write
- * R(r**2) = r*(exp(r)+1)/(exp(r)-1) = 2 + r*r/6 - r**4/360 + ...
- * We use a special Remes algorithm on [0,0.34658] to generate
- * a polynomial of degree 5 to approximate R. The maximum error
- * of this polynomial approximation is bounded by 2**-59. In
- * other words,
- * R(z) ~ 2.0 + P1*z + P2*z**2 + P3*z**3 + P4*z**4 + P5*z**5
- * (where z=r*r, and the values of P1 to P5 are listed below)
- * and
- * | 5 | -59
- * | 2.0+P1*z+...+P5*z - R(z) | <= 2
- * | |
- * The computation of exp(r) thus becomes
- * 2*r
- * exp(r) = 1 + -------
- * R - r
- * r*R1(r)
- * = 1 + r + ----------- (for better accuracy)
- * 2 - R1(r)
- * where
- * 2 4 10
- * R1(r) = r - (P1*r + P2*r + ... + P5*r ).
- *
- * 3. Scale back to obtain exp(x):
- * From step 1, we have
- * exp(x) = 2^k * exp(r)
- *
- * Special cases:
- * exp(INF) is INF, exp(NaN) is NaN;
- * exp(-INF) is 0, and
- * for finite argument, only exp(0)=1 is exact.
- *
- * Accuracy:
- * according to an error analysis, the error is always less than
- * 1 ulp (unit in the last place).
- *
- * Misc. info.
- * For IEEE double
- * if x > 7.09782712893383973096e+02 then exp(x) overflow
- * if x < -7.45133219101941108420e+02 then exp(x) underflow
- *
- * Constants:
- * The hexadecimal values are the intended ones for the following
- * constants. The decimal values may be used, provided that the
- * compiler will convert from decimal to binary accurately enough
- * to produce the hexadecimal values shown.
+ * Copyright (c) 2018, Arm Limited.
+ * SPDX-License-Identifier: MIT
*/
+#include <math.h>
+#include <stdint.h>
#include "libm.h"
+#include "exp_data.h"
-static const double
-halF[2] = {0.5,-0.5,},
-huge = 1.0e+300,
-o_threshold = 7.09782712893383973096e+02, /* 0x40862E42, 0xFEFA39EF */
-u_threshold = -7.45133219101941108420e+02, /* 0xc0874910, 0xD52D3051 */
-ln2HI[2] = { 6.93147180369123816490e-01, /* 0x3fe62e42, 0xfee00000 */
- -6.93147180369123816490e-01},/* 0xbfe62e42, 0xfee00000 */
-ln2LO[2] = { 1.90821492927058770002e-10, /* 0x3dea39ef, 0x35793c76 */
- -1.90821492927058770002e-10},/* 0xbdea39ef, 0x35793c76 */
-invln2 = 1.44269504088896338700e+00, /* 0x3ff71547, 0x652b82fe */
-P1 = 1.66666666666666019037e-01, /* 0x3FC55555, 0x5555553E */
-P2 = -2.77777777770155933842e-03, /* 0xBF66C16C, 0x16BEBD93 */
-P3 = 6.61375632143793436117e-05, /* 0x3F11566A, 0xAF25DE2C */
-P4 = -1.65339022054652515390e-06, /* 0xBEBBBD41, 0xC5D26BF1 */
-P5 = 4.13813679705723846039e-08; /* 0x3E663769, 0x72BEA4D0 */
+#define N (1 << EXP_TABLE_BITS)
+#define InvLn2N __exp_data.invln2N
+#define NegLn2hiN __exp_data.negln2hiN
+#define NegLn2loN __exp_data.negln2loN
+#define Shift __exp_data.shift
+#define T __exp_data.tab
+#define C2 __exp_data.poly[5 - EXP_POLY_ORDER]
+#define C3 __exp_data.poly[6 - EXP_POLY_ORDER]
+#define C4 __exp_data.poly[7 - EXP_POLY_ORDER]
+#define C5 __exp_data.poly[8 - EXP_POLY_ORDER]
-static const volatile double
-twom1000 = 9.33263618503218878990e-302; /* 2**-1000=0x01700000,0 */
+/* Handle cases that may overflow or underflow when computing the result that
+ is scale*(1+TMP) without intermediate rounding. The bit representation of
+ scale is in SBITS, however it has a computed exponent that may have
+ overflown into the sign bit so that needs to be adjusted before using it as
+ a double. (int32_t)KI is the k used in the argument reduction and exponent
+ adjustment of scale, positive k here means the result may overflow and
+ negative k means the result may underflow. */
+static inline double specialcase(double_t tmp, uint64_t sbits, uint64_t ki)
+{
+ double_t scale, y;
-double exp(double x)
+ if ((ki & 0x80000000) == 0) {
+ /* k > 0, the exponent of scale might have overflowed by <= 460. */
+ sbits -= 1009ull << 52;
+ scale = asdouble(sbits);
+ y = 0x1p1009 * (scale + scale * tmp);
+ return eval_as_double(y);
+ }
+ /* k < 0, need special care in the subnormal range. */
+ sbits += 1022ull << 52;
+ scale = asdouble(sbits);
+ y = scale + scale * tmp;
+ if (y < 1.0) {
+ /* Round y to the right precision before scaling it into the subnormal
+ range to avoid double rounding that can cause 0.5+E/2 ulp error where
+ E is the worst-case ulp error outside the subnormal range. So this
+ is only useful if the goal is better than 1 ulp worst-case error. */
+ double_t hi, lo;
+ lo = scale - y + scale * tmp;
+ hi = 1.0 + y;
+ lo = 1.0 - hi + y + lo;
+ y = eval_as_double(hi + lo) - 1.0;
+ /* Avoid -0.0 with downward rounding. */
+ if (WANT_ROUNDING && y == 0.0)
+ y = 0.0;
+ /* The underflow exception needs to be signaled explicitly. */
+ fp_force_eval(fp_barrier(0x1p-1022) * 0x1p-1022);
+ }
+ y = 0x1p-1022 * y;
+ return eval_as_double(y);
+}
+
+/* Top 12 bits of a double (sign and exponent bits). */
+static inline uint32_t top12(double x)
{
- double y,hi=0.0,lo=0.0,c,t,twopk;
- int32_t k=0,xsb;
- uint32_t hx;
+ return asuint64(x) >> 52;
+}
- GET_HIGH_WORD(hx, x);
- xsb = (hx>>31)&1; /* sign bit of x */
- hx &= 0x7fffffff; /* high word of |x| */
+double exp(double x)
+{
+ uint32_t abstop;
+ uint64_t ki, idx, top, sbits;
+ double_t kd, z, r, r2, scale, tail, tmp;
- /* filter out non-finite argument */
- if (hx >= 0x40862E42) { /* if |x| >= 709.78... */
- if (hx >= 0x7ff00000) {
- uint32_t lx;
-
- GET_LOW_WORD(lx,x);
- if (((hx&0xfffff)|lx) != 0) /* NaN */
- return x+x;
- return xsb==0 ? x : 0.0; /* exp(+-inf)={inf,0} */
+ abstop = top12(x) & 0x7ff;
+ if (predict_false(abstop - top12(0x1p-54) >= top12(512.0) - top12(0x1p-54))) {
+ if (abstop - top12(0x1p-54) >= 0x80000000)
+ /* Avoid spurious underflow for tiny x. */
+ /* Note: 0 is common input. */
+ return WANT_ROUNDING ? 1.0 + x : 1.0;
+ if (abstop >= top12(1024.0)) {
+ if (asuint64(x) == asuint64(-INFINITY))
+ return 0.0;
+ if (abstop >= top12(INFINITY))
+ return 1.0 + x;
+ if (asuint64(x) >> 63)
+ return __math_uflow(0);
+ else
+ return __math_oflow(0);
}
- if (x > o_threshold)
- return huge*huge; /* overflow */
- if (x < u_threshold)
- return twom1000*twom1000; /* underflow */
+ /* Large x is special cased below. */
+ abstop = 0;
}
- /* argument reduction */
- if (hx > 0x3fd62e42) { /* if |x| > 0.5 ln2 */
- if (hx < 0x3FF0A2B2) { /* and |x| < 1.5 ln2 */
- hi = x-ln2HI[xsb];
- lo = ln2LO[xsb];
- k = 1 - xsb - xsb;
- } else {
- k = (int)(invln2*x+halF[xsb]);
- t = k;
- hi = x - t*ln2HI[0]; /* t*ln2HI is exact here */
- lo = t*ln2LO[0];
- }
- STRICT_ASSIGN(double, x, hi - lo);
- } else if(hx < 0x3e300000) { /* |x| < 2**-28 */
- /* raise inexact */
- if (huge+x > 1.0)
- return 1.0+x;
- } else
- k = 0;
-
- /* x is now in primary range */
- t = x*x;
- if (k >= -1021)
- INSERT_WORDS(twopk, 0x3ff00000+(k<<20), 0);
- else
- INSERT_WORDS(twopk, 0x3ff00000+((k+1000)<<20), 0);
- c = x - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
- if (k == 0)
- return 1.0 - ((x*c)/(c-2.0) - x);
- y = 1.0-((lo-(x*c)/(2.0-c))-hi);
- if (k < -1021)
- return y*twopk*twom1000;
- if (k == 1024)
- return y*2.0*0x1p1023;
- return y*twopk;
+ /* exp(x) = 2^(k/N) * exp(r), with exp(r) in [2^(-1/2N),2^(1/2N)]. */
+ /* x = ln2/N*k + r, with int k and r in [-ln2/2N, ln2/2N]. */
+ z = InvLn2N * x;
+#if TOINT_INTRINSICS
+ kd = roundtoint(z);
+ ki = converttoint(z);
+#elif EXP_USE_TOINT_NARROW
+ /* z - kd is in [-0.5-2^-16, 0.5] in all rounding modes. */
+ kd = eval_as_double(z + Shift);
+ ki = asuint64(kd) >> 16;
+ kd = (double_t)(int32_t)ki;
+#else
+ /* z - kd is in [-1, 1] in non-nearest rounding modes. */
+ kd = eval_as_double(z + Shift);
+ ki = asuint64(kd);
+ kd -= Shift;
+#endif
+ r = x + kd * NegLn2hiN + kd * NegLn2loN;
+ /* 2^(k/N) ~= scale * (1 + tail). */
+ idx = 2 * (ki % N);
+ top = ki << (52 - EXP_TABLE_BITS);
+ tail = asdouble(T[idx]);
+ /* This is only a valid scale when -1023*N < k < 1024*N. */
+ sbits = T[idx + 1] + top;
+ /* exp(x) = 2^(k/N) * exp(r) ~= scale + scale * (tail + exp(r) - 1). */
+ /* Evaluation is optimized assuming superscalar pipelined execution. */
+ r2 = r * r;
+ /* Without fma the worst case error is 0.25/N ulp larger. */
+ /* Worst case error is less than 0.5+1.11/N+(abs poly error * 2^53) ulp. */
+ tmp = tail + r + r2 * (C2 + r * C3) + r2 * r2 * (C4 + r * C5);
+ if (predict_false(abstop == 0))
+ return specialcase(tmp, sbits, ki);
+ scale = asdouble(sbits);
+ /* Note: tmp == 0 or |tmp| > 2^-200 and scale > 2^-739, so there
+ is no spurious underflow here even without fma. */
+ return eval_as_double(scale + scale * tmp);
}
projects / musl / blobdiff