X-Git-Url: http://nsz.repo.hu/git/?a=blobdiff_plain;f=src%2Fmath%2Flogf.c;h=7ee5d7fe623d6ef97204207c81ebd9f04e99b878;hb=964104f9f0e056cf58d9defa0b716d7756f040f6;hp=285ee6151eaf9df68b961ff078a55bf63e2aad28;hpb=b69f695acedd4ce2798ef9ea28d834ceccc789bd;p=musl

diff --git a/src/math/logf.c b/src/math/logf.c
index 285ee615..7ee5d7fe 100644
--- a/src/math/logf.c
+++ b/src/math/logf.c
@@ -1,89 +1,71 @@
-/* origin: FreeBSD /usr/src/lib/msun/src/e_logf.c */
 /*
- * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
- */
-/*
- * ====================================================
- * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ * Single-precision log function.
  *
- * 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.
- * ====================================================
+ * Copyright (c) 2017-2018, Arm Limited.
+ * SPDX-License-Identifier: MIT
  */
 
+#include <math.h>
+#include <stdint.h>
 #include "libm.h"
+#include "logf_data.h"
 
-static const float
-ln2_hi = 6.9313812256e-01, /* 0x3f317180 */
-ln2_lo = 9.0580006145e-06, /* 0x3717f7d1 */
-two25  = 3.355443200e+07,  /* 0x4c000000 */
-/* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */
-Lg1 = 0xaaaaaa.0p-24, /* 0.66666662693 */
-Lg2 = 0xccce13.0p-25, /* 0.40000972152 */
-Lg3 = 0x91e9ee.0p-25, /* 0.28498786688 */
-Lg4 = 0xf89e26.0p-26; /* 0.24279078841 */
+/*
+LOGF_TABLE_BITS = 4
+LOGF_POLY_ORDER = 4
 
-static const float zero = 0.0;
+ULP error: 0.818 (nearest rounding.)
+Relative error: 1.957 * 2^-26 (before rounding.)
+*/
+
+#define T __logf_data.tab
+#define A __logf_data.poly
+#define Ln2 __logf_data.ln2
+#define N (1 << LOGF_TABLE_BITS)
+#define OFF 0x3f330000
 
 float logf(float x)
 {
-	float hfsq,f,s,z,R,w,t1,t2,dk;
-	int32_t k,ix,i,j;
-
-	GET_FLOAT_WORD(ix, x);
+	double_t z, r, r2, y, y0, invc, logc;
+	uint32_t ix, iz, tmp;
+	int k, i;
 
-	k = 0;
-	if (ix < 0x00800000) {  /* x < 2**-126  */
-		if ((ix & 0x7fffffff) == 0)
-			return -two25/zero;  /* log(+-0)=-inf */
-		if (ix < 0)
-			return (x-x)/zero;   /* log(-#) = NaN */
-		/* subnormal number, scale up x */
-		k -= 25;
-		x *= two25;
-		GET_FLOAT_WORD(ix, x);
-	}
-	if (ix >= 0x7f800000)
-		return x+x;
-	k += (ix>>23) - 127;
-	ix &= 0x007fffff;
-	i = (ix + (0x95f64<<3)) & 0x800000;
-	SET_FLOAT_WORD(x, ix|(i^0x3f800000));  /* normalize x or x/2 */
-	k += i>>23;
-	f = x - (float)1.0;
-	if ((0x007fffff & (0x8000 + ix)) < 0xc000) {  /* -2**-9 <= f < 2**-9 */
-		if (f == zero) {
-			if (k == 0)
-				return zero;
-			dk = (float)k;
-			return dk*ln2_hi + dk*ln2_lo;
-		}
-		R = f*f*((float)0.5 - (float)0.33333333333333333*f);
-		if (k == 0)
-			return f-R;
-		dk = (float)k;
-		return dk*ln2_hi - ((R-dk*ln2_lo)-f);
-	}
-	s = f/((float)2.0+f);
-	dk = (float)k;
-	z = s*s;
-	i = ix-(0x6147a<<3);
-	w = z*z;
-	j = (0x6b851<<3)-ix;
-	t1= w*(Lg2+w*Lg4);
-	t2= z*(Lg1+w*Lg3);
-	i |= j;
-	R = t2 + t1;
-	if (i > 0) {
-		hfsq = (float)0.5*f*f;
-		if (k == 0)
-			return f - (hfsq-s*(hfsq+R));
-		return dk*ln2_hi - ((hfsq-(s*(hfsq+R)+dk*ln2_lo))-f);
-	} else {
-		if (k == 0)
-			return f - s*(f-R);
-		return dk*ln2_hi - ((s*(f-R)-dk*ln2_lo)-f);
+	ix = asuint(x);
+	/* Fix sign of zero with downward rounding when x==1.  */
+	if (WANT_ROUNDING && predict_false(ix == 0x3f800000))
+		return 0;
+	if (predict_false(ix - 0x00800000 >= 0x7f800000 - 0x00800000)) {
+		/* x < 0x1p-126 or inf or nan.  */
+		if (ix * 2 == 0)
+			return __math_divzerof(1);
+		if (ix == 0x7f800000) /* log(inf) == inf.  */
+			return x;
+		if ((ix & 0x80000000) || ix * 2 >= 0xff000000)
+			return __math_invalidf(x);
+		/* x is subnormal, normalize it.  */
+		ix = asuint(x * 0x1p23f);
+		ix -= 23 << 23;
 	}
+
+	/* x = 2^k z; where z is in range [OFF,2*OFF] and exact.
+	   The range is split into N subintervals.
+	   The ith subinterval contains z and c is near its center.  */
+	tmp = ix - OFF;
+	i = (tmp >> (23 - LOGF_TABLE_BITS)) % N;
+	k = (int32_t)tmp >> 23; /* arithmetic shift */
+	iz = ix - (tmp & 0x1ff << 23);
+	invc = T[i].invc;
+	logc = T[i].logc;
+	z = (double_t)asfloat(iz);
+
+	/* log(x) = log1p(z/c-1) + log(c) + k*Ln2 */
+	r = z * invc - 1;
+	y0 = logc + (double_t)k * Ln2;
+
+	/* Pipelined polynomial evaluation to approximate log1p(r).  */
+	r2 = r * r;
+	y = A[1] * r + A[2];
+	y = A[0] * r2 + y;
+	y = y * r2 + (y0 + r);
+	return eval_as_float(y);
 }