X-Git-Url: http://nsz.repo.hu/git/?a=blobdiff_plain;f=src%2Fmath%2Flog2f.c;h=c368f88f33f5cc4eb261a247ffebf6df760c63ce;hb=9532ae1318201d66b235a618df16aac0b3386630;hp=c9b9282ccd2d91a83eb4ad94422eda186b7ed8dc;hpb=93a50a26cd0f9efc59cc83daae7b2d916b327ab1;p=musl

diff --git a/src/math/log2f.c b/src/math/log2f.c
index c9b9282c..c368f88f 100644
--- a/src/math/log2f.c
+++ b/src/math/log2f.c
@@ -1,81 +1,72 @@
-/* origin: FreeBSD /usr/src/lib/msun/src/e_log2f.c */
 /*
- * ====================================================
- * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ * Single-precision log2 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.
- * ====================================================
- */
-/*
- * See comments in log2.c.
+ * Copyright (c) 2017-2018, Arm Limited.
+ * SPDX-License-Identifier: MIT
  */
 
+#include <math.h>
+#include <stdint.h>
 #include "libm.h"
-#include "__log1pf.h"
+#include "log2f_data.h"
+
+/*
+LOG2F_TABLE_BITS = 4
+LOG2F_POLY_ORDER = 4
 
-static const float
-two25   =  3.3554432000e+07, /* 0x4c000000 */
-ivln2hi =  1.4428710938e+00, /* 0x3fb8b000 */
-ivln2lo = -1.7605285393e-04; /* 0xb9389ad4 */
+ULP error: 0.752 (nearest rounding.)
+Relative error: 1.9 * 2^-26 (before rounding.)
+*/
 
-static const float zero = 0.0;
+#define N (1 << LOG2F_TABLE_BITS)
+#define T __log2f_data.tab
+#define A __log2f_data.poly
+#define OFF 0x3f330000
 
 float log2f(float x)
 {
-	float f,hfsq,hi,lo,r,y;
-	int32_t i,k,hx;
-
-	GET_FLOAT_WORD(hx, x);
+	double_t z, r, r2, p, y, y0, invc, logc;
+	uint32_t ix, iz, top, tmp;
+	int k, i;
 
-	k = 0;
-	if (hx < 0x00800000) {  /* x < 2**-126  */
-		if ((hx&0x7fffffff) == 0)
-			return -two25/zero;  /* log(+-0)=-inf */
-		if (hx < 0)
-			return (x-x)/zero;   /* log(-#) = NaN */
-		/* subnormal number, scale up x */
-		k -= 25;
-		x *= two25;
-		GET_FLOAT_WORD(hx, x);
+	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) /* log2(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;
 	}
-	if (hx >= 0x7f800000)
-		return x+x;
-	if (hx == 0x3f800000)
-		return zero;  /* log(1) = +0 */
-	k += (hx>>23) - 127;
-	hx &= 0x007fffff;
-	i = (hx+(0x4afb0d))&0x800000;
-	SET_FLOAT_WORD(x, hx|(i^0x3f800000));  /* normalize x or x/2 */
-	k += i>>23;
-	y = (float)k;
-	f = x - 1.0f;
-	hfsq = 0.5f * f * f;
-	r = __log1pf(f);
 
-	/*
-	 * We no longer need to avoid falling into the multi-precision
-	 * calculations due to compiler bugs breaking Dekker's theorem.
-	 * Keep avoiding this as an optimization.  See log2.c for more
-	 * details (some details are here only because the optimization
-	 * is not yet available in double precision).
-	 *
-	 * Another compiler bug turned up.  With gcc on i386,
-	 * (ivln2lo + ivln2hi) would be evaluated in float precision
-	 * despite runtime evaluations using double precision.  So we
-	 * must cast one of its terms to float_t.  This makes the whole
-	 * expression have type float_t, so return is forced to waste
-	 * time clobbering its extra precision.
-	 */
-// FIXME
-//      if (sizeof(float_t) > sizeof(float))
-//              return (r - hfsq + f) * ((float_t)ivln2lo + ivln2hi) + y;
+	/* 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 - LOG2F_TABLE_BITS)) % N;
+	top = tmp & 0xff800000;
+	iz = ix - top;
+	k = (int32_t)tmp >> 23; /* arithmetic shift */
+	invc = T[i].invc;
+	logc = T[i].logc;
+	z = (double_t)asfloat(iz);
+
+	/* log2(x) = log1p(z/c-1)/ln2 + log2(c) + k */
+	r = z * invc - 1;
+	y0 = logc + (double_t)k;
 
-	hi = f - hfsq;
-	GET_FLOAT_WORD(hx,hi);
-	SET_FLOAT_WORD(hi,hx&0xfffff000);
-	lo = (f - hi) - hfsq + r;
-	return (lo+hi)*ivln2lo + lo*ivln2hi + hi*ivln2hi + y;
+	/* Pipelined polynomial evaluation to approximate log1p(r)/ln2.  */
+	r2 = r * r;
+	y = A[1] * r + A[2];
+	y = A[0] * r2 + y;
+	p = A[3] * r + y0;
+	y = y * r2 + p;
+	return eval_as_float(y);
 }