Module Name:    src
Committed By:   dsl
Date:           Sun Mar 16 22:44:48 UTC 2014

Modified Files:
        src/lib/libm/src: s_exp2.c

Log Message:
Simplify somewhat: this is C not FORTRAN-IV - we have structures!
Directly us 'ieee_double_shape_type' when ripping apart the fp number
  to avoid an extra store-load in 'STRICT_ASSIGN'.
Keep 'k' as the exponent, only do 'k << 20' when generating the fp number
  from it.
Fix infinity and underflow returns on i386 - because the value is returned
  in %st0 (x87 stack) we have to generate a long double error value.
The returned value for integers -1023 and below (which should be denormal
  fp valuesr) are 'just plain wrong' (tm).


To generate a diff of this commit:
cvs rdiff -u -r1.3 -r1.4 src/lib/libm/src/s_exp2.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.

Modified files:

Index: src/lib/libm/src/s_exp2.c
diff -u src/lib/libm/src/s_exp2.c:1.3 src/lib/libm/src/s_exp2.c:1.4
--- src/lib/libm/src/s_exp2.c:1.3	Wed Nov  6 16:49:21 2013
+++ src/lib/libm/src/s_exp2.c	Sun Mar 16 22:44:48 2014
@@ -25,7 +25,7 @@
  */
 
 #include <sys/cdefs.h>
-__RCSID("$NetBSD: s_exp2.c,v 1.3 2013/11/06 16:49:21 christos Exp $");
+__RCSID("$NetBSD: s_exp2.c,v 1.4 2014/03/16 22:44:48 dsl Exp $");
 #ifdef __FBSDID
 __FBSDID("$FreeBSD: src/lib/msun/src/s_exp2.c,v 1.7 2008/02/22 02:27:34 das Exp $");
 #endif
@@ -39,7 +39,6 @@ __FBSDID("$FreeBSD: src/lib/msun/src/s_e
 #define	TBLSIZE	(1 << TBLBITS)
 
 static const double
-    huge     = 0x1p1000,
     redux    = 0x1.8p52 / TBLSIZE,
     P1	     = 0x1.62e42fefa39efp-1,
     P2	     = 0x1.ebfbdff82c575p-3,
@@ -47,266 +46,276 @@ static const double
     P4	     = 0x1.3b2ab88f70400p-7,
     P5	     = 0x1.5d88003875c74p-10;
 
-static volatile double twom1000 = 0x1p-1000;
+#ifdef __i386__
+/* The return value is in %st(0) - we need a 'long double' error. */
+static volatile long double huge = 0x1p+16000L;
+static volatile long double tiny = 0x1p-16000L;
+#else
+static volatile double huge = 0x1p+1000;
+static volatile double tiny = 0x1p-1000;
+#endif
 
-static const double tbl[TBLSIZE * 2] = {
+static const struct {
+    double exp2;	/* exp2(z + eps) */
+    double eps;		/* eps */
+} tbl[TBLSIZE] = {
 /*	exp2(z + eps)		eps	*/
-	0x1.6a09e667f3d5dp-1,	 0x1.9880p-44,
-	0x1.6b052fa751744p-1,	 0x1.8000p-50,
-	0x1.6c012750bd9fep-1,	-0x1.8780p-45,
-	0x1.6cfdcddd476bfp-1,	 0x1.ec00p-46,
-	0x1.6dfb23c651a29p-1,	-0x1.8000p-50,
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+	{ 0x1.4a32af0d7d372p+0,	-0x1.e500p-46 },
+	{ 0x1.4b17dea6db801p+0,	 0x1.7800p-47 },
+	{ 0x1.4bfdad53629e1p+0,	-0x1.3800p-46 },
+	{ 0x1.4ce41b817c132p+0,	 0x1.0800p-47 },
+	{ 0x1.4dcb299fddddbp+0,	 0x1.c700p-45 },
+	{ 0x1.4eb2d81d8ab96p+0,	-0x1.ce00p-46 },
+	{ 0x1.4f9b2769d2d02p+0,	 0x1.9200p-46 },
+	{ 0x1.508417f4531c1p+0,	-0x1.8c00p-47 },
+	{ 0x1.516daa2cf662ap+0,	-0x1.a000p-48 },
+	{ 0x1.5257de83f51eap+0,	 0x1.a080p-43 },
+	{ 0x1.5342b569d4edap+0,	-0x1.6d80p-45 },
+	{ 0x1.542e2f4f6ac1ap+0,	-0x1.2440p-44 },
+	{ 0x1.551a4ca5d94dbp+0,	 0x1.83c0p-43 },
+	{ 0x1.56070dde9116bp+0,	 0x1.4b00p-45 },
+	{ 0x1.56f4736b529dep+0,	 0x1.15a0p-43 },
+	{ 0x1.57e27dbe2c40ep+0,	-0x1.9e00p-45 },
+	{ 0x1.58d12d497c76fp+0,	-0x1.3080p-45 },
+	{ 0x1.59c0827ff0b4cp+0,	 0x1.dec0p-43 },
+	{ 0x1.5ab07dd485427p+0,	-0x1.4000p-51 },
+	{ 0x1.5ba11fba87af4p+0,	 0x1.0080p-44 },
+	{ 0x1.5c9268a59460bp+0,	-0x1.6c80p-45 },
+	{ 0x1.5d84590998e3fp+0,	 0x1.69a0p-43 },
+	{ 0x1.5e76f15ad20e1p+0,	-0x1.b400p-46 },
+	{ 0x1.5f6a320dcebcap+0,	 0x1.7700p-46 },
+	{ 0x1.605e1b976dcb8p+0,	 0x1.6f80p-45 },
+	{ 0x1.6152ae6cdf715p+0,	 0x1.1000p-47 },
+	{ 0x1.6247eb03a5531p+0,	-0x1.5d00p-46 },
+	{ 0x1.633dd1d1929b5p+0,	-0x1.2d00p-46 },
+	{ 0x1.6434634ccc313p+0,	-0x1.a800p-49 },
+	{ 0x1.652b9febc8efap+0,	-0x1.8600p-45 },
+	{ 0x1.6623882553397p+0,	 0x1.1fe0p-40 },
+	{ 0x1.671c1c708328ep+0,	-0x1.7200p-44 },
+	{ 0x1.68155d44ca97ep+0,	 0x1.6800p-49 },
+	{ 0x1.690f4b19e9471p+0,	-0x1.9780p-45 },
 };
 
 /*
@@ -332,8 +341,7 @@ static const double tbl[TBLSIZE * 2] = {
  *   that exp2t[i] is accurate to 2**-64.
  *
  *   Note that the range of i is +-TBLSIZE/2, so we actually index the tables
- *   by i0 = i + TBLSIZE/2.  For cache efficiency, exp2t[] and eps[] are
- *   virtual tables, interleaved in the real table tbl[].
+ *   by i0 = i + TBLSIZE/2.
  *
  *   This method is due to Gal, with many details due to Gal and Bachelis:
  *
@@ -343,15 +351,17 @@ static const double tbl[TBLSIZE * 2] = {
 double
 exp2(double x)
 {
+	volatile ieee_double_shape_type x_p_redux;
 	double r, t, twopk, z;
 	uint32_t hx, ix, lx, i0;
-	int k, big;
+	int k;
 
 	/* Filter out exceptional cases. */
 	GET_HIGH_WORD(hx,x);
 	ix = hx & 0x7fffffff;		/* high word of |x| */
 	if(ix >= 0x40900000) {			/* |x| >= 1024 */
 		if(ix >= 0x7ff00000) {
+			/* x is infinity or NaN */
 			GET_LOW_WORD(lx,x);
 			if(((ix & 0xfffff) | lx) != 0 || (hx & 0x80000000) == 0)
 				return (x + x);	/* x is NaN or +Inf */
@@ -361,38 +371,42 @@ exp2(double x)
 		if(x >= 0x1.0p10)
 			return (huge * huge); /* overflow */
 		if(x <= -0x1.0ccp10)
-			return (twom1000 * twom1000); /* underflow */
+			return (tiny * tiny); /* underflow */
 	} else if (ix < 0x3c900000) {		/* |x| < 0x1p-54 */
 		return (1.0 + x);
 	}
 
 	/* Reduce x, computing z, i0, and k. */
-	STRICT_ASSIGN(double, t, x + redux);
-	GET_LOW_WORD(i0, t);
+	x_p_redux.value = x + redux;
+	i0 = x_p_redux.parts.lsw;
+
 	i0 += TBLSIZE / 2;
-	k = (i0 >> TBLBITS) << 20;
-	i0 = (i0 & (TBLSIZE - 1)) << 1;
-	t -= redux;
-	z = x - t;
+	k = i0 >> TBLBITS;
+	i0 &= TBLSIZE - 1;
+	z = x - (x_p_redux.value - redux);
 
 	/* Compute r = exp2(y) = exp2t[i0] * p(z - eps[i]). */
-	t = tbl[i0];		/* exp2t[i0] */
-	z -= tbl[i0 + 1];	/* eps[i0]   */
-	big = k >= -1021 << 20;
-	if (big)
-		INSERT_WORDS(twopk, 0x3ff00000 + k, 0);
-	else
-		INSERT_WORDS(twopk, 0x3ff00000 + k + (1000 << 20), 0);
+	t = tbl[i0].exp2;
+	z -= tbl[i0].eps;
 	r = t + t * z * (P1 + z * (P2 + z * (P3 + z * (P4 + z * P5))));
 
-	/* Scale by 2**(k>>20). */
-	if (big) {
-		if (k == 1024 << 20)
+	/*
+	 * Scale by 2**k.
+	 * For 'normal' values of 'k' we just multply be the correct value,
+	 * but at the extremes this doesn't work and the multiply must
+	 * be split.
+	 */
+	if (k >= -1021) {
+		if (k == 1024)
+			/* Overflow */
 			return (r * 2.0 * 0x1p1023);
-		return (r * twopk);
 	} else {
-		return (r * twopk * twom1000);
+		/* result might be subnormal */
+		r *= 0x1p-1000;
+		k += 1000;
 	}
+	INSERT_WORDS(twopk, 0x3ff00000 + (k << 20), 0);
+	return (r * twopk);
 }
 
 #ifdef notyet

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