xref: /illumos-gate/usr/src/lib/libm/common/C/logb.c (revision ddc0e0b5)
125c28e83SPiotr Jasiukajtis /*
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325c28e83SPiotr Jasiukajtis  *
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525c28e83SPiotr Jasiukajtis  * Common Development and Distribution License (the "License").
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725c28e83SPiotr Jasiukajtis  *
825c28e83SPiotr Jasiukajtis  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
925c28e83SPiotr Jasiukajtis  * or http://www.opensolaris.org/os/licensing.
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2025c28e83SPiotr Jasiukajtis  */
2125c28e83SPiotr Jasiukajtis 
2225c28e83SPiotr Jasiukajtis /*
2325c28e83SPiotr Jasiukajtis  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
2425c28e83SPiotr Jasiukajtis  */
2525c28e83SPiotr Jasiukajtis /*
2625c28e83SPiotr Jasiukajtis  * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
2725c28e83SPiotr Jasiukajtis  * Use is subject to license terms.
2825c28e83SPiotr Jasiukajtis  */
2925c28e83SPiotr Jasiukajtis 
30*ddc0e0b5SRichard Lowe #pragma weak __logb = logb
31*ddc0e0b5SRichard Lowe #pragma weak _logb = logb
3225c28e83SPiotr Jasiukajtis 
3325c28e83SPiotr Jasiukajtis #include "libm.h"
3425c28e83SPiotr Jasiukajtis #include "xpg6.h"	/* __xpg6 */
3525c28e83SPiotr Jasiukajtis #define	_C99SUSv3_logb	_C99SUSv3_logb_subnormal_is_like_ilogb
3625c28e83SPiotr Jasiukajtis 
371ec68d33SRichard Lowe #if defined(__x86)
3825c28e83SPiotr Jasiukajtis static const double two52 = 4503599627370496.0;
3925c28e83SPiotr Jasiukajtis #else
4025c28e83SPiotr Jasiukajtis /*
4125c28e83SPiotr Jasiukajtis  * v: high part of a non-zero subnormal |x|; w: low part of |x|
4225c28e83SPiotr Jasiukajtis  */
4325c28e83SPiotr Jasiukajtis static int
ilogb_subnormal(unsigned v,unsigned w)4425c28e83SPiotr Jasiukajtis ilogb_subnormal(unsigned v, unsigned w) {
4525c28e83SPiotr Jasiukajtis 	int r = -1022 - 52;
4625c28e83SPiotr Jasiukajtis 
4725c28e83SPiotr Jasiukajtis 	if (v)
4825c28e83SPiotr Jasiukajtis 		r += 32;
4925c28e83SPiotr Jasiukajtis 	else
5025c28e83SPiotr Jasiukajtis 		v = w;
5125c28e83SPiotr Jasiukajtis 	if (v & 0xffff0000)
5225c28e83SPiotr Jasiukajtis 		r += 16, v >>= 16;
5325c28e83SPiotr Jasiukajtis 	if (v & 0xff00)
5425c28e83SPiotr Jasiukajtis 		r += 8, v >>= 8;
5525c28e83SPiotr Jasiukajtis 	if (v & 0xf0)
5625c28e83SPiotr Jasiukajtis 		r += 4, v >>= 4;
5725c28e83SPiotr Jasiukajtis 	v <<= 1;
5825c28e83SPiotr Jasiukajtis 	return (r + ((0xffffaa50 >> v) & 0x3));
5925c28e83SPiotr Jasiukajtis }
601ec68d33SRichard Lowe #endif	/* defined(__x86) */
6125c28e83SPiotr Jasiukajtis 
6225c28e83SPiotr Jasiukajtis double
logb(double x)6325c28e83SPiotr Jasiukajtis logb(double x) {
6425c28e83SPiotr Jasiukajtis 	int *px = (int *) &x, k = px[HIWORD] & ~0x80000000;
6525c28e83SPiotr Jasiukajtis 
6625c28e83SPiotr Jasiukajtis 	if (k < 0x00100000) {
6725c28e83SPiotr Jasiukajtis 		if ((px[LOWORD] | k) == 0)
6825c28e83SPiotr Jasiukajtis 			return (_SVID_libm_err(x, x, 45));
6925c28e83SPiotr Jasiukajtis 		else if ((__xpg6 & _C99SUSv3_logb) != 0) {
701ec68d33SRichard Lowe #if defined(__x86)
7125c28e83SPiotr Jasiukajtis 			x *= two52;
7225c28e83SPiotr Jasiukajtis 			return ((double) (((px[HIWORD] & 0x7ff00000) >> 20)
7325c28e83SPiotr Jasiukajtis 				- 1075));
7425c28e83SPiotr Jasiukajtis #else
7525c28e83SPiotr Jasiukajtis 			return ((double) ilogb_subnormal(k, px[LOWORD]));
7625c28e83SPiotr Jasiukajtis #endif
7725c28e83SPiotr Jasiukajtis 		} else
7825c28e83SPiotr Jasiukajtis 			return (-1022.0);
7925c28e83SPiotr Jasiukajtis 	} else if (k < 0x7ff00000)
8025c28e83SPiotr Jasiukajtis 		return ((double) ((k >> 20) - 1023));
8125c28e83SPiotr Jasiukajtis 	else
8225c28e83SPiotr Jasiukajtis 		return (x * x);
8325c28e83SPiotr Jasiukajtis }