s_scalbn.c revision 1.14
1/* @(#)s_scalbn.c 5.1 93/09/24 */ 2/* 3 * ==================================================== 4 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 5 * 6 * Developed at SunPro, a Sun Microsystems, Inc. business. 7 * Permission to use, copy, modify, and distribute this 8 * software is freely granted, provided that this notice 9 * is preserved. 10 * ==================================================== 11 */ 12 13#include <sys/cdefs.h> 14#if defined(LIBM_SCCS) && !defined(lint) 15__RCSID("$NetBSD: s_scalbn.c,v 1.14 2010/04/23 19:17:07 drochner Exp $"); 16#endif 17 18/* 19 * scalbn (double x, int n) 20 * scalbn(x,n) returns x* 2**n computed by exponent 21 * manipulation rather than by actually performing an 22 * exponentiation or a multiplication. 23 */ 24 25#include "namespace.h" 26#include "math.h" 27#include "math_private.h" 28 29#ifdef __weak_alias 30__weak_alias(scalbn, _scalbn) 31#endif 32 33static const double 34two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 35twom54 = 5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */ 36huge = 1.0e+300, 37tiny = 1.0e-300; 38 39double 40scalbn(double x, int n) 41{ 42 int32_t k,hx,lx; 43 EXTRACT_WORDS(hx,lx,x); 44 k = ((uint32_t)hx&0x7ff00000)>>20; /* extract exponent */ 45 if (k==0) { /* 0 or subnormal x */ 46 if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */ 47 x *= two54; 48 GET_HIGH_WORD(hx,x); 49 k = (((uint32_t)hx&0x7ff00000)>>20) - 54; 50 if (n< -50000) return tiny*x; /*underflow*/ 51 } 52 if (k==0x7ff) return x+x; /* NaN or Inf */ 53 k = k+n; 54 if (k > 0x7fe) return huge*copysign(huge,x); /* overflow */ 55 if (k > 0) /* normal result */ 56 {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;} 57 if (k <= -54) { 58 if (n > 50000) /* in case integer overflow in n+k */ 59 return huge*copysign(huge,x); /*overflow*/ 60 else return tiny*copysign(tiny,x); /*underflow*/ 61 } 62 k += 54; /* subnormal result */ 63 SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); 64 return x*twom54; 65} 66