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 1.12 2002/05/26 22:01:58 wiz 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 "math.h" 26 #include "math_private.h" 27 28 static const double 29 two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 30 twom54 = 5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */ 31 huge = 1.0e+300, 32 tiny = 1.0e-300; 33 34 double 35 scalbn(double x, int n) 36 { 37 int32_t k,hx,lx; 38 EXTRACT_WORDS(hx,lx,x); 39 k = (hx&0x7ff00000)>>20; /* extract exponent */ 40 if (k==0) { /* 0 or subnormal x */ 41 if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */ 42 x *= two54; 43 GET_HIGH_WORD(hx,x); 44 k = ((hx&0x7ff00000)>>20) - 54; 45 if (n< -50000) return tiny*x; /*underflow*/ 46 } 47 if (k==0x7ff) return x+x; /* NaN or Inf */ 48 k = k+n; 49 if (k > 0x7fe) return huge*copysign(huge,x); /* overflow */ 50 if (k > 0) /* normal result */ 51 {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;} 52 if (k <= -54) { 53 if (n > 50000) /* in case integer overflow in n+k */ 54 return huge*copysign(huge,x); /*overflow*/ 55 else return tiny*copysign(tiny,x); /*underflow*/ 56 } 57 k += 54; /* subnormal result */ 58 SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); 59 return x*twom54; 60 } 61