s_scalbn.c revision 1.4
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#ifndef lint 14static char rcsid[] = "$Id: s_scalbn.c,v 1.4 1994/03/03 17:04:46 jtc Exp $"; 15#endif 16 17/* 18 * scalbn (double x, int n) 19 * scalbn(x,n) returns x* 2**n computed by exponent 20 * manipulation rather than by actually performing an 21 * exponentiation or a multiplication. 22 */ 23 24#include <math.h> 25#include <machine/endian.h> 26 27#if BYTE_ORDER == LITTLE_ENDIAN 28#define n0 1 29#else 30#define n0 0 31#endif 32 33#ifdef __STDC__ 34static const double 35#else 36static double 37#endif 38two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 39twom54 = 5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */ 40huge = 1.0e+300, 41tiny = 1.0e-300; 42 43#ifdef __STDC__ 44 double scalbn (double x, int n) 45#else 46 double scalbn (x,n) 47 double x; int n; 48#endif 49{ 50 int k,hx,lx; 51 hx = *(n0+(int*)&x); 52 lx = *(1-n0+(int*)&x); 53 k = (hx&0x7ff00000)>>20; /* extract exponent */ 54 if (k==0) { /* 0 or subnormal x */ 55 if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */ 56 x *= two54; 57 hx = *(n0+(int*)&x); 58 k = ((hx&0x7ff00000)>>20) - 54; 59 if (n< -50000) return tiny*x; /*underflow*/ 60 } 61 if (k==0x7ff) return x+x; /* NaN or Inf */ 62 k = k+n; 63 if (k > 0x7fe) return huge*copysign(huge,x); /* overflow */ 64 if (k > 0) /* normal result */ 65 {*(n0+(int*)&x) = (hx&0x800fffff)|(k<<20); return x;} 66 if (k <= -54) 67 if (n > 50000) /* in case integer overflow in n+k */ 68 return huge*copysign(huge,x); /*overflow*/ 69 else return tiny*copysign(tiny,x); /*underflow*/ 70 k += 54; /* subnormal result */ 71 *(n0+(int*)&x) = (hx&0x800fffff)|(k<<20); 72 return x*twom54; 73} 74