s_scalbn.c revision 1.2
1 2/* @(#)s_scalbn.c 5.1 93/09/24 */ 3/* 4 * ==================================================== 5 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 6 * 7 * Developed at SunPro, a Sun Microsystems, Inc. business. 8 * Permission to use, copy, modify, and distribute this 9 * software is freely granted, provided that this notice 10 * is preserved. 11 * ==================================================== 12 */ 13 14/* 15 * scalbn (double x, int n) 16 * scalbn(x,n) returns x* 2**n computed by exponent 17 * manipulation rather than by actually performing an 18 * exponentiation or a multiplication. 19 */ 20 21#include <math.h> 22 23#ifdef __STDC__ 24static const double 25#else 26static double 27#endif 28two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 29twom54 = 5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */ 30huge = 1.0e+300, 31tiny = 1.0e-300; 32 33#ifdef __STDC__ 34 double scalbn (double x, int n) 35#else 36 double scalbn (x,n) 37 double x; int n; 38#endif 39{ 40 int k,n0,hx,lx; 41 n0 = ((*(int*)&two54)>>30)^1; /* high word index */ 42 hx = *(n0+(int*)&x); 43 lx = *(1-n0+(int*)&x); 44 k = (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 hx = *(n0+(int*)&x); 49 k = ((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 {*(n0+(int*)&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 k += 54; /* subnormal result */ 62 *(n0+(int*)&x) = (hx&0x800fffff)|(k<<20); 63 return x*twom54; 64} 65