1 /* derived from /netlib/fdlibm */ 2 3 /* @(#)s_scalbn.c 1.3 95/01/18 */ 4 /* 5 * ==================================================== 6 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 7 * 8 * Developed at SunSoft, a Sun Microsystems, Inc. business. 9 * Permission to use, copy, modify, and distribute this 10 * software is freely granted, provided that this notice 11 * is preserved. 12 * ==================================================== 13 */ 14 15 /* 16 * scalbn (double x, int n) 17 * scalbn(x,n) returns x* 2**n computed by exponent 18 * manipulation rather than by actually performing an 19 * exponentiation or a multiplication. 20 */ 21 22 #include "fdlibm.h" 23 24 static const double 25 two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 26 twom54 = 5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */ 27 Huge = 1.0e+300, 28 tiny = 1.0e-300; 29 scalbn(double x,int n)30 double scalbn (double x, int n) 31 { 32 int k,hx,lx; 33 hx = __HI(x); 34 lx = __LO(x); 35 k = (hx&0x7ff00000)>>20; /* extract exponent */ 36 if (k==0) { /* 0 or subnormal x */ 37 if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */ 38 x *= two54; 39 hx = __HI(x); 40 k = ((hx&0x7ff00000)>>20) - 54; 41 if (n< -50000) return tiny*x; /*underflow*/ 42 } 43 if (k==0x7ff) return x+x; /* NaN or Inf */ 44 k = k+n; 45 if (k > 0x7fe) return Huge*copysign(Huge,x); /* overflow */ 46 if (k > 0) /* normal result */ 47 {__HI(x) = (hx&0x800fffff)|(k<<20); return x;} 48 if (k <= -54) 49 if (n > 50000) /* in case integer overflow in n+k */ 50 return Huge*copysign(Huge,x); /*overflow*/ 51 else return tiny*copysign(tiny,x); /*underflow*/ 52 k += 54; /* subnormal result */ 53 __HI(x) = (hx&0x800fffff)|(k<<20); 54 return x*twom54; 55 } 56