s_scalbn.c (1887B)
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 14 static char rcsid[] = "$FreeBSD: src/lib/msun/src/s_scalbn.c,v 1.9 2003/07/23 04:53:47 peter 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 "math_private.h" 26 27 static const double 28 two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 29 twom54 = 5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */ 30 huge = 1.0e+300, 31 tiny = 1.0e-300; 32 33 double 34 scalbn (double x, int n) 35 { 36 int32_t k,hx,lx; 37 EXTRACT_WORDS(hx,lx,x); 38 k = (hx&0x7ff00000)>>20; /* extract exponent */ 39 if (k==0) { /* 0 or subnormal x */ 40 if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */ 41 x *= two54; 42 GET_HIGH_WORD(hx,x); 43 k = ((hx&0x7ff00000)>>20) - 54; 44 if (n< -50000) return tiny*x; /*underflow*/ 45 } 46 if (k==0x7ff) return x+x; /* NaN or Inf */ 47 k = k+n; 48 if (k > 0x7fe) return huge*copysign(huge,x); /* overflow */ 49 if (k > 0) /* normal result */ 50 {SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;} 51 if (k <= -54) 52 if (n > 50000) /* in case integer overflow in n+k */ 53 return huge*copysign(huge,x); /*overflow*/ 54 else return tiny*copysign(tiny,x); /*underflow*/ 55 k += 54; /* subnormal result */ 56 SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); 57 return x*twom54; 58 }