lib/libc/gen/fixunsgen_ieee754.c

   1 /*      $NetBSD: fixunsgen_ieee754.c,v 1.3 2012/03/25 19:53:41 christos Exp $   */
   2
   3 /*-
   4  * Copyright (c) 1992, 1993
   5  *      The Regents of the University of California.  All rights reserved.
   6  *
   7  * This software was developed by the Computer Systems Engineering group
   8  * at Lawrence Berkeley Laboratory under DARPA contract BG 91-66 and
   9  * contributed to Berkeley.
  10  *
  11  * Redistribution and use in source and binary forms, with or without
  12  * modification, are permitted provided that the following conditions
  13  * are met:
  14  * 1. Redistributions of source code must retain the above copyright
  15  *    notice, this list of conditions and the following disclaimer.
  16  * 2. Redistributions in binary form must reproduce the above copyright
  17  *    notice, this list of conditions and the following disclaimer in the
  18  *    documentation and/or other materials provided with the distribution.
  19  * 3. Neither the name of the University nor the names of its contributors
  20  *    may be used to endorse or promote products derived from this software
  21  *    without specific prior written permission.
  22  *
  23  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  24  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  25  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  26  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  27  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  28  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  29  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  30  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  31  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  32  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  33  * SUCH DAMAGE.
  34  */
  35
  36 #include <sys/cdefs.h>
  37
  38 #if !defined(FIXUNSNAME) && defined(LIBC_SCCS) && !defined(lint)
  39 __RCSID("$NetBSD: fixunsgen_ieee754.c,v 1.3 2012/03/25 19:53:41 christos Exp $");
  40 #endif /* LIBC_SCCS and not lint */
  41
  42 #include <stddef.h>
  43 #include <stdint.h>
  44 #include <stdbool.h>
  45 #include <float.h>
  46
  47 #ifndef FIXUNSNAME
  48 #define FIXUNSNAME(n)   n##32
  49 #define UINTXX_T        uint32_t
  50 #endif
  51
  52 __dso_hidden UINTXX_T
  53         FIXUNSNAME(__fixunsgen)(int, bool, size_t, size_t, const uint32_t *);
  54
  55 /*
  56  * Convert double to (unsigned) int.  All operations are done module 2^32.
  57  */
  58 UINTXX_T
  59 FIXUNSNAME(__fixunsgen)(int exp, bool sign, size_t mant_dig, size_t fracbits,
  60         const uint32_t *frac)
  61 {
  62         UINTXX_T tmp;
  63
  64         /*
  65          * If it's less than 1 (negative exponent), it's going to round
  66          * to zero.  If the exponent is so large that it is a multiple of
  67          * 2^N, then x module 2^N will be 0.  (we use the fact treating a
  68          * negative value as unsigned will be greater than nonnegative value)
  69          */
  70         if (__predict_false((size_t)exp >= mant_dig + sizeof(UINTXX_T)*8))
  71                 return 0;
  72
  73         /*
  74          * This is simplier than it seems.  Basically we are constructing
  75          * fixed binary representation of the floating point number tossing
  76          * away bits that wont be in the modulis we return.
  77          */
  78         tmp = 1;
  79         for (size_t ebits = exp;;) {
  80                 if (ebits <= fracbits) {
  81                         /*
  82                          * The current fraction has more bits than we need.
  83                          * Shift the current value over and insert the bits
  84                          * we want.  We're done.
  85                          */
  86                         tmp <<= (unsigned int)ebits;
  87                         tmp |= *frac >> (fracbits - ebits);
  88                         break;
  89                 }
  90                 if (fracbits == sizeof(tmp)*4) {
  91                         /*
  92                          * Shifts must be < sizeof(type).  If it's going to be
  93                          * sizeof(type), just replace the value.
  94                          */
  95                         tmp = *frac--;
  96                 } else {
  97                         tmp <<= (unsigned int)fracbits;
  98                         tmp |= *frac--;
  99                 }
 100                 ebits -= fracbits;
 101                 fracbits = sizeof(frac[0]) * 4;
 102         }
 103
 104         /*
 105          * If the input was negative, make tmp negative module 2^32.
 106          */
 107         if (sign)
 108                 tmp = -tmp;
 109
 110         return tmp;
 111 }