sys/arch/sparc/fpu/fpu_mul.c

   1 /*      $NetBSD: fpu_mul.c,v 1.4 2003/08/07 16:29:37 agc 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  * All advertising materials mentioning features or use of this software
  12  * must display the following acknowledgement:
  13  *      This product includes software developed by the University of
  14  *      California, Lawrence Berkeley Laboratory.
  15  *
  16  * Redistribution and use in source and binary forms, with or without
  17  * modification, are permitted provided that the following conditions
  18  * are met:
  19  * 1. Redistributions of source code must retain the above copyright
  20  *    notice, this list of conditions and the following disclaimer.
  21  * 2. Redistributions in binary form must reproduce the above copyright
  22  *    notice, this list of conditions and the following disclaimer in the
  23  *    documentation and/or other materials provided with the distribution.
  24  * 3. Neither the name of the University nor the names of its contributors
  25  *    may be used to endorse or promote products derived from this software
  26  *    without specific prior written permission.
  27  *
  28  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  29  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  30  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  31  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  32  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  33  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  34  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  35  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  36  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  37  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  38  * SUCH DAMAGE.
  39  *
  40  *      @(#)fpu_mul.c   8.1 (Berkeley) 6/11/93
  41  */
  42
  43 /*
  44  * Perform an FPU multiply (return x * y).
  45  */
  46
  47 #include <sys/cdefs.h>
  48 __KERNEL_RCSID(0, "$NetBSD: fpu_mul.c,v 1.4 2003/08/07 16:29:37 agc Exp $");
  49
  50 #include <sys/types.h>
  51
  52 #include <machine/reg.h>
  53
  54 #include <sparc/fpu/fpu_arith.h>
  55 #include <sparc/fpu/fpu_emu.h>
  56
  57 /*
  58  * The multiplication algorithm for normal numbers is as follows:
  59  *
  60  * The fraction of the product is built in the usual stepwise fashion.
  61  * Each step consists of shifting the accumulator right one bit
  62  * (maintaining any guard bits) and, if the next bit in y is set,
  63  * adding the multiplicand (x) to the accumulator.  Then, in any case,
  64  * we advance one bit leftward in y.  Algorithmically:
  65  *
  66  *      A = 0;
  67  *      for (bit = 0; bit < FP_NMANT; bit++) {
  68  *              sticky |= A & 1, A >>= 1;
  69  *              if (Y & (1 << bit))
  70  *                      A += X;
  71  *      }
  72  *
  73  * (X and Y here represent the mantissas of x and y respectively.)
  74  * The resultant accumulator (A) is the product's mantissa.  It may
  75  * be as large as 11.11111... in binary and hence may need to be
  76  * shifted right, but at most one bit.
  77  *
  78  * Since we do not have efficient multiword arithmetic, we code the
  79  * accumulator as four separate words, just like any other mantissa.
  80  * We use local `register' variables in the hope that this is faster
  81  * than memory.  We keep x->fp_mant in locals for the same reason.
  82  *
  83  * In the algorithm above, the bits in y are inspected one at a time.
  84  * We will pick them up 32 at a time and then deal with those 32, one
  85  * at a time.  Note, however, that we know several things about y:
  86  *
  87  *    - the guard and round bits at the bottom are sure to be zero;
  88  *
  89  *    - often many low bits are zero (y is often from a single or double
  90  *      precision source);
  91  *
  92  *    - bit FP_NMANT-1 is set, and FP_1*2 fits in a word.
  93  *
  94  * We can also test for 32-zero-bits swiftly.  In this case, the center
  95  * part of the loop---setting sticky, shifting A, and not adding---will
  96  * run 32 times without adding X to A.  We can do a 32-bit shift faster
  97  * by simply moving words.  Since zeros are common, we optimize this case.
  98  * Furthermore, since A is initially zero, we can omit the shift as well
  99  * until we reach a nonzero word.
 100  */
 101 struct fpn *
 102 fpu_mul(struct fpemu *fe)
 103 {
 104         register struct fpn *x = &fe->fe_f1, *y = &fe->fe_f2;
 105         register u_int a3, a2, a1, a0, x3, x2, x1, x0, bit, m;
 106         register int sticky;
 107         FPU_DECL_CARRY
 108
 109         /*
 110          * Put the `heavier' operand on the right (see fpu_emu.h).
 111          * Then we will have one of the following cases, taken in the
 112          * following order:
 113          *
 114          *  - y = NaN.  Implied: if only one is a signalling NaN, y is.
 115          *      The result is y.
 116          *  - y = Inf.  Implied: x != NaN (is 0, number, or Inf: the NaN
 117          *    case was taken care of earlier).
 118          *      If x = 0, the result is NaN.  Otherwise the result
 119          *      is y, with its sign reversed if x is negative.
 120          *  - x = 0.  Implied: y is 0 or number.
 121          *      The result is 0 (with XORed sign as usual).
 122          *  - other.  Implied: both x and y are numbers.
 123          *      The result is x * y (XOR sign, multiply bits, add exponents).
 124          */
 125         ORDER(x, y);
 126         if (ISNAN(y)) {
 127                 y->fp_sign ^= x->fp_sign;
 128                 return (y);
 129         }
 130         if (ISINF(y)) {
 131                 if (ISZERO(x))
 132                         return (fpu_newnan(fe));
 133                 y->fp_sign ^= x->fp_sign;
 134                 return (y);
 135         }
 136         if (ISZERO(x)) {
 137                 x->fp_sign ^= y->fp_sign;
 138                 return (x);
 139         }
 140
 141         /*
 142          * Setup.  In the code below, the mask `m' will hold the current
 143          * mantissa byte from y.  The variable `bit' denotes the bit
 144          * within m.  We also define some macros to deal with everything.
 145          */
 146         x3 = x->fp_mant[3];
 147         x2 = x->fp_mant[2];
 148         x1 = x->fp_mant[1];
 149         x0 = x->fp_mant[0];
 150         sticky = a3 = a2 = a1 = a0 = 0;
 151
 152 #define ADD     /* A += X */ \
 153         FPU_ADDS(a3, a3, x3); \
 154         FPU_ADDCS(a2, a2, x2); \
 155         FPU_ADDCS(a1, a1, x1); \
 156         FPU_ADDC(a0, a0, x0)
 157
 158 #define SHR1    /* A >>= 1, with sticky */ \
 159         sticky |= a3 & 1, a3 = (a3 >> 1) | (a2 << 31), \
 160         a2 = (a2 >> 1) | (a1 << 31), a1 = (a1 >> 1) | (a0 << 31), a0 >>= 1
 161
 162 #define SHR32   /* A >>= 32, with sticky */ \
 163         sticky |= a3, a3 = a2, a2 = a1, a1 = a0, a0 = 0
 164
 165 #define STEP    /* each 1-bit step of the multiplication */ \
 166         SHR1; if (bit & m) { ADD; }; bit <<= 1
 167
 168         /*
 169          * We are ready to begin.  The multiply loop runs once for each
 170          * of the four 32-bit words.  Some words, however, are special.
 171          * As noted above, the low order bits of Y are often zero.  Even
 172          * if not, the first loop can certainly skip the guard bits.
 173          * The last word of y has its highest 1-bit in position FP_NMANT-1,
 174          * so we stop the loop when we move past that bit.
 175          */
 176         if ((m = y->fp_mant[3]) == 0) {
 177                 /* SHR32; */                    /* unneeded since A==0 */
 178         } else {
 179                 bit = 1 << FP_NG;
 180                 do {
 181                         STEP;
 182                 } while (bit != 0);
 183         }
 184         if ((m = y->fp_mant[2]) == 0) {
 185                 SHR32;
 186         } else {
 187                 bit = 1;
 188                 do {
 189                         STEP;
 190                 } while (bit != 0);
 191         }
 192         if ((m = y->fp_mant[1]) == 0) {
 193                 SHR32;
 194         } else {
 195                 bit = 1;
 196                 do {
 197                         STEP;
 198                 } while (bit != 0);
 199         }
 200         m = y->fp_mant[0];              /* definitely != 0 */
 201         bit = 1;
 202         do {
 203                 STEP;
 204         } while (bit <= m);
 205
 206         /*
 207          * Done with mantissa calculation.  Get exponent and handle
 208          * 11.111...1 case, then put result in place.  We reuse x since
 209          * it already has the right class (FP_NUM).
 210          */
 211         m = x->fp_exp + y->fp_exp;
 212         if (a0 >= FP_2) {
 213                 SHR1;
 214                 m++;
 215         }
 216         x->fp_sign ^= y->fp_sign;
 217         x->fp_exp = m;
 218         x->fp_sticky = sticky;
 219         x->fp_mant[3] = a3;
 220         x->fp_mant[2] = a2;
 221         x->fp_mant[1] = a1;
 222         x->fp_mant[0] = a0;
 223         return (x);
 224 }