/* $OpenBSD: fpu_mul.c,v 1.4 2024/03/29 21:07:11 miod Exp $ */
/* $NetBSD: fpu_mul.c,v 1.2 1994/11/20 20:52:44 deraadt Exp $ */
/*
* Copyright (c) 1992, 1993
* The Regents of the University of California. All rights reserved.
*
* This software was developed by the Computer Systems Engineering group
* at Lawrence Berkeley Laboratory under DARPA contract BG 91-66 and
* contributed to Berkeley.
*
* All advertising materials mentioning features or use of this software
* must display the following acknowledgement:
* This product includes software developed by the University of
* California, Lawrence Berkeley Laboratory.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* @(#)fpu_mul.c 8.1 (Berkeley) 6/11/93
*/
/*
* Perform an FPU multiply (return x * y).
*/
#include
#include
#include
#include
/*
* The multiplication algorithm for normal numbers is as follows:
*
* The fraction of the product is built in the usual stepwise fashion.
* Each step consists of shifting the accumulator right one bit
* (maintaining any guard bits) and, if the next bit in y is set,
* adding the multiplicand (x) to the accumulator. Then, in any case,
* we advance one bit leftward in y. Algorithmically:
*
* A = 0;
* for (bit = 0; bit < FP_NMANT; bit++) {
* sticky |= A & 1, A >>= 1;
* if (Y & (1 << bit))
* A += X;
* }
*
* (X and Y here represent the mantissas of x and y respectively.)
* The resultant accumulator (A) is the product's mantissa. It may
* be as large as 11.11111... in binary and hence may need to be
* shifted right, but at most one bit.
*
* Since we do not have efficient multiword arithmetic, we code the
* accumulator as four separate words, just like any other mantissa.
*
* In the algorithm above, the bits in y are inspected one at a time.
* We will pick them up 32 at a time and then deal with those 32, one
* at a time. Note, however, that we know several things about y:
*
* - the guard and round bits at the bottom are sure to be zero;
*
* - often many low bits are zero (y is often from a single or double
* precision source);
*
* - bit FP_NMANT-1 is set, and FP_1*2 fits in a word.
*
* We can also test for 32-zero-bits swiftly. In this case, the center
* part of the loop---setting sticky, shifting A, and not adding---will
* run 32 times without adding X to A. We can do a 32-bit shift faster
* by simply moving words. Since zeros are common, we optimize this case.
* Furthermore, since A is initially zero, we can omit the shift as well
* until we reach a nonzero word.
*/
struct fpn *
fpu_mul(struct fpemu *fe)
{
struct fpn *x = &fe->fe_f1, *y = &fe->fe_f2;
u_int a3, a2, a1, a0, x3, x2, x1, x0, bit, m;
int sticky;
FPU_DECL_CARRY
/*
* Put the `heavier' operand on the right (see fpu_emu.h).
* Then we will have one of the following cases, taken in the
* following order:
*
* - y = NaN. Implied: if only one is a signalling NaN, y is.
* The result is y.
* - y = Inf. Implied: x != NaN (is 0, number, or Inf: the NaN
* case was taken care of earlier).
* If x = 0, the result is NaN. Otherwise the result
* is y, with its sign reversed if x is negative.
* - x = 0. Implied: y is 0 or number.
* The result is 0 (with XORed sign as usual).
* - other. Implied: both x and y are numbers.
* The result is x * y (XOR sign, multiply bits, add exponents).
*/
ORDER(x, y);
if (ISNAN(y)) {
y->fp_sign ^= x->fp_sign;
return (y);
}
if (ISINF(y)) {
if (ISZERO(x))
return (fpu_newnan(fe));
y->fp_sign ^= x->fp_sign;
return (y);
}
if (ISZERO(x)) {
x->fp_sign ^= y->fp_sign;
return (x);
}
/*
* Setup. In the code below, the mask `m' will hold the current
* mantissa byte from y. The variable `bit' denotes the bit
* within m. We also define some macros to deal with everything.
*/
x3 = x->fp_mant[3];
x2 = x->fp_mant[2];
x1 = x->fp_mant[1];
x0 = x->fp_mant[0];
sticky = a3 = a2 = a1 = a0 = 0;
#define ADD /* A += X */ \
FPU_ADDS(a3, a3, x3); \
FPU_ADDCS(a2, a2, x2); \
FPU_ADDCS(a1, a1, x1); \
FPU_ADDC(a0, a0, x0)
#define SHR1 /* A >>= 1, with sticky */ \
sticky |= a3 & 1, a3 = (a3 >> 1) | (a2 << 31), \
a2 = (a2 >> 1) | (a1 << 31), a1 = (a1 >> 1) | (a0 << 31), a0 >>= 1
#define SHR32 /* A >>= 32, with sticky */ \
sticky |= a3, a3 = a2, a2 = a1, a1 = a0, a0 = 0
#define STEP /* each 1-bit step of the multiplication */ \
SHR1; if (bit & m) { ADD; }; bit <<= 1
/*
* We are ready to begin. The multiply loop runs once for each
* of the four 32-bit words. Some words, however, are special.
* As noted above, the low order bits of Y are often zero. Even
* if not, the first loop can certainly skip the guard bits.
* The last word of y has its highest 1-bit in position FP_NMANT-1,
* so we stop the loop when we move past that bit.
*/
if ((m = y->fp_mant[3]) == 0) {
/* SHR32; */ /* unneeded since A==0 */
} else {
bit = 1 << FP_NG;
do {
STEP;
} while (bit != 0);
}
if ((m = y->fp_mant[2]) == 0) {
SHR32;
} else {
bit = 1;
do {
STEP;
} while (bit != 0);
}
if ((m = y->fp_mant[1]) == 0) {
SHR32;
} else {
bit = 1;
do {
STEP;
} while (bit != 0);
}
m = y->fp_mant[0]; /* definitely != 0 */
bit = 1;
do {
STEP;
} while (bit <= m);
/*
* Done with mantissa calculation. Get exponent and handle
* 11.111...1 case, then put result in place. We reuse x since
* it already has the right class (FP_NUM).
*/
m = x->fp_exp + y->fp_exp;
if (a0 >= FP_2) {
SHR1;
m++;
}
x->fp_sign ^= y->fp_sign;
x->fp_exp = m;
x->fp_sticky = sticky;
x->fp_mant[3] = a3;
x->fp_mant[2] = a2;
x->fp_mant[1] = a1;
x->fp_mant[0] = a0;
return (x);
}