How can I multiply and divide using only bit shifting and adding? - c

Question

How can I multiply and divide using only bit shifting and adding?

Answer 1

To multiply in terms of adding and shifting you want to decompose one of the numbers by powers of two, like so:
21 * 5 = 10101_2 * 101_2 (Initial step)
= 10101_2 * (1 * 2^2 + 0 * 2^1 + 1 * 2^0)
= 10101_2 * 2^2 + 10101_2 * 2^0
= 10101_2 << 2 + 10101_2 << 0 (Decomposed)
= 10101_2 * 4 + 10101_2 * 1
= 10101_2 * 5
= 21 * 5 (Same as initial expression)
(_2 means base 2)
As you can see, multiplication can be decomposed into adding and shifting and back again. This is also why multiplication takes longer than bit shifts or adding - it's O(n^2) rather than O(n) in the number of bits. Real computer systems (as opposed to theoretical computer systems) have a finite number of bits, so multiplication takes a constant multiple of time compared to addition and shifting. If I recall correctly, modern processors, if pipelined properly, can do multiplication just about as fast as addition, by messing with the utilization of the ALUs (arithmetic units) in the processor.

Answer 2

The answer by Andrew Toulouse can be extended to division.
The division by integer constants is considered in details in the book "Hacker's Delight" by Henry S. Warren (ISBN 9780201914658).
The first idea for implementing division is to write the inverse value of the denominator in base two.
E.g.,
1/3 = (base-2) 0.0101 0101 0101 0101 0101 0101 0101 0101 .....
So,
a/3 = (a >> 2) + (a >> 4) + (a >> 6) + ... + (a >> 30)
for 32-bit arithmetics.
By combining the terms in an obvious manner we can reduce the number of operations:
b = (a >> 2) + (a >> 4)
b += (b >> 4)
b += (b >> 8)
b += (b >> 16)
There are more exciting ways to calculate division and remainders.
EDIT1:
If the OP means multiplication and division of arbitrary numbers, not the division by a constant number, then this thread might be of use: https://stackoverflow.com/a/12699549/1182653
EDIT2:
One of the fastest ways to divide by integer constants is to exploit the modular arithmetics and Montgomery reduction: What's the fastest way to divide an integer by 3?

Answer 3

X * 2 = 1 bit shift left
X / 2 = 1 bit shift right
X * 3 = shift left 1 bit and then add X

Answer 4

x << k == x multiplied by 2 to the power of k
x >> k == x divided by 2 to the power of k
You can use these shifts to do any multiplication operation. For example:
x * 14 == x * 16 - x * 2 == (x << 4) - (x << 1)
x * 12 == x * 8 + x * 4 == (x << 3) + (x << 2)
To divide a number by a non-power of two, I'm not aware of any easy way, unless you want to implement some low-level logic, use other binary operations and use some form of iteration.

Answer 5

A left shift by 1 position is analogous to multiplying by 2. A right shift is analogous to dividing by 2.
You can add in a loop to multiply. By picking the loop variable and the addition variable correctly, you can bound performance. Once you've explored that, you should use Peasant Multiplication

Answer 6

A procedure for dividing integers that uses shifts and adds can be derived in straightforward fashion from decimal longhand division as taught in elementary school. The selection of each quotient digit is simplified, as the digit is either 0 and 1: if the current remainder is greater than or equal to the divisor, the least significant bit of the partial quotient is 1.
Just as with decimal longhand division, the digits of the dividend are considered from most significant to least significant, one digit at a time. This is easily accomplished by a left shift in binary division. Also, quotient bits are gathered by left shifting the current quotient bits by one position, then appending the new quotient bit.
In a classical arrangement, these two left shifts are combined into left shifting of one register pair. The upper half holds the current remainder, the lower half initial holds the dividend. As the dividend bits are transferred to the remainder register by left shift, the unused least significant bits of the lower half are used to accumulate the quotient bits.
Below is x86 assembly language and C implementations of this algorithm. This particular variant of a shift & add division is sometimes referred to as the "non-performing" variant, as the subtraction of the divisor from the current remainder is not performed unless the remainder is greater than or equal to the divisor (Otto Spaniol, "Computer Arithmetic: Logic and Design." Chichester: Wiley 1981, p. 144). In C, there is no notion of the carry flag used by the assembly version in the register pair left shift. Instead, it is emulated, based on the observation that the result of an addition modulo 2n can be smaller that either addend only if there was a carry out.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#define USE_ASM 0
#if USE_ASM
uint32_t bitwise_division (uint32_t dividend, uint32_t divisor)
{
uint32_t quot;
__asm {
mov eax, [dividend];// quot = dividend
mov ecx, [divisor]; // divisor
mov edx, 32; // bits_left
mov ebx, 0; // rem
$div_loop:
add eax, eax; // (rem:quot) << 1
adc ebx, ebx; // ...
cmp ebx, ecx; // rem >= divisor ?
jb $quot_bit_is_0; // if (rem < divisor)
$quot_bit_is_1: //
sub ebx, ecx; // rem = rem - divisor
add eax, 1; // quot++
$quot_bit_is_0:
dec edx; // bits_left--
jnz $div_loop; // while (bits_left)
mov [quot], eax; // quot
}
return quot;
}
#else
uint32_t bitwise_division (uint32_t dividend, uint32_t divisor)
{
uint32_t quot, rem, t;
int bits_left = CHAR_BIT * sizeof (uint32_t);
quot = dividend;
rem = 0;
do {
// (rem:quot) << 1
t = quot;
quot = quot + quot;
rem = rem + rem + (quot < t);
if (rem >= divisor) {
rem = rem - divisor;
quot = quot + 1;
}
bits_left--;
} while (bits_left);
return quot;
}
#endif

Answer 7

I translated the Python code to C. The example given had a minor flaw. If the dividend value that took up all the 32 bits, the shift would fail. I just used 64-bit variables internally to work around the problem:
int No_divide(int nDivisor, int nDividend, int *nRemainder)
{
int nQuotient = 0;
int nPos = -1;
unsigned long long ullDivisor = nDivisor;
unsigned long long ullDividend = nDividend;
while (ullDivisor < ullDividend)
{
ullDivisor <<= 1;
nPos ++;
}
ullDivisor >>= 1;
while (nPos > -1)
{
if (ullDividend >= ullDivisor)
{
nQuotient += (1 << nPos);
ullDividend -= ullDivisor;
}
ullDivisor >>= 1;
nPos -= 1;
}
*nRemainder = (int) ullDividend;
return nQuotient;
}

Answer 8

Take two numbers, lets say 9 and 10, write them as binary - 1001 and 1010.
Start with a result, R, of 0.
Take one of the numbers, 1010 in this case, we'll call it A, and shift it right by one bit, if you shift out a one, add the first number, we'll call it B, to R.
Now shift B left by one bit and repeat until all bits have been shifted out of A.
It's easier to see what's going on if you see it written out, this is the example:
0
0000 0
10010 1
000000 0
1001000 1
------
1011010

Answer 9

Taken from here.
This is only for division:
int add(int a, int b) {
int partialSum, carry;
do {
partialSum = a ^ b;
carry = (a & b) << 1;
a = partialSum;
b = carry;
} while (carry != 0);
return partialSum;
}
int subtract(int a, int b) {
return add(a, add(~b, 1));
}
int division(int dividend, int divisor) {
boolean negative = false;
if ((dividend & (1 << 31)) == (1 << 31)) { // Check for signed bit
negative = !negative;
dividend = add(~dividend, 1); // Negation
}
if ((divisor & (1 << 31)) == (1 << 31)) {
negative = !negative;
divisor = add(~divisor, 1); // Negation
}
int quotient = 0;
long r;
for (int i = 30; i >= 0; i = subtract(i, 1)) {
r = (divisor << i);
// Left shift divisor until it's smaller than dividend
if (r < Integer.MAX_VALUE && r >= 0) { // Avoid cases where comparison between long and int doesn't make sense
if (r <= dividend) {
quotient |= (1 << i);
dividend = subtract(dividend, (int) r);
}
}
}
if (negative) {
quotient = add(~quotient, 1);
}
return quotient;
}

Answer 10

This should work for multiplication:
.data
.text
.globl main
main:
# $4 * $5 = $2
addi $4, $0, 0x9
addi $5, $0, 0x6
add $2, $0, $0 # initialize product to zero
Loop:
beq $5, $0, Exit # if multiplier is 0,terminate loop
andi $3, $5, 1 # mask out the 0th bit in multiplier
beq $3, $0, Shift # if the bit is 0, skip add
addu $2, $2, $4 # add (shifted) multiplicand to product
Shift:
sll $4, $4, 1 # shift up the multiplicand 1 bit
srl $5, $5, 1 # shift down the multiplier 1 bit
j Loop # go for next
Exit: #
EXIT:
li $v0,10
syscall

Answer 11

The below method is the implementation of binary divide considering both numbers are positive. If subtraction is a concern we can implement that as well using binary operators.
Code
-(int)binaryDivide:(int)numerator with:(int)denominator
{
if (numerator == 0 || denominator == 1) {
return numerator;
}
if (denominator == 0) {
#ifdef DEBUG
NSAssert(denominator==0, #"denominator should be greater then 0");
#endif
return INFINITY;
}
// if (numerator <0) {
// numerator = abs(numerator);
// }
int maxBitDenom = [self getMaxBit:denominator];
int maxBitNumerator = [self getMaxBit:numerator];
int msbNumber = [self getMSB:maxBitDenom ofNumber:numerator];
int qoutient = 0;
int subResult = 0;
int remainingBits = maxBitNumerator-maxBitDenom;
if (msbNumber >= denominator) {
qoutient |=1;
subResult = msbNumber - denominator;
}
else {
subResult = msbNumber;
}
while (remainingBits > 0) {
int msbBit = (numerator & (1 << (remainingBits-1)))>0?1:0;
subResult = (subResult << 1) | msbBit;
if(subResult >= denominator) {
subResult = subResult - denominator;
qoutient= (qoutient << 1) | 1;
}
else{
qoutient = qoutient << 1;
}
remainingBits--;
}
return qoutient;
}
-(int)getMaxBit:(int)inputNumber
{
int maxBit = 0;
BOOL isMaxBitSet = NO;
for (int i=0; i<sizeof(inputNumber)*8; i++) {
if (inputNumber & (1<<i)) {
maxBit = i;
isMaxBitSet=YES;
}
}
if (isMaxBitSet) {
maxBit+=1;
}
return maxBit;
}
-(int)getMSB:(int)bits ofNumber:(int)number
{
int numbeMaxBit = [self getMaxBit:number];
return number >> (numbeMaxBit - bits);
}
For multiplication:
-(int)multiplyNumber:(int)num1 withNumber:(int)num2
{
int mulResult = 0;
int ithBit;
BOOL isNegativeSign = (num1<0 && num2>0) || (num1>0 && num2<0);
num1 = abs(num1);
num2 = abs(num2);
for (int i=0; i<sizeof(num2)*8; i++)
{
ithBit = num2 & (1<<i);
if (ithBit>0) {
mulResult += (num1 << i);
}
}
if (isNegativeSign) {
mulResult = ((~mulResult)+1);
}
return mulResult;
}

Answer 12

it is basically multiplying and dividing with the base power 2
shift left = x * 2 ^ y
shift right = x / 2 ^ y
shl eax,2 = 2 * 2 ^ 2 = 8
shr eax,3 = 2 / 2 ^ 3 = 1/4

Answer 13

For anyone interested in a 16-bit x86 solution, there is a piece of code by JasonKnight here1 (he also includes a signed multiply piece, which I haven't tested). However, that code has issues with large inputs, where the "add bx,bx" part would overflow.
The fixed version:
softwareMultiply:
; INPUT CX,BX
; OUTPUT DX:AX - 32 bits
; CLOBBERS BX,CX,DI
xor ax,ax ; cheap way to zero a reg
mov dx,ax ; 1 clock faster than xor
mov di,cx
or di,bx ; cheap way to test for zero on both regs
jz #done
mov di,ax ; DI used for reg,reg adc
#loop:
shr cx,1 ; divide by two, bottom bit moved to carry flag
jnc #skipAddToResult
add ax,bx
adc dx,di ; reg,reg is faster than reg,imm16
#skipAddToResult:
add bx,bx ; faster than shift or mul
adc di,di
or cx,cx ; fast zero check
jnz #loop
#done:
ret
Or the same in GCC inline assembly:
asm("mov $0,%%ax\n\t"
"mov $0,%%dx\n\t"
"mov %%cx,%%di\n\t"
"or %%bx,%%di\n\t"
"jz done\n\t"
"mov %%ax,%%di\n\t"
"loop:\n\t"
"shr $1,%%cx\n\t"
"jnc skipAddToResult\n\t"
"add %%bx,%%ax\n\t"
"adc %%di,%%dx\n\t"
"skipAddToResult:\n\t"
"add %%bx,%%bx\n\t"
"adc %%di,%%di\n\t"
"or %%cx,%%cx\n\t"
"jnz loop\n\t"
"done:\n\t"
: "=d" (dx), "=a" (ax)
: "b" (bx), "c" (cx)
: "ecx", "edi"
);

Answer 14

Try this. https://gist.github.com/swguru/5219592
import sys
# implement divide operation without using built-in divide operator
def divAndMod_slow(y,x, debug=0):
r = 0
while y >= x:
r += 1
y -= x
return r,y
# implement divide operation without using built-in divide operator
def divAndMod(y,x, debug=0):
## find the highest position of positive bit of the ratio
pos = -1
while y >= x:
pos += 1
x <<= 1
x >>= 1
if debug: print "y=%d, x=%d, pos=%d" % (y,x,pos)
if pos == -1:
return 0, y
r = 0
while pos >= 0:
if y >= x:
r += (1 << pos)
y -= x
if debug: print "y=%d, x=%d, r=%d, pos=%d" % (y,x,r,pos)
x >>= 1
pos -= 1
return r, y
if __name__ =="__main__":
if len(sys.argv) == 3:
y = int(sys.argv[1])
x = int(sys.argv[2])
else:
y = 313271356
x = 7
print "=== Slow Version ...."
res = divAndMod_slow( y, x)
print "%d = %d * %d + %d" % (y, x, res[0], res[1])
print "=== Fast Version ...."
res = divAndMod( y, x, debug=1)
print "%d = %d * %d + %d" % (y, x, res[0], res[1])