How to replace /(division) operator

[spl]

To increase the performance, how to change the / operator with bitwise
operators?
for ex: 25/5, 225/25 or 25/3 or any division, but I am not bothered
of any remainder.

 

[jacob navia]

To increase the performance, how to change the / operator with bitwise
operators?
for ex: 25/5, 225/25 or 25/3 or any division, but I am not bothered
of any remainder.

This is not meaningful if you do not say which CPU
are you using. Division is not that expensive anymore,
and the extra code for implementing division with bitwise
operators could very well be MUCH slower.

The lcc-win compiler will replace certain kinds of division ( divisions
by an integer constant) with 3-4 instructions with shifts and adds. This
is only possible when the divisor is known at compile time.

 

[spl]

I use normal Microsoft visual C++ compiler only. Due to lots of more
frequency in accessing / operator, I feel to change it with bitwise
operator, as it is always faster then / operator. So, If you know
bitwise manipulation, please suggest me!!

 

[Chris Dollin]

I use normal Microsoft visual C++ compiler only. Due to lots of more
frequency in accessing / operator, I feel to change it with bitwise
operator, as it is always faster then / operator.

Does your application have an actual, measured, performance problem
that you've tracked down to your use of `/`?

Sure, individual bitwise operations are often faster than individual
`/` operations. That's because they do simpler -- different -- things.
To replace your `/`s with bitwise ops, you'll have to do /multiple/,
/dependent/ bitwise ops, so it's not longer obvious that this is
faster.

 

[spl]

Does your application have an actual, measured, performance problem
that you've tracked down to your use of `/`?

YES. I have.

 

[Bartc]

I use normal Microsoft visual C++ compiler only. Due to lots of more
frequency in accessing / operator, I feel to change it with bitwise
operator, as it is always faster then / operator. So, If you know
bitwise manipulation, please suggest me!!

Do you know what sort of numbers are being operated on?

Are the numbers you divide by, constants? If not things are more difficult.

Dividing by a power of 2 can sometimes be replaced by a shift, if you know
the number. (If not, but is a power of 2, you can keep track of the shift
count needed.) However, if it's something obvious, the compiler will already
have done it!

How much runtime is given over to division anyway? On what processor (as
they are all different)?

Can you post some test code that demonstrates the problem you have?

Sometimes you can rearrange the code to reduce or eliminate division.

 

[Chris Dollin]

YES. I have.

Then you can tell us the numbers and the context and the code
that is slow, and get advice suited to your actual problem
rather than speculation into the vacuum.

(And note that you'll get /C/ advice; if you have a C++ program,
it's possible that there will be better C++-oriented answers
in A Different Group.)

--
"It was the first really clever thing the King had /Alice in Wonderland/
said that day."

Hewlett-Packard Limited registered office: Cain Road, Bracknell,
registered no: 690597 England Berks RG12 1HN

 

[Martin Ambuhl]

I use normal Microsoft visual C++ compiler only. Due to lots of more
frequency in accessing / operator, I feel to change it with bitwise
operator, as it is always faster then / operator. So, If you know
bitwise manipulation, please suggest me!!

You are probably confused about what language you are using. The C
programing language (the one discussed in the newsgroup you posted to,
<does not allow you to redefine the meaning of
operators like '/'.

There are other puzzling things about your post. If you actually knew
the "bitwise operator, as it is always faster than / operator", then you
would already have the implementation you want. In that case, you would
have no need of anyone to "please suggest me!!". And if your claim were
in any sense true, why would you think that the Microsoft compiler
developers did not already use this supposed "always faster" approach
for their implementation of the '/' operator?

I fear that you are very confused, or erroneously parroting someone
else, or simply a troll.

 

[spl]

Sorry for the delay in getting back to your questions, Actually
changing the division operator to bitwise operators is suggested by my
tech lead. As she done so many such improvement by doing this and she
is having enough evidence for the same. She suggested me to do the
same in my current project too. Actually I want to divide some big
number with constant number, say 1024 always. Please give me your
suggestion please.

 

[Bartc]

Sorry for the delay in getting back to your questions, Actually
changing the division operator to bitwise operators is suggested by my
tech lead. As she done so many such improvement by doing this and she
is having enough evidence for the same. She suggested me to do the
same in my current project too. Actually I want to divide some big
number with constant number, say 1024 always. Please give me your
suggestion please.

My suggestion is to just divide by 1024.

Your compiler will use the most appropriate coding, you probably don't even
have to tell it to optimise.

Only if your compiler is so basic that you might try using (A>>10) instead
of A/1024, if A is an appropriate type like int, followed by a comment as to
why you are doing this.

 

[spl]

Thanks Bartc & Eric. Your suggestions are very useful. By the way all
my variables are unsigned int only. So right shift gives me exact
value.

 

[thomas.mertes]

Pay close attention to that "appropriate type" part, and
view "like int" with caution. The problem is that the result
of right-shifting a negative number is implementation-defined,
and is usually not the same as the result of dividing by two.
For example, on the two's complement machines that are all
but universal nowadays the representation of -1 is a string
of 1-bits. Shift the string one place to the right and you
may get another string of 1-bits ("arithmetic shift") or a
single 0-bit followed by 1-bits ("logical shift"). The first
thus gives -1 again, while the second gives a large positive
result -- but neither gives the correct result -1 / 2 == 0.

So: "appropriate type" means either an *unsigned* integer
or a signed integer that you happen to know is non-negative.

Agreed.
There can be even more problems with negative numbers.
IMHO the definition of the division in C89 allows also
-1 / 2 == -1. Although I did not find a C compiler which
does this, it is theoretically possible since in C89 the
division is defined as follows:

The binary / operator yields the quotient, and the % operator
the remainder, of the first operand by the second; if the
second operand is 0, the result is undefined. Otherwise, it
is always true that (a/b)*b + a%b is equal to a. If both
operands are non-negative, the remainder is non-negative and
smaller than the divisor; if not it is guaranteed only that
the absolute value of the remainder is smaller than the
absolute value of the divisor.

As said before this definition allows that the integer
division can be rounded towards minus infinite.
Note that when -1 / 2 == -1 at the same time -1 % 2 == 1

IMHO this definition was chosen to allow integer operations
with one machine instruction.

Greetings Thomas Mertes

Seed7 Homepage: http://seed7.sourceforge.net
Seed7 - The extensible programming language: User defined statements
and operators, abstract data types, templates without special
syntax, OO with interfaces and multiple dispatch, statically typed,
interpreted or compiled, portable, runs under linux/unix/windows.

 

[christian.bau]

Sorry for the delay in getting back to your questions, Actually
changing the division operator to bitwise operators is suggested by my
tech lead. As she done so many such improvement by doing this and she
is having enough evidence for the same. She suggested me to do the
same in my current project too. Actually I want to divide some big
number with constant number, say 1024 always. Please give me your
suggestion please.

Write two functions

unsigned int f1 (unsigned int x) { return x / 1024; }
unsigned int f2 (unsigned int x) { return x >> 10; }

Find a way to make the compiler show the assembler code that it
generates and compare. If you get different code, tell us which
compiler you are using so we can all avoid it. I can't actually
remember any C compiler that wouldn't produce optimal code for the
division, and that is going back more than 20 years.

 

[christian.bau]

Write two functions

  unsigned int f1 (unsigned int x) { return x / 1024; }
  unsigned int f2 (unsigned int x) { return x >> 10; }

Find a way to make the compiler show the assembler code that it
generates and compare. If you get different code, tell us which
compiler you are using so we can all avoid it. I can't actually
remember any C compiler that wouldn't produce optimal code for the
division, and that is going back more than 20 years.

Oh, I just noticed you mentioned using a reasonably modern Microsoft
compiler.

In that case, you should also check what code is produced for

unsigned int f3 (unsigned int x) { return x / 1000; }

and ask your lead to explain the code that is generated. Have fun.

 

[Tomás Ó hÉilidhe]

I use normal Microsoft visual C++ compiler only. Due to lots of more
frequency in accessing / operator, I feel to change it with bitwise
operator, as it is always faster then / operator. So, If you know
bitwise manipulation, please suggest me!!

If you can do it faster on your own, I'll take out a loan and buy you
a Porsche.

As others have said, a bitwise instruction might take less CPU cycles
on your platform than a division instruction, but I can guarantee that
the amount of bitwise instruction you use will result in the division
instruction being faster.

In fact, it it were possible to get better performance using your own
bitwise instructions, then I think a lot of people would be ringing up
the CPU manufacturer to ask why the division instruction isn't done
efficiently.