RSS

C++ Performance Tips

Please support our C++ advertiser: Programming Forums

Thread Tools Display Modes

Views: 42532 | Replies: 42

Page 4 of 5

« First

•

Join Date: Apr 2007

Posts: 6

Reputation: kmillen is an unknown quantity at this point

Rep Power: 0

Solved Threads: 0

kmillen

Offline

Newbie Poster

Re: C++ Performance Tips

#31

Jul 12th, 2007

Using an example will help.
For instance let a = 6 and b = 13.

a = a + b
Now a = 19 and b is unchanged at 13.

b - a - b or b = 19 - 13
Now a = 19 (unchanged) and b = 6.

a = a - b or a = 19 - 6
Now a = 13 and b = 6. The values are swapped.

For pure functionality, I don't recommend this method because it only works with summable values. Swapping using a temporary variable works in all cases.

•

Join Date: Sep 2004

Posts: 6,585

Reputation: Narue has much to be proud of

Rep Power: 31

Solved Threads: 501

Narue

Offline

Expert Meanie

Re: C++ Performance Tips

#32

Jul 12th, 2007

>I don't recommend this method because it only works with summable values.
Not to mention that you also risk integer overflow and undefined behavior.

I'm here to prove you wrong.

•

Join Date: Apr 2004

Location: Tracy

Posts: 744

Reputation: Killer_Typo will become famous soon enough

Rep Power: 7

Solved Threads: 32

Killer_Typo Killer_Typo is offline

Offline

Master Poster

Re: C++ Performance Tips

#33

Sep 6th, 2007

•

Originally Posted by yumvpvip

I don't think so. the result will be the fllowigs:
a=b;
b=0;
I am a beginner please give me some advice

actually it does work

assume a = 3 and b = 7

a = a + b

a = 10
b = 7

b = a - b
a = 10
b = 3

a = a - b
a = 7
b = 3

however from the responses this doesnt seem to be a very safe way of handling things.

!!!!! WARNING YOUR COMPUTER MAY BE INFECTED WITH SPYWARE!!!! PAY AN OVER PRICED AMMOUNT TO HAVE SOMTHING FIXED WE PLACED THERE IN THE FIRST PLACE!!!!!!!!!

sound familiar, know how to block yourself and keep yourself clean.
_____________________
http://www.lavasoftusa.com/ -->adaware
http://www.safer-networking.org/en/index.html -->spybot S&D
http://www.javacoolsoftware.com/spywareblaster.html -->spywareblaster
http://www.javacoolsoftware.com/spywareguard.html -->spywareguard
_____________________
and dont forget to spread the reputation to those that deserve!

•

Join Date: Dec 2007

Location: Chennai

Posts: 22

Reputation: Rajith Cherian is an unknown quantity at this point

Rep Power: 1

Solved Threads: 4

Rajith Cherian Rajith Cherian is offline

Offline

Newbie Poster

Re: C++ Performance Tips

#34

Jan 8th, 2008

The most efficient way for swapping whole numbers is

int a = 10;
int b = 15;

a ^= b;
b ^= a;
a ^= b;

y because xor is an assembly level instruction and its much faster than any move operation.

•

Join Date: Sep 2004

Posts: 6,585

Reputation: Narue has much to be proud of

Rep Power: 31

Solved Threads: 501

Narue

Offline

Expert Meanie

Re: C++ Performance Tips

#35

Jan 8th, 2008

>The most efficient way for swapping whole numbers is <snip xor swap>
>because xor is an assembly level instruction and its much faster than any move operation.
An empirical test disproves your claim. The swap using a temporary is noticeably faster on at least one compiler (Microsoft C++) given any optimization level.

 Help with Code Tags 
 cplusplus Syntax (Toggle Plain Text) 
#include <climits>
#include <ctime>
#include <iostream>
 
void swap_temp ( int& a, int& b )
{
  int temp = a;
  a = b;
  b = temp;
}
 
void swap_xor ( int& a, int& b )
{
  a ^= b;
  b ^= a;
  a ^= b;
}
 
int main()
{
  int a1 = 10, a2 = 20;
  int b1 = 10, b2 = 20;
  std::clock_t start;
 
  std::cout<< a1 <<"\t"<< a2 <<'\n';
  std::cout<< b1 <<"\t"<< b2 <<'\n';
 
  swap_temp ( a1, a2 );
  swap_temp ( b1, b2 );
 
  std::cout<< a1 <<"\t"<< a2 <<'\n';
  std::cout<< b1 <<"\t"<< b2 <<'\n';
 
  std::cout<<"swap_temp: ";
  start = std::clock();
  for ( unsigned i = 0; i < UINT_MAX; i++ )
    swap_temp ( a1, a2 );
  std::cout<< static_cast<double> ( std::clock() ) - start / CLOCKS_PER_SEC <<'\n';
 
  std::cout<<"swap_xor: ";
  start = std::clock();
  for ( unsigned i = 0; i < UINT_MAX; i++ )
    swap_temp ( b1, b2 );
  std::cout<< static_cast<double> ( std::clock() ) - start / CLOCKS_PER_SEC <<'\n';
}
#include <climits>
#include <ctime>
#include <iostream>

void swap_temp ( int& a, int& b )
{
  int temp = a;
  a = b;
  b = temp;
}

void swap_xor ( int& a, int& b )
{
  a ^= b;
  b ^= a;
  a ^= b;
}

int main()
{
  int a1 = 10, a2 = 20;
  int b1 = 10, b2 = 20;
  std::clock_t start;

  std::cout<< a1 <<"\t"<< a2 <<'\n';
  std::cout<< b1 <<"\t"<< b2 <<'\n';

  swap_temp ( a1, a2 );
  swap_temp ( b1, b2 );

  std::cout<< a1 <<"\t"<< a2 <<'\n';
  std::cout<< b1 <<"\t"<< b2 <<'\n';

  std::cout<<"swap_temp: ";
  start = std::clock();
  for ( unsigned i = 0; i < UINT_MAX; i++ )
    swap_temp ( a1, a2 );
  std::cout<< static_cast<double> ( std::clock() ) - start / CLOCKS_PER_SEC <<'\n';

  std::cout<<"swap_xor: ";
  start = std::clock();
  for ( unsigned i = 0; i < UINT_MAX; i++ )
    swap_temp ( b1, b2 );
  std::cout<< static_cast<double> ( std::clock() ) - start / CLOCKS_PER_SEC <<'\n';
}

I'm here to prove you wrong.

•

Join Date: Feb 2008

Location: Singapore

Posts: 16

Reputation: sohguanh is an unknown quantity at this point

Rep Power: 1

Solved Threads: 2

sohguanh

Offline

Newbie Poster

Re: C++ Performance Tips

#36

Feb 28th, 2008

This sticky topic is a very interesting read but I think most of the articles are dwelling very low level C++ stuff which is not all wrong but in the current modern age and days and unless you are into compiler or device driver or OS kernel level programming, such esoteric constructs do not really apply to business application level programming.

Why was STL declared as part of the C++ standard? Simply becuz enough wasted years are spent on re-inventing code like vector, list, map, common algorithms starting from scratch. By mandating STL as part of C++ standard, it make us C++ programmer very fast up and running and devote more time to focus on our own company business domain logic!!!!

With ACE, I am now looking to deploy a portable multi-threaded server program in weeks instead of months or even years previously. I hope soon ACE will be part of C++ standard too. Java success is becuz Java Collections, Multi-thread, Logging, Socket, Generics, ImageIO,NIO etc etc are PART OF THE SDK. It make Java programmers more efficient in producing application level code as compared to traditional C++.

Back to performance tip. My working experience with a mission critical system using BEA Tuxedo is software design and IO are the two main culprits of low performance. A properly designed and efficient choice of data structures and algorithms of the module will yield magnitudes more performance than tweaking the C++ loop constructs, bitwise operations etc etc etc.

IO is the other killer. If possible structure your program such that just the correct number of times we need to do IO is enough. Additional IO accesses should be forbidden (if possible!!!). Despite commercial database boast of their performance, my working experience is doing the correct data structures and algorithms in programs will already give me the much higher performance I need. The database performance will be the icing on the cake though.

Lastly, I hope Java Hibernate and Swing concept can be part of C++ standard too. It is time C++ standard catch up with Java in the database access area and GUI. C++ still suffer from the syndrome of being too low level and not abstracted high enough for fast application level programming and uses.

•

Join Date: Sep 2004

Posts: 6,585

Reputation: Narue has much to be proud of

Rep Power: 31

Solved Threads: 501

Narue

Offline

Expert Meanie

Re: C++ Performance Tips

#37

Feb 28th, 2008

>C++ still suffer from the syndrome of being too low level and not
>abstracted high enough for fast application level programming and uses.
You're missing the point, I think. C++ and Java aren't competing for the same market. Think of it this way: Java is good for high level applications. C++ is good for the frameworks (OS, JVM, compilers, etc...) that Java needs to exist. Don't forget that even though it can be used for applications, C++ is and always has been a systems programming language.

I'm here to prove you wrong.

•

Join Date: Mar 2008

Location: India

Posts: 5

Reputation: patelmiteshb is an unknown quantity at this point

Rep Power: 0

Solved Threads: 0

patelmiteshb patelmiteshb is offline

Offline

Newbie Poster

Re: C++ Performance Tips

#38

Apr 28th, 2008

Great Job Guys !! Thanks.

Offshore Outsourcing
SEO Services

•

Join Date: May 2008

Posts: 1

Reputation: AyaToujo is an unknown quantity at this point

Rep Power: 0

Solved Threads: 0

AyaToujo

Offline

Newbie Poster

Re: C++ Performance Tips

#39

May 5th, 2008

Wow! I learned a lot of things from this thread! ... Thanks for the info everyone!!

malibuhorizon

•

Join Date: Jun 2008

Posts: 3

Reputation: Cait is an unknown quantity at this point

Rep Power: 0

Solved Threads: 1

Cait

Offline

Newbie Poster

Re: C++ Performance Tips

#40

Jun 20th, 2008

Hello,
Thanks to share the tips of c++,you have taken these tips from the book Efficient c++,I think, Thinking in c++ is also a good book for c++,To Swap two variables, you use simple logic:
a=a+b;
b=a-b;
a=a-b;

Last edited by Cait : Jun 20th, 2008 at 3:39 am.

Website development
Free templates