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C++ Performance Tips

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Salem

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Re: C++ Performance Tips

#41

Jun 20th, 2008

> 1. In VS 6.0 (on Intel H/W) (are unsigned better?)
So, you pick one specific processor / compiler combination out of the many 10's of processors (or maybe hundreds of variants, or thousands of compilers + flag variations) and decide that proves it?
Interesting it may be, but it's not in my "useful" things to know when it comes to getting the best out of the code.

The whole point of using a HLL is to stop you from having to worry about the minutia of how specific machines handle specific cases, and thus allow you to concentrate on the bigger issues.

Any (let me say it again for the hard of reading, ANY) attempt at performance optimisation before you've finished the program and done some meaningful profiling is a waste of time. Focus your effort on choosing good data structures and algorithms, which will save hours (or months) of computing time, and let the compiler worry about the microseconds.

Your fast loop won't mean squat if you were dumb enough to put it in a bubble sort for example. There isn't an optimiser out there which can spot a bubble sort and decide to replace it with quicksort. That's YOUR job, so do it.

If you're finding your "optimised" code hard to read, then for sure so will the compilers' optimiser, and it will just chicken out and do exactly what you asked for.

Take those ridiculous attempts at swapping variables (without a temp) for example.

What could possibly be wrong with

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 C++ Syntax (Toggle Plain Text) 
{ int temp = a ; a = b ; b = temp; }
printf( "%d %d\n", a, b );
{ int temp = a ; a = b ; b = temp; }
printf( "%d %d\n", a, b );

Simple, obvious, and more importantly, TRANSPARENT to the optimiser. My recent version of gcc for example optimised this out to (wait for it)
printf( "%d %d\n", b, a ); That's right, the swap is GONE!

An extreme case maybe, but it does illustrate that if the code is clear in it's purpose, then the optimiser may just get rid of it and find another way to get to where you want to be.

With all those swap tricks, the optimiser is left with "Huh?, WTF is that" and you get exactly what you asked for.

Compilers have evolved a lot since the 1980's, so stop recycling all those 1980's tricks. They either don't work, are unnecessary or just make it worse.

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Re: C++ Performance Tips

#42

Sep 17th, 2008

I have picked some ideas here about c++. Thanks gurus.

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Re: C++ Performance Tips

#43

Nov 13th, 2008

Optimizing Software in C++
http://www.agner.org/optimize/optimizing_cpp.pdf

Agner has a few other manuals, on his site too.

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Re: C++ Performance Tips

-3

#44

Dec 29th, 2008

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Originally Posted by bitforce

An easy way to swap 2 variables without using another variable:
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a=a+b;
b=a-b;
a=a-b;
a=a+b; b=a-b; a=a-b;

here's a one-statement version:

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a = a + b - (b = a);

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Salem

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Re: C++ Performance Tips

#45

Dec 29th, 2008

> here's a one-statement version:
Which is just as broken as all the other 1-statement attempts, which result in a variable (in this case b) being assigned before (or maybe after, who knows) the rest of the expression.
Read the thread, learn about sequence points.

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Re: C++ Performance Tips

#46

Jan 18th, 2009

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Originally Posted by Narue

>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.
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C++ 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'; }