Tips to make your program more efficient

[asadullah]

How to make your program to be fast.

1. Use Pre Increment or Pre Decrement in place of Post Increment or Post Decrement operator.

i.e. use ++i or --i in place of i++ or i--.

Because ++i operator take increment and save it i.e. One CPU Instruction is required for it. and in case of i++ first save the value of i in register and then Increment, i.e. It takes two instruction set.

So My dear just imagine If you have a loop of 10000 . Then you can save 10000 CPU Instruction to be waste.

PRACTICE: Make Practice to write simple Loop as
Code:

for( i=0; i<n; ++i)

in place of
Code:

for( i=0;i<n; i++)

2. Use either for( i=n; i>=0;--i) or for( i=0;i<=n;++i)

Depends on CPU which are you using. Just check CPU Ticks in both cases which for loop is taking more time. mostly CPU give more efficiently on for( i=n; i>=0;--i)

3. Never use function call inside for loop. Make it redesign to be a for loop inside one function.

Example: In place of

for( i=n;i<=0;--i)
sum = Fun4Add(i);

use
Code:

Fun4Add(int num)
{
sum+=num;
}

Because No Overhead of creating stack frame, saving register etc in every loop and don't declare a variable in a loop and avoid to put any expression in a loop If that expression can be without loop with same output.

4. Don't Use Math library function pow(num1,num2) if num1 is multiple of 2. Because POW() library function is very costly. You can imagine about this arithmetic operation.

If num1==2 then use " Left shift Operator" num1< num2 . It is 100 times fast that Pow() library Function.

5. Always Declare any multi dimensions Array with the power Of 2.

example
int arr[2][1024] in place of int arr[1][1023] etc
int arr[32][128] in place of int arr[30][128] etc.

Because Reason is same as i have given in 4.

Asadullah Ansari
( Trueth can'nt be Changed)

[Narue]

Thanks for the tips. However, I'm going to disagree with them to an extent.

>How to make your program to be fast.
Don't waste your time with micro-optimizations. Instead, focus on algorithms and I/O. That's where the big wins in performance are found.

>1. Use Pre Increment or Pre Decrement in place of Post Increment or Post Decrement operator.
That's generally good advice, and as a guideline it's a good idea if only for consistency across the board. However, keep in mind that this guideline really only applies to user-defined types. Prefix and postfix increment on built-in types will be optimized into the same machine code by your compiler.

>2. Use either for( i=n; i>=0;--i) or for( i=0;i<=n;++i)
Yea, that's helpful. :icon_rolleyes: I bet everyone was using one or the other anyway, so perhaps you should reword this to "Prefer a loop that counts down because it's generally converted to more efficient machine code". Unfortunately, this particular optimization is often not worth the decrease in clarity that you get for all but the simplest of loops. However, I've found that in a lot of cases like the following:

for ( int i = 0; i < n; i++ ) {
  // Do something that doesn't use n or rely on ascending i
}

You can pare it down into this:

while ( --n >= 0 ) {
  // Use n instead of i
}

or this:

while ( n-- > 0 ) {
  // Use n instead of i
}

>3. Never use function call inside for loop.
Not only is this silly, it's also quite impossible to follow. If you're that worried about stack frames, use preprocessor macros or inline functions and suffer the problems that they give you. But don't be surprised when you manage to slow your code down by avoiding function calls.

>Don't Use Math library function pow(num1,num2) if num1 is multiple of 2.
Of course, this only works if both num1 and num2 are integers.

>5. Always Declare any multi dimensions Array with the power Of 2.
It's rarely worth the trouble of doing this unless you needed a power of two size anyway. You're not likely to see any significant speed difference.