Safer Code – Secure Coding In C \ C++ And More..

Subscribe To Our Feed | Follow Us On Twitter | Get Updates on Email

One of the ways to optimize your code for speed is to unroll the loops you have in your program. If you don’t know what loop unrolling is, then see the following simple example, where we are trying to copy a 5 element long string:
A Simple Loop:

for (i = 0 ; i < 5; ++i)
{
    *j++ = *k++;
}

The same loop unrolled:

*j++ = *k++;
*j++ = *k++;
*j++ = *k++;
*j++ = *k++;
*j++ = *k++;

Now, the unrolled version is definitely going to be faster because of the following reasons:

1. There are no branches any more. In a loop, the compiler has to insert branches to jump back to the for statement.

2. There are no condition checks. In a loop, the value of i is checked every time to see what to do next.

3. A third rather hidden, but really important, advantage is that unrolled code can be pipelined efficiently by the processor hence resulting in faster execution.

The above code in itself might not show you any perceivable difference in execution time, but it does become crucial when the loops are of much higher magnitude and especially in embedded/real time scenarios.

Now, the loop unrolling can be done by the compiler or manually by the coder. In this part, we will see how to facilitate compiler to carry out loop unrolling efficiently. (more...)

© Safer Code | Tweak Your Code For Speed: Unroll Those Loops Part 1

Liked this post? Get FREE Updates Subscribe to RSS feed Or Enter Your E-mail ID below

Subscribe To Our Feed | Follow Us On Twitter | Get Updates on Email

The biggest bottlenecks in making efficient code today are jumps or branches. You must have always heard of people telling you to use switch-case blocks instead of cascadin if-else’es. They were right, but partially.

This is because a cascading if-else block will check for each condition in each iteration until it reaches the correct one. On the other hand, switch-case blocks are “supposed to” (why supposed to, we’ll see just now) just execute the correct statement and move on.

Optimization Technique 1: Keep the case label values close and small (Answer to “supposed to”)

Actually switch-case statements are also many times changed into if-else cascades by the compilers. This happens when the case label values are too far apart and/or are spread over a fairly large range. If you have a small switch-case statement like below, then the compiler will generate a jump-table instead of the if-else cascade.

switch(a)
{
  case 0: /*do something*/
  case 1: /*do something*/
  case 2: /*do something*/
  case 3: /*do something*/
  default:
}

A jump table is like a “lookup table” or a “dispatch table”. You can assume that compiler creates an internal array of addresses. The array is indexed by the case lable values and the value at that index is the address of instructions corresponding to that case.

Optimization Technique 2: Place those cases first which are more likely to occur

(more…)

© Safer Code | Optimizing Switch-Case Statements In C For Speed

Liked this post? Get FREE Updates Subscribe to RSS feed Or Enter Your E-mail ID below