Code Optimization | Code Optimization Techniques

Code Optimization-

 

Code Optimization is an approach to enhance the performance of the code.

 

The process of code optimization involves-

  • Eliminating the unwanted code lines
  • Rearranging the statements of the code

 

Advantages-

 

The optimized code has the following advantages-

  • Optimized code has faster execution speed.
  • Optimized code utilizes the memory efficiently.
  • Optimized code gives better performance.

 

Code Optimization Techniques-

 

Important code optimization techniques are-

 

 

  1. Compile Time Evaluation
  2. Common sub-expression elimination
  3. Dead Code Elimination
  4. Code Movement
  5. Strength Reduction

 

1. Compile Time Evaluation-

 

Two techniques that falls under compile time evaluation are-

 

A) Constant Folding-

 

In this technique,

  • As the name suggests, it involves folding the constants.
  • The expressions that contain the operands having constant values at compile time are evaluated.
  • Those expressions are then replaced with their respective results.

 

Example-

 

Circumference of Circle  = (22/7) x Diameter

 

Here,

  • This technique evaluates the expression 22/7 at compile time.
  • The expression is then replaced with its result 3.14.
  • This saves the time at run time.

 

B) Constant Propagation-

 

In this technique,

  • If some variable has been assigned some constant value, then it replaces that variable with its constant value in the further program during compilation.
  • The condition is that the value of variable must not get alter in between.

 

Example-

 

pi = 3.14

radius = 10

Area of circle = pi x radius x radius

 

Here,

  • This technique substitutes the value of variables ‘pi’ and ‘radius’ at compile time.
  • It then evaluates the expression 3.14 x 10 x 10.
  • The expression is then replaced with its result 314.
  • This saves the time at run time.

 

2. Common Sub-Expression Elimination-

 

The expression that has been already computed before and appears again in the code for computation

is called as Common Sub-Expression.

 

In this technique,

  • As the name suggests, it involves eliminating the common sub expressions.
  • The redundant expressions are eliminated to avoid their re-computation.
  • The already computed result is used in the further program when required.

 

Example-

 

Code Before OptimizationCode After Optimization
S1 = 4 x i

S2 = a[S1]

S3 = 4 x j

S4 = 4 x i  // Redundant  Expression

S5 = n

S6 = b[S4] + S5

S1 = 4 x i

S2 = a[S1]

S3 = 4 x j

S5 = n

S6 = b[S1] + S5

 

3. Code Movement-

 

In this technique,

  • As the name suggests, it involves movement of the code.
  • The code present inside the loop is moved out if it does not matter whether it is present inside or outside.
  • Such a code unnecessarily gets execute again and again with each iteration of the loop.
  • This leads to the wastage of time at run time.

 

Example-

 

Code Before OptimizationCode After Optimization
for ( int j = 0 ; j < n ; j ++)

{

x = y + z ;

a[j] = 6 x j;

}

x = y + z ;

for ( int j = 0 ; j < n ; j ++)

{

a[j] = 6 x j;

}

 

4. Dead Code Elimination-

 

In this technique,

  • As the name suggests, it involves eliminating the dead code.
  • The statements of the code which either never executes or are unreachable or their output is never used are eliminated.

 

Example-

 

Code Before OptimizationCode After Optimization
i = 0 ;

if (i == 1)

{

a = x + 5 ;

}

i = 0 ;

 

5. Strength Reduction-

 

In this technique,

  • As the name suggests, it involves reducing the strength of expressions.
  • This technique replaces the expensive and costly operators with the simple and cheaper ones.

 

Example-

 

Code Before OptimizationCode After Optimization
B = A x 2B = A + A

 

Here,

  • The expression “A x 2” is replaced with the expression “A + A”.
  • This is because the cost of multiplication operator is higher than that of addition operator.

 

To gain better understanding about Code Optimization Techniques,

Watch this Video Lecture

 

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Summary
Code Optimization | Code Optimization Techniques
Article Name
Code Optimization | Code Optimization Techniques
Description
In Compiler design, Code Optimization is an approach for enhancing the performance of the code. Various Code optimization techniques are- Compile Time Evaluation, Common Sub expression elimination, Code movement, Dead code elimination, Strength reduction.
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Gate Vidyalay
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