Code Optimization Technique
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Common Subexpressions Elimination
- Compile-time evaluation means shifting of computations from runtime to compile time.computations
- There are two methods used to obtain the compile time evaluation.
- In the folding technique, the computation of constant done at compile time instead of runtime.
Example : length = (22/7)*d
- Here folding is implied by performing the computation of 22/7 at compile time.
- In this technique, the value of the variable replaced and computation of expression is expression done at compilation time.
pi = 3.14; r = 5;
Area = pi * r * rr;
- Here at the compilation time, the value of pi is replaced by 3.14 and r by five then the computation of 3.14 * 5 * 5 is done during compilation.
Common Subexpressions Elimination: Code Optimization Technique
- The common subexpression is an expression repeatedly appearing in the program which computed previously.
- If the operands of this subexpression do not get changed at all, then the result of such subexpression used instead of rere-computing it each time.
t1 := 4 * it1=4*i
t2 := a[t1]t2=a[t1]
t3 := 4 * jt3=4*j
t4 : = 4 * it5=n
t6 := b[t4]+t5
- The common sub expression t4:=t4:=4*i is eliminated as its computation is already in t1 and value of i is not been changed from definition to use.
Variable Propagation: Code Optimization Technique
- Variable propagation means a use of one variable instead of another.
x = pi;
area = x * r * r;
- The optimisation using variable propagation can do as follows, area = pi * r * r;
- Here the variable x eliminated. Here the necessary condition that a variable must assign to another variable or some constant.
Code Movement: Code Optimization Technique
There are two basic goals of code movement:
I. To reduce the size of the code.
II.To reduce the frequency of execution of code.
}k=(temp) + 50;
Loop Invariant Computation
- Loop invariant optimization can obtain by moving some amount of code outside the loop and placing it just before entering the loop.
- This method also called code motion.
Strength Reduction: Code Optimization Technique
- The strength of certain operators is higher than others.
- For instance strength of * is higher than +.
- In this technique, the higher strength operators can replace by lower strength operators.
count = i*7;
- Here we get the count values as 7, 14, 21 and so on up to less than 50.
- This code can replaced by using strength reduction as follows
count = temp;
temp = temp+7;
Dead Code Elimination (Code Optimization Technique)
- A variable said to be live in a program if the value contained into is subsequently.
- On the other hand, the variable said to be dead at a point in a program if the value contained in it never used. The code comprising such a variable supposed to be a dead code. And an optimization can perform by eliminating such a dead code.
- If statement dead code as this condition will never get satisfied hence, statement can eliminat and ooptimization can be done.