Machine-Independent Optimization

• Machine independent optimization attempts to improve the intermediate code to get a better target code. The part of the code which is transformed here does not involve any absolute memory location or any CPU registers.
• The process of intermediate code generation introduces much inefficiency like: using variable instead of constants, extra copies of variable, repeated evaluation of expression. Through the code optimization, you can remove such efficiencies and improves code.
• It can change the structure of program sometimes of beyond recognition like: unrolls loops, inline functions, eliminates some variables that are programmer defined.

Code Optimization can perform in the following different ways:

(1) Compile Time Evaluation:

(a) z = 5*(45.0/5.0)*r
     Perform 5*(45.0/5.0)*r at compile time.

(b) x = 5.7
     y = x/3.6
    Evaluate x/3.6 as 5.7/3.6 at compile time.

(2) Variable Propagation:

Before Optimization the code is:

c = a * b                                              
x = a                                                 
till                                                          
d = x * b + 4 

After Optimization the code is:

c = a * b  
x = a
till
	d = a * b + 4

Here, after variable propagation a*b and x*b identified as common sub expression.

(3) Dead code elimination:

Before elimination the code is:

c = a * b                                                
x = b                                               
till                                                        
d = a * b + 4  

After elimination the code is:

c = a * b
till
d = a * b + 4

Here, x= b is a dead state because it will never subsequently used in the program. So, we can eliminate this state.

(4) Code Motion:

• It reduces the evaluation frequency of expression.
• It brings loop invariant statements out of the loop.

do
{
   item = 10;
   value = value + item; 
} while(value<100);
 
 
//This code can be further optimized as

item = 10;
do
{
   value = value + item; 
} while(value<100);

(5) Induction Variable and Strength Reduction:

• Strength reduction is used to replace the high strength operator by the low strength.
• An induction variable is used in loop for the following kind of assignment like i = i + constant.

Before reduction the code is:

i = 1;                                                  
while(i<10)                                              
{                                                      
    y = i * 4; 
}

After Reduction the code is:

i = 1
t = 4
{ 
   while( t<40) 
  y = t; 
  t = t + 4;
}