Compiler Design- Machine Independent Optimizations

Compiler Design- Machine
Independent Optimizations
Dr R Jegadeesan Prof-CSE
Jyothishmathi Institute of Technology and Science,
Karimnagar
1

SYLLABUS
U N I T 5
2
Machine-Independent Optimizations: The Principal Sources of
Optimization, Introduction to Data-Flow Analysis, Foundations of
Data-Flow Analysis, Constant Propagation, Partial Redundancy
Elimination, Loops in Flow Graphs.

UNIT 5: MACHINE INDEPENDENT
OPTIMIZATION
The principle source of optimization
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Optimization of the code is often performed at the end of the development stage
since it reduces readability and adds code that is used to increase the
performance.
Types of Code Optimization –The optimization process can be broadly classified
into two types :
Machine Independent Optimization – This code optimization phase attempts to
improve the intermediate code to get a better target code as the output. The part
of the intermediate code which is transformed here does not involve any CPU
registers or absolute memory locations.
Topic Name : The principle source of optimization
Aim & Objective : This code optimization phase attempts improve the
intermediate code to get a better target code as the output
Principle & Operation/ Detailed Explanation

OPTIMIZATION
4
Machine Dependent Optimization – Machine-dependent optimization is done
after the target code has been generated and when the code is transformed
according to the target machine architecture. It involves CPU registers and may
have absolute memory references rather than relative references. Machine-
dependent optimizers put efforts to take maximum advantage of the memory
hierarchy
Topic Name : The principle source of optimization
Aim & Objective : This code optimization phase attempts improve the
intermediate code to get a better target code as the output

Code Optimization is done in the following different ways :
Compile Time Evaluation
(i) A = 2*(22.0/7.0)*r
Perform 2*(22.0/7.0)*r at compile time.
(ii) x = 12.4
y = x/2.3
Evaluate x/2.3 as 12.4/2.3 at compile time.
Variable Propagation
//Before Optimization
c = a * b
x = a
till
d = x * b + 4 5
OPTIMIZATION

//After Optimization
c = a * b
x = a
till
d = a * b + 4
Hence, after variable propagation, a*b and x*b will be identified as common sub-expression.
Dead code elimination : Variable propagation often leads to making assignment statement into dead
code
brightness_4
c = a * b
x = a
till
d = a * b + 4
//After elimination :
c = a * b
till
d = a * b + 4
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OPTIMIZATION

Code Motion :
• Reduce the evaluation frequency of expression.
• Bring loop invariant statements out of the loop.
a = 200;
while(a>0)
{
b = x + y;
if (a % b == 0}
printf(“%d”, a);
}
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OPTIMIZATION

//This code can be further optimized as
a = 200;
b = x + y;
while(a>0)
{
if (a % b == 0}
printf(“%d”, a);
}
Induction Variable and Strength Reduction :
• An induction variable is used in loop for the following kind of assignment i = i + constant.
• Strength reduction means replacing the high strength operator by the low strength.
i = 1;
while (i<10)
{
y = i * 4;
}
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OPTIMIZATION

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

UNIT 5: DATA-FLOW ANALYSIS
Data flow analysis in Compiler
It is the analysis of flow of data in control flow graph, i.e., the analysis that determines the
information regarding the definition and use of data in program. With the help of this analysis
optimization can be done. In general, its process in which values are computed using data flow
analysis.
The data flow property represents information which can be used for optimization.
Basic Terminologies –
Definition Point: a point in a program containing some definition.
Reference Point: a point in a program containing a reference to a data item.
Evaluation Point: a point in a program containing evaluation of expression.
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Topic Name : Data flow analysis in Compiler
Aim & Objective : The data flow property represents information which
can be used for optimization.

Basic Terminologies
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Data Flow Properties –
Available Expression – A expression is said to be available at a program point x iff along paths its
reaching to x. A Expression is available at its evaluation point.
A expression a+b is said to be available if none of the operands gets modified before their use.
Example –
Data Flow Properties
12

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Advantage –
It is used to eliminate common sub expressions
•Reaching Definition – A definition D is reaches a point x if there is path from D to x in
which D is not killed, i.e., not redefined.
Example –

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Advantage –
It is used in constant and variable propagation.
Live variable – A variable is said to be live at some point p if from p to end the variable is used before
it is redefined else it becomes dead. Example –
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Advantage –
It is useful for register allocation.
It is used in dead code elimination.
Busy Expression – An expression is busy along a path iff its evaluation exists along that path and none
of its operand definition exists before its evaluation along the path.
Advantage –
It is used for performing code movement optimization.
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Constant Propagation
If a variable is known to contain a particular constant value at a particular
point in the program, replace references to the variable at that point with that
constant value.
After the assignment of one variable to another, a reference to one variable may be
replaced with the value of the other variable (until one or the other of the
variables is reassigned).
Dead Code Elimination
Expressions or statements whose values or effects are unused may be eliminated.
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Topic Name : Constant Propagation
Aim & Objective : machine independent optimization.
Principle & Operation/ Detailed Explanation :
Constant Propagation
Universities & Important Questions:
1. Explain constant propagation?

UNIT 1 : HEADING
Partial Redundancy Elimination
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we consider in detail how to minimize the number of expression evaluations.
That is, we want to consider all possible execution sequences in a flow graph, and look at the
number of times an expression
such as x + y is evaluated. By moving around the places where x + y is evaluated and keeping
the result in a temporary variable when necessary, we often can reduce the number of evaluations
of this expression
along many of the execution paths, while not increasing that number along any path.
Note that the number of different places in the flow graph where x + y is evaluated may increase,
but that is relatively unimportant, as long as the number of evaluations of the expression x + y is
reduced.

Loop Optimization
Loop Optimization
Most programs run as a loop in the system. It becomes necessary to optimize the loops in
order to save CPU cycles and memory. Loops can be optimized by the following
techniques:
Invariant code : A fragment of code that resides in the loop and computes the same
value at each iteration is called a loop-invariant code. This code can be moved out of the
loop by saving it to be computed only once, rather than with each iteration.
Induction analysis : A variable is called an induction variable if its value is altered within
the loop by a loop-invariant value.
Strength reduction : There are expressions that consume more CPU cycles, time, and
memory. These expressions should be replaced with cheaper expressions without
compromising the output of expression. For example, multiplication (x * 2) is expensive
in terms of CPU cycles than (x << 1) and yields the same result.
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Universities & Important Questions:
1. Explain loop optimization?

Compiler Design- Machine Independent Optimizations

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