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Chapter: Principles of Compiler Design : Code optimization

Important Short Questions and Answers: Code optimization

Principles of Compiler Design - Code optimization Important Short Questions and Answers: Code optimization


1. How the quality of object program is measured?


The quality of an object program is measured by its Size or its running time. For large computation running time is particularly important. For small computations size may be as important or even more.


2. What is the more accurate term for code optimization?

The more accurate term for code optimization would be “code improvement”


3. Explain the principle sources of optimization.


Code optimization techniques are generally applied after syntax analysis, usually both before and during code generation. The techniques consist of detecting patterns in the program and replacing these patterns by equivalent and more efficient constructs.


4. What are the patterns used for code optimization?


The patterns may be local or global and replacement strategy may be a machine dependent or independent


5.     What are the 3 areas of code optimization?

·     Local optimization

·     Loop optimization


·     Data flow analysis

6.     Define local optimization.

The optimization performed within a block of code is called a local optimization.


7. Define constant folding.


Deducing at compile time that the value of an expression is a constant and using the constant instead is known as constant folding.


8. What do you mean by inner loops?


The most heavily traveled parts of a program, the inner loops, are an obvious target for optimization. Typical loop optimizations are the removal of loop invariant computations and the elimination of induction variables.

9. What is code motion?

Code motion is an important modification that decreases the amount of code in a




10.            What are the properties of optimizing compilers?

·     Transformation must preserve the meaning of programs.


·     Transformation must, on the average, speed up the programs by a measurable amount


·     A Transformation must be worth the effort.


11.            Give the block diagram of organization of code optimizer.

12.            What are the advantages of the organization of code optimizer?


a.     The operations needed to implement high level constructs are made explicit in the intermediate code, so it is possible to optimize them.


b.     The intermediate code can be independent of the target machine, so the optimizer does not have to change much if the code generator is replaced by one for a different machine


13.            Define Local transformation & Global Transformation.


A transformation of a program is called Local, if it can be performed by looking only at the statements in a basic block otherwise it is called global.

 Give examples for function preserving transformations.

·     Common subexpression elimination

·     Copy propagation

·     Dead – code elimination


·     Constant folding


15.            What is meant by Common Subexpressions?


An occurrence of an expression E is called a common subexpression, if E was previously computed, and the values of variables in E have not changed since the previous computation.

16. What is meant by Dead Code?


A variable is live at a point in a program if its value can be used subsequently otherwise, it is dead at that point. The statement that computes values that never get used is known Dead code or useless code.


17.            What are the techniques used for loop optimization?

i)                   Code motion

ii)                Induction variable elimination

iii)              Reduction in strength


18.            What is meant by Reduction in strength?


Reduction in strength is the one which replaces an expensive operation by a cheaper one such as a multiplication by an addition.


19. What is meant by loop invariant computation?


An expression that yields the same result independent of the number of times the loop is executed is known as loop invariant computation.


20. Define data flow equations.

A typical equation has the form


Out[S] = gen[S] U (In[S] – kill[S])


and can be read as, “ the information at the end of a statement is either generated within


the statement, or enters at the beginning and is not killed as control flows through the statement”. Such equations are called data flow equations.


29.            What are the two standard storage allocation strategies?

The two standard allocation strategies are


1.     Static allocation.

2.     Stack allocation


30.            Discuss about static allocation.

In static allocation the position of an activation record in memory is fixed at run time.


31.            Write short notes on activation tree. Nov/Dec 2007

·     A tree which depicts the way of control enters and leaves activations.

·     In an activation tree

i.     Each node represents an activation of an procedure

ii.     The root represents the activation of the main program.


iii.     The node for a is the parent of the node for b , if and only if control flows from activation a to b


iv.    Node for a is to the left of the node for b, if and only if the lifetime of a occurs before the lifetime of b.


32.            Define control stack.

A stack which is used to keep track of live procedure actions is known as control



33. Define heap.

A separate area of run-time memory which holds all other information is called a




34. Give the structure of general activation record


Returned value




Actual parameters




Optional control link




Optional access link




Saved machine status




Local data








35. Discuss about stack allocation.


In stack allocation a new activation record is pushed on to the stack for each execution of a procedure. The record is popped when the activation ends.


36.            What are the 2 approaches to implement dynamic scope?

·     Deep access

·     Shallow access


37.            What is padding?

Space left unused due to alignment consideration is referred to as padding.


38.            What are the 3 areas used by storage allocation strategies?

·     Static allocation

Stack allocation


·     Heap allocation


39.            What are the limitations of using static allocation?


·     The size of a data object and constraints on its position in memory must be known at compile time.


·     Recursive procedure are restricted, because all activations of a procedure use the same bindings for local name


·     Data structures cannot be created dynamically since there is no mechanism for storage allocation at run time


40.            Define calling sequence and return sequence.


·     A call sequence allocates an activation record and enters information into its fields


·     A return sequence restores the state of the machine so that calling procedure can continue execution.



41.            When dangling reference occurs?

·     A dangling reference occurs when there is storage that has been deallocated.


·     It is logical error to use dangling references, since the value of deallocated storage is undefined according to the semantics of most languages.


42.            Define static scope rule and dynamic rule


·     Lexical or static scope rule determines the declaration that applies to a name by a examining the program text alone.


·     Dynamic scope rule determines the declaration applicable to name at runtime, by considering the current activations.


43.            What is block? Give its syntax.

·     A block is a statement containing its own data declaration.

·     Syntax:



Declaration statements




44.            What is access link?


·     An access link is a pointer to each activation record which obtains a direct implementation of lexical scope for nested procedure.


45.            What is known as environment and state?

·     The term environment refers to a function that maps a name to a storage location.


·     The term state refers to a function that maps a storage location to the value held there.


46.            How the run-time memory is sub-divided?

·     Generated target code

·     Data objects


·     A counterpart of the control stack to keep track of procedure activations



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