Compiler Design and Construction - Old Questions

6.  What do you mean by code optimization? Explain the basic blocks and their optimization.

Code optimization is used to improve the intermediate code so that the output of the program could run faster and takes less space. It removes the unnecessary lines of the code and arranges the sequence of statements in order to speed up the program execution without wasting resources. It tries to improve the code by making it consume less resources (i.e. CPU, Memory) and deliver high speed.

A basic block is a sequence of consecutive instructions in which flow of control enters by one entry point and exit to another point without halt or branching except at the end. 

Basic block optimization techniques are:

1. Common-Subexpression Elimination: In the common sub-expression, we don't need to be computed it over and over again. Instead of this we can compute it once and kept in store from where it's referenced when encountered again. For e.g.

2. Dead Code Elimination: The dead code may be a variable or the result of some expression computed by the programmer that may not have any further uses. By eliminating these useless things from a code, the code will get optimized. For e.g.

3. Renaming Temporary Variables: Temporary variables that are dead at the end of a block can be safely renamed. The basic block is transforms into an equivalent block in which each statement that defines a temporary defines a new temporary. Such a basic block is called normal-form block or simple block. For e.g.

4. Interchange of Statements: Independent statements can be reordered without effecting the value of block to make its optimal use. For e.g.

5. Algebraic Transformations: In the algebraic transformation, we can change the set of expression into an algebraically equivalent set. Simplify expression or replace expensive expressions by cheaper ones. For e.g.