Compiler Design and Construction - Old Questions

5.  What are the different issues in the design of code generator? Explain with example about the optimization of basic blocks.

The issues to code generator design includes:

1. Input to the code generator: The input to the code generator is intermediate representation together with the information in the symbol table. Intermediate representation has the several choices: 

• Postfix notation, 
• Syntax tree or DAG,
• Three address code

The code generation phase needs complete error-free intermediate code as an input requires.

2. Target Program: The target program is the output of the code generator. The output can be:

• Assembly language: It allows subprogram to be separately compiled.
• Relocatable machine language: It makes the process of code generation easier.
• Absolute machine language: It can be placed in a fixed location in memory and can be executed immediately.

3. Target Machine: Implementing code generation requires thorough understanding of the target machine architecture and its instruction set. 

4. Instruction Selection: Efficient and low cost instruction selection is important to obtain efficient code. When we consider the efficiency of target machine then the instruction speed and machine idioms are important factors. The quality of the generated code can be determined by its speed and size.

5. Register Allocation: Proper utilization of registers improve code efficiency. Use of registers make the computations faster in comparison to that of memory, so efficient utilization of registers is important. The use of registers are subdivided into two subproblems:

• During Register allocation, we select only those set of variables that will reside in the registers at each point in the program.
• During a subsequent Register assignment phase, the specific register is picked to access the variable.

6. Choice of Evaluation order: The efficiency of the target code can be affected by the order in which the computations are performed. 

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.