The document discusses issues in the design of code generators within compiler design, focusing on the conversion of intermediate representation to assembly output. Key topics include register allocation, instruction selection, evaluation order, and memory management, emphasizing the importance of code quality and error-free input. The document outlines the roles of the symbol table and the various forms of target programs, such as absolute and relocatable machine languages.

1. Bangladesh University
Department of Computer Science & Engineering
Subject: Compiler Design
Course code: CSE-3201
Issues in The Design of Code Generator
Presented By:
 Md. Sabbir Hossen
Id: 202011056022
 Md. Saiduzzaman Sojib Id:
202011056021
 Taslima Patwary Id:
202011056028
Submitted To:
 Md Nazim Uddin
Lecturer,
Department of CSE
Bangladesh University
Tuesday, March 14, 2023
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2. Code Generator
 Code generator converts the intermediate representation of the source
program and produces as output an equivalent program.
 Code generator received optimized code from code optimizer and convert
into a expected assembly code.
 Its not necessary that code generator always will accept optimized code, it
also can accept 3 address codes from intermediate code generator.
 The final phase in compiler model is the code generator.
 Designing of the code generator should be done in such a way so that it
can be easily implemented, tested and maintained.
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3. Lets have a look at the position of a Code generator
During this operation some issues arise which will be discussed further.
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Intermediate Code
Generator
Code Optimizer Target Code
Generator
Symbol table
3 address code
Intermediate
representation
Target Program
Figure 1: Position of Code Generator

4. The following issues arise during code generation
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Code
Generator
Register
Allocation
Issue
Target Program
Input to code
generator
Instruction
Selection Issue
Evaluation
Order
Memory
Management

5. Input to Code Generator
 Input to code generator is nothing but you will get the intermediate code.
 Intermediate code will be look like linear representation like:
* Postfix Expression
* Three Address Code
* DAG (Directed Acyclic Graph)
* Prior to Code Generation
 Make sure the input is free of errors by performing the type checking.
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6. Target Program
 Target program is nothing but output. The output may be in many forms like:
* Absolute Machine Language
* Relocatable Machine Language
* Assembly Language
 Absolute machine language as an output has advantage that is can be placed in a fixed
memory location and can be immediately executed.
 Relocatable object files which are used linkers.
 Assembly language facilities the debuggers. We can generate symbolic instructions and use
macro facilities of assembler is generating code.
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Compiler
Source Program Target Program

7. Memory Management
 The mapping names in the square program to address of data object is done by the front end
and back end also.
 So, here everything the information stored in the symbol table. Symbol table here consisting of
relative address. Whenever the backend it calls just take. The information and place into symbol
table. So, with the help of the relative address it find out variable in the symbol table and
execute the operation.
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Source Program
Back End
Mapping
Symbol Table
Front End

8. Instruction Selection Issue
 The code generator takes intermediate representation as input and convert into target machine
instruction set.
 To choose an appropriate instruction its responsibility for code generator.
 The quality of the generated code determined by its speed and size.
 The former statement can be translated into the letter statement as shown below.
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𝒕𝟏 = 𝒊𝒅𝟑 ∗ 60.0
𝒊𝒅𝟏= 𝒊𝒅𝟐 + 𝒕𝟏
Target Code Generator
LD 𝑹𝟏, 𝒊𝒅𝟑
MUL 𝑹𝟏, 𝑹𝟏, #60.0
𝑹𝟐 𝒊𝒅𝟐
𝑹𝟐 𝑹𝟐 + 𝑹𝟏
LD 𝑹𝟐, 𝒊𝒅𝟐
ADD 𝑹𝟐, 𝑹𝟐, 𝑹𝟏
ST 𝒊𝒅𝟏, 𝑹𝟐
𝑹𝟏 𝒊𝒅𝟑
𝑹𝟏 𝑹𝟏, * 60.0

9. Register Allocation Issue
 Register is faster than memory element which hold operand in a expression for some short
period of time.
 The use of registers are subdivided into two subproblems:
Register Allocation – The set of variables that will reside the registers at a point in the
program is selected.
Register Assignment – The specific register that a value picked.
 Certain machine requires even-odd register pairs for some operands and results.
For Example:
 This types of multiplicative instruction involve register pairs where ‘a’ the multiplicand is an
even register and ‘b’ the multiplier is the odd register of even-odd register pair.
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M a, b

10. Evaluation Order
 The order in which computations are performed can effect the efficiency of the target code.
 When instructions are independent their evaluation order can be change.
 When reorder the statements number of TAG (Three Address Code) statements remain same
but converted assembly code statements can reduce or increase.
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11. Have you any Questions?
Feel Free to Ask.
THANKYOU
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