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Presented by,
M.Selva Vinitha
B.E/CSE
M.Kumarasamy college of engineering
Karur.
Code generator
Final phase of compiler design
Optimized intermediate code is provided as input
It generates target code
2/27/2015 2Issues in design of code generator
Code generation and Instruction Selection
4
Symboltable
input output
Frontend
IntermediateCode
generatorCode
generator
2/27/2015 Issues in design of code generator
Contd…
output code must be correct
output code must be of high quality
code generator should run efficiently
2/27/2015 4Issues in design of code generator
Pre-requisites Instruction set of target machine.
Instruction addressing modes.
No. of registers.
Configuration of ALU
2/27/2015 5Issues in design of code generator
Issues in the design of a code
generator
Input to the code generator
Memory management
Target programs
Instruction selection
Register allocation
Evaluation order
Approaches to code generation
2/27/2015 6Issues in design of code generator
Input to the code generator The intermediate representation of the source program
produced by the front end
Several choices for the intermediate language
Linear - postfix nottion
3 address - quadruples
Virtual machie - stack machine code
Graphical - syntax tree &dags
2/27/2015 7Issues in design of code generator
Memory management
Mapping names in the source program to addresses of data
objects in run-time memory
Done by the front end and the code generator.
A name in a three- address statement refers to a symbol-
table entry for the name.
A relative address can be determined
2/27/2015 8Issues in design of code generator
Target programs
Absolute machine language
Relocatable machine language
Assembly language
2/27/2015 9Issues in design of code generator
Contd..
Absolute machine language:
Produce an absolute machine language program
can be placed in a fixed location in memory and
immediately executed.
2/27/2015 10Issues in design of code generator
Contd..Relocatable machine language:
Producing a relocatable machine language program
subprograms to be compiled separately.
relocatable object modules can be linked together and loaded for execution by a linking loader.
must provide explicit relocation information to the loader, to link the separately compiled program segments.
2/27/2015 11Issues in design of code generator
Contd…Assembly language:
Producing an assembly language program
makes the process of code generation easier
2/27/2015 12Issues in design of code generator
Instruction selection
The factors to be considered during instruction selection are:
The uniformity and completeness of the instruction set.
Instruction speed and machine idioms.
Size of the instruction set.
2/27/2015 13Issues in design of code generator
Contd…Eg., for the following address code is:
a := b + cd := a + e
inefficient assembly code is:
MOV b, R0 R0 ← b
ADD c, R0 R0 ← c + R0
MOV R0, a a ← R0
MOV a, R0 R0 ← a
ADD e, R0 R0 ← e + R0
MOV R0 , d d ← R0
Here the fourth statement is redundant, and so is the third statement if ,
'a' is not subsequently used.
2/27/2015 14Issues in design of code generator
Register allocation
• Instructions with register operands are usually shorter and
faster
• Efficient utilization of registers is important in
generating good code.
2/27/2015 15Issues in design of code generator
Contd..Register allocation phase:
• Select the set of variables that will reside in registers
Register assignment phase:
• Pick the specific register that a variable will reside in.
2/27/2015 16Issues in design of code generator
Evaluation order The order in which computations are performed
Affect the efficiency of the target code.
Some computation orders require fewer registers to hold
intermediate results
2/27/2015 17Issues in design of code generator
Approaches to code generation Most important criteria for code generator is that it
produces correct code
Correctness takes on special signification
It contains a straightforward code generation algorithm
The output of such code generator can be improved by
peephole optimization technique
2/27/2015 18Issues in design of code generator
Conclusion
Output of code generator phase is dependent on:
Target language
Operating system
Memory management system
Instruction selection
Register allocation
Evaluation order
2/27/2015 19Issues in design of code generator