Memory Management
Both the front
end and code generator perform the task of mapping the names in the source
program to addresses to the data objects in runtime memory
The names used
in the three address code refer to the entries in the symbol table
The type in a
declaration statement determines the amount of storage (memory) needed to store
the declared identifier
Using the symbol
table information about memory requirements code generator determines the
addresses in the target code
Similarly, if
the three address code contains the labels then those labels can be converted
into equivalent memory address
Instruction Selection:
The uniformity
and completeness of instruction set is an important factor for the code generator
The selection of
instruction depends upon the instruction set of target machine
The speed of
instruction and machine idioms are two important factors in selection of
instructions
If we do not
consider the efficiency of target code then the instruction selection becomes a
straightforward task
For each type of
three address code, the code skeleton can be prepared which ultimately gives
the target code for the corresponding construct
Register Allocation
The usage of
operands becomes shorter and faster if the instruction contains “Register
Operands” when compared to the usage of operands in memory. The effective
utilization of register becomes an important factor while generating a good
code
There are two
important activities done while using registers:
Register Allocation: During
register allocation, select appropriate set of variable that will reside in
registers
Register Assignment: During
register assignment, pick up the specific register in which corresponding
variable will reside
Choice of Evaluation Order
The evaluation
order is an important factor in generating an efficient target code
Some orders
require less number of registers to hold the intermediate results than others.
Picking up the best order is one difficulty in code generation
Mostly, avoid
this problem by referring the order in which the three address code is
generated by semantic actions
Approaches to Code Generation
The most
important factor for a code generation is that it produces the correct code
With this
approach of code generation, various algorithms for generating code are
designed
Object Code Forms
The output
generated by code generator is an object code or machine code. This code
typically appears in the below forms
Absolute code
Relocatable Machine Code
Assemble Code
Figure 1
Absolute code:
Absolute code is
a machine code that consists of reference to actual addresses within programs
address space
The generated
code can be put directly in the memory, and execution begins quickly
With the help of
absolute code generation, the compilation and execution of small program is
done rapidly
Example:
A number of
“student-job” compilers like WATFIV (WATerlodo FORTRAM IV) and PL/C generate
absolute code.
Relocatable Machine Code:
The compilation
of subprograms can be carried out separately whenever relocatable machine
language program is generated as output
A set of
relocatable object modules can be linked together and can be loaded for
execution with the aid of a linking loader
The prime
advantage of producing relocatable machine code is that one gains more
flexibility in the process of compiling subroutines independently and calling
other previously compiled projects from an object module
In the case when
the target machine does not handle relocation automatically, the compiler must
give explicit relocation data to the loader to link the independently compiled
program segments
Assemble code:
The code
generation process becomes little easier when an assembly language program is
produced as output.
In the process
of generation of code, the symbolic instructions can be generated, and
assembler’s macro facilities can be used.
But the
generation of assembler code as an output slows down the code generation
process, since assembling, linking and loading are required.
