1 Code Generation Code generator phase ... - VTU e-Learning

Code Optimization has to mainly achieve two goals
1. Preserve the meaning of code – The output generated before (without) Code Optimization
should be same as the code after optimization.
2. Optimization should reduce the cost of execution considerably. The effort spent on code
optimization should be worth it.
It implies that amount of time taken for optimization should be very less when compared to the
reduction of overall execution time. Generally, a fast non optimizing compilers are preferred for
debugging programs
Code improvement always need not be in code optimization phase. It can be incorporated in source
program or in intermediate code or on target code. In source program say, for sorting program, user
can choose different algorithm based on the cost function like minimum space or minimum time.
Each algorithm can be efficient it its own way or other, like quick sort is very fast on unsorted/random
array where as other sorting like bubble sort is efficient on partially sorted array. Intermediate code
can be improved by improving loops and efficient address calculation may give better results. In final
code generation phase, optimized code can be efficiently generated by selecting appropriate
instruction, use registers efficiently and some instruction transformations. Example: Keeping most
used variables in registers which avoids frequent fetching and storing in memory location. This
chapter deals with optimization of intermediate code represented as three address code. Intermediate
code is relatively independent at target machine so optimization is machine independent.
Programs are represented as flow graphs to study control flow and temporary variables are used to
store intermediate results help in data flow analysis. It is seen that compilation speed is proportional to
the size at program being compiled hence amount of time taken for code optimization should be
relatively less.
Principal of code optimizations
This sections deals with identifying that part of the program where optimization is required. By using
the concept of proper register allocation, elimination of dead code and finding the cost of instruction,
it is possible to improve the efficiency of program statements.
Unnecessary Operation
In a program there may be some part of code which never executes. It would be waste to generate
code for these statements. It may also happen that some of the values of temporary variables may
never be used. These are called as dead codes, it has to be removed. There can be some subexpression
whose value is computed many times. This can be optimized by calculating the value of
sub-expression only once and other statements can just use this value.
Example:
x = 1
while (x != 1)
{ …}
Statements of while never executed hence do not generate code for statements within while statement.
Example:
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