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

Each instruction takes some machine cycles to perform the operation. The optimization strategies
should be such that it should reduce the number of machine cycles or in other words the strength of
instruction should be reduced to have better optimization.
Example:
x 2 can be replaced by expression x * x.
Expressions like adding 0 or multiplying by 1 can be removed, as these do not change the value of
variable.
Example:
1) x = x + 0
2) a = a * 1
These instructions can be eliminated as they do not change the value of x and a.
This is called algebraic transformation.
Data Structure:
Syntax trees can be used for some of the optimization techniques like constant folding, constant
propagation etc., but for optimization like eliminating loop invariant, or dead code elimination, it is
not very efficient, Specially for global optimization syntax tree is not efficient as it requires the study
for control flow. Hence flow graphs are used. Flow graphs consist of basic blocks as nodes and edges
connecting basic blocks indicate the control flow. The sequence of three address statement is
converted to flow graph using following steps.
1. Construct basic block
2. Generate flow graph
1. Construction of Basic Blocks
a) Determine set of header statements. Header statements are the first statement of each basic
block.
b) First statement is a header statement
c) Any statement which is target of conditional or unconditional jump is a header statement.
d) Any statement following conditional or unconditional jump is a header statement.
2. Construct flow graph
Construct graph with B1 as the stating node where B1 is basic block which has first statement of the
program. Generate edge from Bi to Bj if control flows from block Bi to Bj. Entry for any block Bk
will be from the first statement of Bk and exit from Bk will be from last statement only. No
intermediate jump or return can happen in the basic block.
Example: Consider the following C statement
for i = 1 to n do
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