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

}
t1 = 4 * i;
The statement with in while loop, will be executed until ‘i’ greater than 10. Initially if i = 0, for the
first iteration i = 1 and t1 = 4, for the 2 nd instruction i = 2 and t1 = 4 * 2 = 8
or t1 = 4 * (i + 1)
t1 = 4 * i + 4 (∴ t1 = 4 * i )
t1 = t1 + 4
As the expression for evaluating t1 which requires multiplication is reduced to addition, its execution
is faster.
Local, Global & Inter-Procedural Optimization:
In case of local optimization straight line codes with in basic block are optimized. The basic block
consists of only assignment statements with no jumps or loops. Some of the optimization techniques
that can be used for local optimization are constant folding, constant propagations and algebraic
transformations.
Optimization considering many basic blocks of single procedure is called global optimization. They
use optimization techniques like code motion, elimination of induction variables and reduction in
strength of expression. Global optimization requires data flow analysis to detect jump boundaries
before optimization.
Inter-procedural optimization deals with optimization of entire program as a whole. This is very
difficult to achieve as it has to take care of different parameters passing mechanization and non local
variable access. The advantage of inter procedural optimization is that each procedure can be
optimized independently and linked together at the end with the help of linker which performs
optimization later on.
Machine dependent optimization
Some of the optimizations are machine independent, like register allocation and cost of instruction.
Register Allocation:
Number of times variable in each block of program may vary, but there are fixed number at register in
the system. Hence these registers are to be efficiently used. As far as possible the temporary variable
or intermediate values should be present in register this reduces the load and store to memory.
Example:
x = y + z
a = x + 10
b = x + 20
As the value of x is used after it has been assigned a value. Retain the value of x in the register, to
avoid storing and reloading from memory.
Cost of Instructions:
12