Unit-5: Web technologies lecture notes P.R.Engineering college

Unit-5: Web technologies lecture notes in Web technology - Subject uploaded at P.R.Engineering college

Unit-5: Web technologies lecture notes in Web technology - Subject at P.R.Engineering college P.R.Engineering college Staff uploaded in lecturenotes.net

Summary / document preview text:

UNIT 5
CODE OPTIMIZATION
5.1 INTRODUCTION
 The code produced by the straight forward compiling algorithms can often be 
made to run faster or take less space, or both. This improvement is achieved by 
program transformations that are traditionally called optimizations. Compilers that 
apply code-improving transformations are called optimizing compilers. 
 Optimizations are classified into two categories. They are 
o Machine independent optimizations: 
o Machine dependant optimizations: 
Machine independent optimizations:
 Machine independent optimizations are program transformations that improve the 
target code without taking into consideration any properties of the target machine. 
Machine dependant optimizations:
 Machine dependant optimizations are based on register allocation and utilization of 
special machine- instruction sequences. 
The criteria for code improvement transformations:
 Simply stated, the best program transformations are those that yield the most benefit 
for the least effort. 
 The transformation must preserve the meaning of programs. That is, the optimization 
must not change the output produced by a program for a given input, or cause an
error such as division by zero, that was not present in the original source program. At 
all times we take the “safe” approach of missing an opportunity to apply a 
transformation rather than risk changing what the program does. 
 A transformation must, on the average, speed up programs by a measurable amount. 
We are also interested in reducing the size of the compiled code although the size of 
the code has less importance than it once had. Not every transformation succeeds in 
improving every program, occasionally an “optimization” may slow down a program 
slightly. 
 The transformation must be worth the effort. It does not make sense for a compiler 
writer to expend the intellectual effort to implement a code improving transformation 
and to have the compiler expend the additional time compiling source programs if this 
effort is not repaid when the target programs are executed. “Peephole” 
transformations of this kind are simple enough and beneficial enough to be included 
in any compiler.  Code Optimization unit V
Compiler Design Page 2
Organization for an Optimizing Compiler:
 Flow analysis is a fundamental prerequisite for many important types of code 
improvement. 
 Generally control flow analysis precedes data flow analysis. 
 Control flow analysis (CFA) represents flow of control usually in form of
graphs, CFA constructs such as 
 control flow graph 
 Call graph 
 Data flow analysis (DFA) is the process of ascerting and collecting information 
prior to program execution about the possible modification, preservation, and use 
of certain entities (such as values or attributes of variables) in a computer 
program. 
5.2 PRINCIPAL SOURCES OF OPTIMISATION
 A transformation of a program is called local if it can be performed by looking
only at the statements in a basic block; otherwise, it is called global. 
 Many transformations can be performed at both the local and global levels. Local 
transformations are usually performed first. 
Function-Preserving Transformations
 There are a number of ways in which a compiler can improve a program without 
changing the function it computes. 
 The transformations 
 Common sub expression elimination, 
 Copy propagation, 
 Dead-code elimination, and 
 Constant folding  Code Optimization unit V
Compiler Design Page 3
are common examples of such function-preserving transformations. The other 
transformations come up primarily when global optimizations are performed.
Frequently, a program will include several calculations of the same value, such as 
an offset in an array. Some of the duplicate calculations cannot be avoided by the 
programmer because they lie below the level of detail accessible within the source 
language. 
Common Sub expressions elimination: 
An occurrence of an expression E is called a common sub-expression if E was 
previously computed, and the values of variables in E have not changed since the 
previous computation. We can avoid re-computing the expression if we can use the 
previously computed value. 
For example 
t1: =4*i
t2: =a [t1]
t3: =4*j
t4:=4*i
t5: =n
t6: =b [t4] +t5
The above code can be optimized using the common subexpression elimination as
t1: =4*i
t2: =a [t1]
t3: =4*j
t5: =n
t6: =b [t1] +t5
The common sub expression t 4: =4*i is eliminated as its computation is already in 
t1. And value of i is not been changed from definition to use.
Copy Propagation: 
 Assignments of the form f : = g called copy statements, or copies for short. The 
idea behind the copy-propagation transformation is to use g for f, whenever
possible after the copy statement f: = g. Copy propagation means use of one 
variable instead of another. This may not appear to be an improvement, but as we 
shall see it gives us an opportunity to eliminate x. 
 For example: x=Pi; 
……
A=x*r*r;
The optimization using copy propagation can be done as follows:
A=Pi*r*r;
Here the variable x is eliminated
Dead-Code Eliminations:
 A variable is live at a point in a program if its value can be used subsequently; 
otherwise, it is dead at that point. A related idea is dead or useless code, statements 
that compute values that never get used. While the programmer is unlikely to 
introduce any dead code intentionally, it may appear as the result of previous  Code Optimization unit V
Compiler Design Page 4
transformations. An optimization can be done byeliminating dead code.
Example:
i=0;
if(i=1)
{
a=b+5;
}
Here, ‘if’statement is dead code because this condit

Title:
Unit-5: Web technologies lecture notes

Subject:
Web technology - Subject

Institute:
P.R.Engineering college

Unit-5: Web technologies lecture notes P.R.Engineering college

Unit-5: Web technologies lecture notes in Web technology - Subject uploaded at P.R.Engineering college

Summary / document preview text:

Modal

Are you sure to delete this information ?