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code optimization

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Sanjeev Raaz

This document discusses code optimization techniques at various levels including the design level, compile level, assembly level, and runtime level. It describes common subexpression elimination as an optimization that identifies identical expressions and replaces them with a single variable to improve efficiency. It also covers dead code elimination and loop optimization techniques like removing loop invariants and induction variables. An example of applying common subexpression elimination to optimize the three address code of quicksort is provided.

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CODE OPTIMIZATION PRSENTED BY: SANJEEV KUMAR…… DEPT:-IT. ASSAM UNIVERSITY ,SILCHAR
Design Of a Compiler Lexical Analysis Syntax Analysis Intermediate Code Generation Code Generation Code Optimization Table Mgmt Routine Error Handling Routine Source code Object code
What is optimization?  In computing, optimization is the process of modifying a system to make some aspect of it work more efficiently or use fewer resources. For instance, a computer program may be optimized so that it executes more rapidly, or is capable of operating with less memory storage or other resources, or draw less power. The system may be a single computer program, a collection of computers or even an entire network such as the internet.
code optimization
Levels' of optimization Optimization can occur at a number of 'levels':  Design level At the highest level, the design may be optimized to make best use of the available resources. The implementation of this design will benefit from the use of suitable efficient algorithms and the implementation of these algorithms will benefit from writing good quality code. The architectural design of a system overwhelmingly affects its performance. The choice of algorithm affects efficiency more than any other item of the design.  Compile level Use of an optimizing compiler tends to ensure that the executable program is optimized at least as much as the compiler can predict.
 Assembly level At the lowest level, writing code using an Assembly language designed for a particular hardware platform will normally produce the most efficient code since the programmer can take advantage of the full repertoire of machine instructions. The operating systems of most machines has been traditionally written in Assembler code for this reason.  Runtime Just In Time Compiler and assembler programmers are able to perform runtime optimization.
When to optimize ? Optimization is often performed at the end of the development stage since it • reduces readability • adds code that is used to improve the performance.
Criteria For optimization An optimization must preserve the meaning of a program : -Cannot change the output produced for any input -Can not introduce an error optimization should, on average, speed up programs Transformation should be worth the effort
Improvements can be made at various phases: Source Code: -Algorithms transformations can produce spectacular improvements -Profiling can be helpful to focus a programmer’s attention on important code. Intermediate Code: -Compiler can improve loops, procedure calls and address calculations -Typically only optimizing compilers include this phase Target Code: -Compilers can use registers efficiently -Peephole transformation can be applied
Types of Code optimization  Common Sub-expression Removal  Dead Code Optimization  Loop Optimization
Common Sub expression elimination Common Sub expression elimination is a optimization that searches for instances of identical expressions (i.e they all evaluate the same value), and analyses whether it is worthwhile replacing with a single variable holding the computed value. a=b * c + g d=b * c * d temp=b * c a=temp + g d=temp * d
Dead Code elimination is a compiler optimization that removes code that does not affect a program. Removing such code has two benefits It shrinks program size, an important consideration in some contexts. It lets the running program avoid executing irrelevant operations, which reduces its running time. Dead Code elimination is of two types Unreachable Code Redundant statement Dead code Optimization:
Unreachable Code In Computer Programming, Unreachable Code or dead code is code that exists in the source code of a program but can never be executed. Program Code If (a>b) m=a elseif (a<b) m=b elseif (a==b) m=0 else m=-1 Optimized Code If (a>b) m=a elseif (a<b) m=b else m=0
Redundant Code Redundant Code is code that is executed but has no effect on the output from a program main(){ int a,b,c,r; a=5; b=6; c=a + b; r=2; r++; printf(“%d”,c); } Adding time & space complexity
Loop optimization Loop optimization plays an important role in improving the performance of the source code by reducing overheads associated with executing loops. Loop Optimization can be done by removing: • Loop invariant • Induction variables
Loop Invariant i = 1 s= 0 do{ s= s + i a =5 i = i + 1 { while (i < =n) i = 1 s= 0 a =5 do{ s= s + i i = i + 1 { while (i < =n) Bringing a=5 outside the do while loop, is called code motion.
Induction variables i = 1 s= 0 S1=0 S2=0 while (i < =n) { s= s + a[ i ] t1 = i * 4 s= s + b[ t1 ] t2 = t1 +2 s2= s2 + c[ t2 ] i = i + 1 } i = 1 s= 0 S1=0 S2=0 t2=0 while (i < =n) { s= s + a[ i ] t1 = t1+ 4 s= s + b[ t1 ] s2= s2 + c[t1 +2 ] i = i + 1 } t1,t2 are induction variables. i is inducing t1 and t1 is inducing t2 “+” replaced “ * ”, t1 was made independent of i
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Common Sub-expression Removal  It is used to remove redundant computations which usually improves the execution time of a program.
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Three Address Code of Quick Sort i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto (5)< v goto (5) j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto (9)> v goto (9) if i >= j goto (23)if i >= j goto (23) tt66 = 4 * i= 4 * i x = a[tx = a[t66]] 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 tt77 = 4 * I= 4 * I tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[ta[t77] = t] = t99 tt1010 = 4 * j= 4 * j a[ta[t1010] = x] = x goto (5)goto (5) tt1111 = 4 * I= 4 * I x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t ] = x] = x 1616 1717 1818 1919 2020 2121 2222 2323 2424 2525 2626 2727 2828 2929 3030
Find The Basic Block i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto (5)< v goto (5) j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto (9)> v goto (9) if i >= j goto (23)if i >= j goto (23) tt66 = 4 * i= 4 * i x = a[tx = a[t66]] 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 tt77 = 4 * I= 4 * I tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[ta[t77] = t] = t99 tt1010 = 4 * j= 4 * j a[ta[t1010] = x] = x goto (5)goto (5) tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t ] = x] = x 1616 1717 1818 1919 2020 2121 2222 2323 2424 2525 2626 2727 2828 2929 3030
Flow Graph i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt77 = 4 * i= 4 * i tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[ta[t77] = t] = t99 tt1010 = 4 * j= 4 * j a[ta[t1010] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt77 = 4 * i= 4 * i tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[ta[t77] = t] = t99 tt1010 = 4 * j= 4 * j a[ta[t1010] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 tt1010 = 4 *= 4 * jj a[ta[t1010] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 *i= 4 *i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = a[x = a[tt22]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt22] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = tx = t33 tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt22] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = tx = t33 a[a[tt22] = t] = t55 a[a[tt44] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = tx = t33 a[a[tt22] = t] = t55 a[a[tt44] = x] = x goto Bgoto B22 x = tx = t33 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = x] = x B1 B2 B3 B4 B5 B6 Similarly for B6
Dead Code Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = tx = t33 a[a[tt22] = t] = t55 a[a[tt44] = x] = x goto Bgoto B22 x = tx = t33 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = x] = x B1 B2 B3 B4 B5 B6
Dead Code Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 a[a[tt22] = t] = t55 a[a[tt44] = t] = t33 goto Bgoto B22 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = t] = t33 B1 B2 B3 B4 B5 B6
Reduction in Strength i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 a[a[tt22] = t] = t55 a[a[tt44] = t] = t33 goto Bgoto B22 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = t] = t33 B1 B2 B3 B4 B5 B6
Reduction in Strength i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] tt22 = 4 * i= 4 * i tt44 = 4 * j= 4 * j tt22 = t= t22 + 4+ 4 tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 tt44 = t= t44 - 4- 4 tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 a[a[tt22] = t] = t55 a[a[tt44] = t] = t33 goto Bgoto B22 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = t] = t33 B1 B2 B3 B4 B5 B6

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code optimization

  • 1. CODE OPTIMIZATION PRSENTED BY: SANJEEV KUMAR…… DEPT:-IT. ASSAM UNIVERSITY ,SILCHAR
  • 2. Design Of a Compiler Lexical Analysis Syntax Analysis Intermediate Code Generation Code Generation Code Optimization Table Mgmt Routine Error Handling Routine Source code Object code
  • 3. What is optimization?  In computing, optimization is the process of modifying a system to make some aspect of it work more efficiently or use fewer resources. For instance, a computer program may be optimized so that it executes more rapidly, or is capable of operating with less memory storage or other resources, or draw less power. The system may be a single computer program, a collection of computers or even an entire network such as the internet.
  • 5. Levels' of optimization Optimization can occur at a number of 'levels':  Design level At the highest level, the design may be optimized to make best use of the available resources. The implementation of this design will benefit from the use of suitable efficient algorithms and the implementation of these algorithms will benefit from writing good quality code. The architectural design of a system overwhelmingly affects its performance. The choice of algorithm affects efficiency more than any other item of the design.  Compile level Use of an optimizing compiler tends to ensure that the executable program is optimized at least as much as the compiler can predict.
  • 6.  Assembly level At the lowest level, writing code using an Assembly language designed for a particular hardware platform will normally produce the most efficient code since the programmer can take advantage of the full repertoire of machine instructions. The operating systems of most machines has been traditionally written in Assembler code for this reason.  Runtime Just In Time Compiler and assembler programmers are able to perform runtime optimization.
  • 7. When to optimize ? Optimization is often performed at the end of the development stage since it • reduces readability • adds code that is used to improve the performance.
  • 8. Criteria For optimization An optimization must preserve the meaning of a program : -Cannot change the output produced for any input -Can not introduce an error optimization should, on average, speed up programs Transformation should be worth the effort
  • 9. Improvements can be made at various phases: Source Code: -Algorithms transformations can produce spectacular improvements -Profiling can be helpful to focus a programmer’s attention on important code. Intermediate Code: -Compiler can improve loops, procedure calls and address calculations -Typically only optimizing compilers include this phase Target Code: -Compilers can use registers efficiently -Peephole transformation can be applied
  • 10. Types of Code optimization  Common Sub-expression Removal  Dead Code Optimization  Loop Optimization
  • 11. Common Sub expression elimination Common Sub expression elimination is a optimization that searches for instances of identical expressions (i.e they all evaluate the same value), and analyses whether it is worthwhile replacing with a single variable holding the computed value. a=b * c + g d=b * c * d temp=b * c a=temp + g d=temp * d
  • 12. Dead Code elimination is a compiler optimization that removes code that does not affect a program. Removing such code has two benefits It shrinks program size, an important consideration in some contexts. It lets the running program avoid executing irrelevant operations, which reduces its running time. Dead Code elimination is of two types Unreachable Code Redundant statement Dead code Optimization:
  • 13. Unreachable Code In Computer Programming, Unreachable Code or dead code is code that exists in the source code of a program but can never be executed. Program Code If (a>b) m=a elseif (a<b) m=b elseif (a==b) m=0 else m=-1 Optimized Code If (a>b) m=a elseif (a<b) m=b else m=0
  • 14. Redundant Code Redundant Code is code that is executed but has no effect on the output from a program main(){ int a,b,c,r; a=5; b=6; c=a + b; r=2; r++; printf(“%d”,c); } Adding time & space complexity
  • 15. Loop optimization Loop optimization plays an important role in improving the performance of the source code by reducing overheads associated with executing loops. Loop Optimization can be done by removing: • Loop invariant • Induction variables
  • 16. Loop Invariant i = 1 s= 0 do{ s= s + i a =5 i = i + 1 { while (i < =n) i = 1 s= 0 a =5 do{ s= s + i i = i + 1 { while (i < =n) Bringing a=5 outside the do while loop, is called code motion.
  • 17. Induction variables i = 1 s= 0 S1=0 S2=0 while (i < =n) { s= s + a[ i ] t1 = i * 4 s= s + b[ t1 ] t2 = t1 +2 s2= s2 + c[ t2 ] i = i + 1 } i = 1 s= 0 S1=0 S2=0 t2=0 while (i < =n) { s= s + a[ i ] t1 = t1+ 4 s= s + b[ t1 ] s2= s2 + c[t1 +2 ] i = i + 1 } t1,t2 are induction variables. i is inducing t1 and t1 is inducing t2 “+” replaced “ * ”, t1 was made independent of i
  • 22. Common Sub-expression Removal  It is used to remove redundant computations which usually improves the execution time of a program.
  • 29. Three Address Code of Quick Sort i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto (5)< v goto (5) j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto (9)> v goto (9) if i >= j goto (23)if i >= j goto (23) tt66 = 4 * i= 4 * i x = a[tx = a[t66]] 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 tt77 = 4 * I= 4 * I tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[ta[t77] = t] = t99 tt1010 = 4 * j= 4 * j a[ta[t1010] = x] = x goto (5)goto (5) tt1111 = 4 * I= 4 * I x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t ] = x] = x 1616 1717 1818 1919 2020 2121 2222 2323 2424 2525 2626 2727 2828 2929 3030
  • 30. Find The Basic Block i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto (5)< v goto (5) j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto (9)> v goto (9) if i >= j goto (23)if i >= j goto (23) tt66 = 4 * i= 4 * i x = a[tx = a[t66]] 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 tt77 = 4 * I= 4 * I tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[ta[t77] = t] = t99 tt1010 = 4 * j= 4 * j a[ta[t1010] = x] = x goto (5)goto (5) tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t ] = x] = x 1616 1717 1818 1919 2020 2121 2222 2323 2424 2525 2626 2727 2828 2929 3030
  • 31. Flow Graph i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt77 = 4 * i= 4 * i tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[ta[t77] = t] = t99 tt1010 = 4 * j= 4 * j a[ta[t1010] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
  • 32. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt77 = 4 * i= 4 * i tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[ta[t77] = t] = t99 tt1010 = 4 * j= 4 * j a[ta[t1010] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
  • 33. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 tt1010 = 4 *= 4 * jj a[ta[t1010] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
  • 34. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 *i= 4 *i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
  • 35. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1212 = 4 * i= 4 * i tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[ta[t1212] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
  • 36. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 tt1515 = 4 * n= 4 * n a[ta[t1515] = x] = x B1 B2 B3 B4 B5 B6
  • 37. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
  • 38. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 tt66 = 4 * i= 4 * i x = a[tx = a[t66]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt66] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
  • 39. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = a[x = a[tt22]] tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt22] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
  • 40. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = tx = t33 tt88 = 4 * j= 4 * j tt99 = a[t= a[t88]] a[a[tt22] = t] = t99 a[a[tt88] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
  • 41. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = tx = t33 a[a[tt22] = t] = t55 a[a[tt44] = x] = x goto Bgoto B22 tt1111 = 4 * i= 4 * i x = a[tx = a[t1111]] tt1313 = 4 * n= 4 * n tt1414 = a[t= a[t1313]] a[a[tt1111] = t] = t1414 a[a[tt1313] = x] = x B1 B2 B3 B4 B5 B6
  • 42. Common Subexpression Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = tx = t33 a[a[tt22] = t] = t55 a[a[tt44] = x] = x goto Bgoto B22 x = tx = t33 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = x] = x B1 B2 B3 B4 B5 B6 Similarly for B6
  • 43. Dead Code Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 x = tx = t33 a[a[tt22] = t] = t55 a[a[tt44] = x] = x goto Bgoto B22 x = tx = t33 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = x] = x B1 B2 B3 B4 B5 B6
  • 44. Dead Code Elimination i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 a[a[tt22] = t] = t55 a[a[tt44] = t] = t33 goto Bgoto B22 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = t] = t33 B1 B2 B3 B4 B5 B6
  • 45. Reduction in Strength i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] i = i + 1i = i + 1 tt22 = 4 * i= 4 * i tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 j = j – 1j = j – 1 tt44 = 4 * j= 4 * j tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 a[a[tt22] = t] = t55 a[a[tt44] = t] = t33 goto Bgoto B22 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = t] = t33 B1 B2 B3 B4 B5 B6
  • 46. Reduction in Strength i = m - 1i = m - 1 j = nj = n tt11 =4 * n=4 * n v = a[tv = a[t11]] tt22 = 4 * i= 4 * i tt44 = 4 * j= 4 * j tt22 = t= t22 + 4+ 4 tt33 = a[t= a[t22]] if tif t33 < v goto B< v goto B22 tt44 = t= t44 - 4- 4 tt55 = a[t= a[t44]] if tif t55 > v goto B> v goto B33 if i >= j goto B6 a[a[tt22] = t] = t55 a[a[tt44] = t] = t33 goto Bgoto B22 tt1414 = a[t= a[t11]] a[a[tt22] = t] = t1414 a[a[tt11] = t] = t33 B1 B2 B3 B4 B5 B6