Code tuning techniques

Code-Tuning Techniques

Code Complete
Author : Steven C. McConnell.

Prof. Asha N

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
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To improve performance in code
Also called as “anti-refactoring”.
Code tuning refers to small-scale changes
rather than changes in larger-scale designs.

Prof. Asha N

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1.
2.
3.

4.
5.
6.

Logic
Loops
Data Transformations
Expressions
Routines
Recoding in Assembler

Prof. Asha N

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1. Logic


Stop Testing When You Know the Answer
 if ( 5 < x ) and ( x < 10 ) then ...
With short-circuit evaluation
if ( 5 < x ) then
if ( x < 10 ) then ...


C++ Example of Not Stopping After You Know the Answer
negativeInputFound = False;
for ( i = 0; i < iCount; i++ )
{
if ( input[ i ] < 0 ) {
negativeInputFound = True;
}
}
Prof. Asha N

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Contd….


Order Tests by Frequency




Arrange tests so that the one that’s fastest and most likely to be true is
performed first. This principle applies to case statements and to chains of ifthen-elses.
Visual Basic Example of a Poorly Ordered Logical Test
Select inputCharacter
Case "+", "="
ProcessMathSymbol( inputCharacter )
Case "0" To "9"
ProcessDigit( inputCharacter )
Case ",", ".", ":", ";", "!", "?"
ProcessPunctuation( inputCharacter )
Case " "
ProcessSpace( inputCharacter )
Case "A" To "Z", "a" To "z"
ProcessAlpha( inputCharacter )
Case Else
ProcessError( inputCharacter )
End Select

Prof. Asha N

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Contd….


Visual Basic Example of a Well-Ordered Logical Test
Select inputCharacter
Case "A" To "Z", "a" To "z"
ProcessAlpha( inputCharacter )
Case " "
ProcessSpace( inputCharacter )
Case ",", ".", ":", ";", "!", "?"
ProcessPunctuation( inputCharacter )
Case "0" To "9"
ProcessDigit( inputCharacter )
Case "+", "="
ProcessMathSymbol( inputCharacter )
Case Else
ProcessError( inputCharacter )
End Select
Prof. Asha N

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Contd….


Substitute Table Lookups for Complicated Expressions

Prof. Asha N

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Contd….
C++ Example of a Complicated Chain of Logic
if ( ( a && !c ) || ( a && b && c ) ) {
category = 1;
}
else if ( ( b && !a ) || ( a && c && !b ) ) {
category = 2;
}
else if ( c && !a && !b ) {
category = 3;
}
else {
category = 0;
}
Prof. Asha N

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Contd….
C++ Example of Using a Table Lookup to Replace Complicated
Logic

Prof. Asha N

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Contd….


Use Lazy Evaluation
 Lazy evaluation is similar to just-in-time strategies
that do the work closest to when it’s needed.

Prof. Asha N

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2. Loops

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Un switching
Jamming
Unrolling
Minimizing the Work Inside Loops
Sentinel Values
Putting the Busiest Loop on the Inside
Strength Reduction

Prof. Asha N

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Contd….


Un switching


putting loops inside the conditional rather than
putting the conditional inside the loop
C++ Example of a Switched Loop

Prof. Asha N

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Contd….
C++ Example of an Unswitched Loop

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Contd….


Jamming
 Jamming, or “fusion,” is the result of combining two loops that
operate on the same set of elements. The gain lies in cutting the
loop overhead from two loops to one.




For i = 0 to employeeCount - 1
employeeName( i ) = ""
Next
...
employeeEarnings( i ) = 0
Next
employeeName( i ) = ""
employeeEarnings( i ) = 0
Next
Prof. Asha N

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Contd….


Unrolling


The goal of loop unrolling is to reduce the amount of loop housekeeping




Java Example of a Loop That Can Be Unrolled
i = 0;
while ( i < count ) {
a[ i ] = i;
i = i + 1;
}

Java Example of a Loop That’s Been Unrolled Once
i = 0;
while ( i < count - 1 ) {
a[ i ] = i;
a[ i + 1 ] = i + 1;
i = i + 2;
}
if ( i == count ) {
a[ count - 1 ] = count - 1;
}

Prof. Asha N

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Contd….


Minimizing the Work Inside Loops




for ( i = 0; i < rateCount; i++ ) {
netRate[ i ] = baseRate[ i ] * rates->discounts->factors->net;
}
quantityDiscount = rates->discounts->factors->net;
for ( i = 0; i < rateCount; i++ ) {
netRate[ i ] = baseRate[ i ] * quantityDiscount;
}

Prof. Asha N

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Contd….


Sentinel Values
 When you have a loop with a compound test, you can often save
time by simplifying the test
found = FALSE;
i = 0;
while ( ( !found ) && ( i < count ) ) {
if ( item[ i ] == testValue ) {
found = TRUE;
}
else {
i++;
}
}
if ( found ) {
...
Prof. Asha N

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Contd….


Putting the Busiest Loop on the Inside
for ( column = 0; column < 100; column++ ) {
for ( row = 0; row < 5; row++ ) {
sum = sum + table[ row ][ column ];
}
}

Prof. Asha N

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Contd….


Strength Reduction


Reducing strength means replacing an expensive
operation such as multiplication with a cheaper
operation such as addition

Prof. Asha N

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3. Data Transformations
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Use Integers Rather Than Floating-Point
Numbers
Use the Fewest Array Dimensions
Possible
Minimize Array References
Use Supplementary Indexes
String-Length Index
Independent, Parallel Index Structure
Use Caching
Prof. Asha N

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4. Expressions
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Exploit Algebraic Identities
Use Strength Reduction
Initialize at Compile Time
Be Wary of System Routines
Use the Correct Type of Constants
Precompute Results
Eliminate Common Subexpressions

Prof. Asha N

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5. Routines



the most powerful tools in code tuning is a
good routine decomposition
Rewrite Routines In Line

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6. Recoding in Assembler



Recoding in assembler tends to improve both speed
and code size
typical approach to optimizing with assembler
1. Write 100 percent of an application in a high-level language.
2. Fully test the application, and verify that it’s correct.
3. If performance improvements are needed after that, profile the
application to identify hot spots. Since about 5 percent of a
program usually accounts for about 50 percent of the running
time, you can usually identify small pieces of the program as
hot spots.
4. Recode a few small pieces in assembler to improve overall
performance.
Prof. Asha N

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Code tuning techniques

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Code tuning techniques