Refreshing Fresher’s

1. Refreshing Fresher’s -Sanket Shah Freelancer:FreshBreezeSoftware Consultancy Services (FreshBreeze.co.in) Corporate Trainer:E-Consultancy FZC (EConsultancyFZC.com) Blogger:http://www.sanket-shah.com E-Consultancy FZC http://www.econsultancyfzc.com 2011

2. Improving String Comparison • Our belief: • “Don’t re-invent the wheel” • To compare string, naturally and with basic instinct, we go for in-built function – strcmp() • But… • What about performance? Can we increase? • Let’s say I’ve two strings as: • String1 = “sanket” • String2 = “Sanket” • The best way to improve performance – Check first character and if they are same, then only proceed. E-Consultancy FZC http://www.econsultancyfzc.com 2011

3. Improving String Comparison • The optimized way can be written as: • return (*s != *t ? *s - *t : strcmp(s, t)); • Execution benefit? • AT LEAST 20% (MIN VALUE) AS TESTED ON VARIOUS COMPILERS !!! • Sample Code: ModFunc.c E-Consultancy FZC http://www.econsultancyfzc.com 2011

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5. Sample Code class A { int x, y, z; public: A(); }; class B { A a; int p, q, r; // Other Variables... public: B() {} }; A::A() {x = 0; y = 0; z = 0;} • Whether class A has any constructor? • Whether class B has any constructor? • Will class B invoke class A’s constructor? • What is the price of Object of class A? • What is the price of Object of class B? • Is constructor body empty for class B? • What effect is there at runtime? E-Consultancy FZC http://www.econsultancyfzc.com 2011

6. Answers to Questions… • Whether class A has any constructor? • Whether class B has any constructor? • Will class B invoke class A’s constructor? • What is the price of Object of class A? • What is the price of Object of class B? • Is constructor body empty for class B? • What effect is there at runtime? • Yes • Yes • Yes • 6 bytes • Size of variables of B + 6 bytes • No. Invokes constructor of A. Such as: B::B() {a.A::A();} • Higher the objects of B, higher the objects of A will be created internally E-Consultancy FZC http://www.econsultancyfzc.com 2011

7. Optimizing Execution • Speed of application can be improved on functions that execute frequently. • The key is to validate data / arguments first and the perform (possibly) tedious jobs. • In C++, create objects / variables once all checks have passed. • Sample Code: • FactImpr.c • FactImpr.cpp E-Consultancy FZC http://www.econsultancyfzc.com 2011

8. Speed Difference in C E-Consultancy FZC http://www.econsultancyfzc.com 2011

9. Speed Difference in C++ E-Consultancy FZC http://www.econsultancyfzc.com 2011

10. Optimizing Console Output • On UNIX: • endl > string > character • On Windows: • string > character > endl • But one thing is always same: • string > character • Reason being in “character”, we deal with only a single value, whereas in “string”, we deal with array (“\n” + Terminating NULL) • Sample Program: • NewLine.cpp E-Consultancy FZC http://www.econsultancyfzc.com 2011

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12. Optimization using shift operators • Always prefer shift operators if multiplication or division is in power of 2 • Shift operators only work on integers • Instead of calculations using Registers and ALU, operation is performed directly by shifting bit in Holding Register • Equivalence: • x << y ó x * 2y • x >> y ó x / 2y • Sample Code: • BitShift.cpp E-Consultancy FZC http://www.econsultancyfzc.com 2011

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14. Optimizing Mathematical Operations • a*b + a*c = a*(b+c) • Gets rid of one multiplication • b/a + c/a = (1/a)*(b+c) • Replacing two divisions by a division and a multiplication. On every platform, divisions are slower than multiplications, so this rewrite will provide a speed improvement • (a || b ) && c ) = c && ( a || b ) • C++ standard requires lazy evaluation. • Whenever “c” happens to be false, in the first case ( a || b ) has to be evaluated, whereas in the second case, it can be skipped as the entire expression can never evaluate to true. E-Consultancy FZC http://www.econsultancyfzc.com 2011

15. Pass Class Parameters by Reference • Pass by Value: • Invokes copy constructor • Pass by Reference: • “dereference” instruction • Beneficial, especially when classes use string data types E-Consultancy FZC http://www.econsultancyfzc.com 2011

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17. Use Initialization over Assignment • Initialize object at the moment it is declared • Initializing object invokes copy constructor • Defining and assigning invokes default constructor and assignment operator • Postpone declaration until you can do an initialization • Saves CPU Cycles also • Improves Readability E-Consultancy FZC http://www.econsultancyfzc.com 2011

18. Return Value Optimization • Original Code: complex<double> Mult(const complex<double>& a, const complex<double>& b) { complex<double> c; double i = (a.real() * b.imag()) + (a.imag() * b.real()); double r = (a.real() * b.real()) - (a.imag() * b.imag()); c.imag(i); c.real(r); return (c); } E-Consultancy FZC http://www.econsultancyfzc.com 2011

19. Return Value Optimization • First Refinement: complex<double> Mult(const complex<double>& a, const complex<double>& b) { complex<double> c((a.real() * b.real()) - (a.imag() * b.imag()), (a.real() * b.imag()) + (a.imag() * b.real())); return (c); } E-Consultancy FZC http://www.econsultancyfzc.com 2011

20. Return Value Optimization • Second Refinement: complex<double> Mult(const complex<double>& a, const complex<double>& b) { return (complex<double>((a.real() * b.real()) + (a.imag() * b.imag()), (a.real() * b.imag()) - (a.imag() * b.real()))); } E-Consultancy FZC http://www.econsultancyfzc.com 2011

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22. Minimize Local Variables • Less Local Variables: • Compiler can probably fit them easily in registers • No / less overhead of setting up and restoring frame pointer E-Consultancy FZC http://www.econsultancyfzc.com 2011

23. Avoid Unnecessary Looping Calculations • Do not perform repetitive tasks in loops for which value will not change or the value of part of expression can be derived as a fixed constant. • Original Code: float summation = 0.0; for(intctr = 0; ctr < maxValue; ctr++) { summation += ctr + (a / b); } • Refined Code: float summation = 0.0; float divisionValue = a / b; for(intctr = 0; ctr < maxValue; ctr++) { summation += ctr + divisionValue; } E-Consultancy FZC http://www.econsultancyfzc.com 2011

24. Prefer “int” over “char” or “short” char sum_char(char a, char b) { char c; c = a + b; return c; } • Convert 2nd parameter into intby sign extension • Push sign extended parameter on stack as b • Convert 1st parameter into intby sign extension • Push sign extended parameter on stack as a • Add a and b • Cast result to char • Store result in char c • Sign extend c • Copy c into return value register and function returns to caller • caller converts from int to char • The result is stored E-Consultancy FZC http://www.econsultancyfzc.com 2011

25. Prefer “int” over “char” or “short” intsum_int(int a, int b) { int c; c = a + b; return c; } • Push int b on stack • Push int a on stack • Called function adds a and b • Result is stored in int c • c is copied into return value register and function returns to caller. • The called function stores the returned value E-Consultancy FZC http://www.econsultancyfzc.com 2011

26. “if…[else if]…else” vs “switch…case” • Use switch case: • Comparison with fixed set of constant values • Use if statements: • Comparison with Sequence • If there are multiple cases associated with single action, do not repeat statements, rather use fallback mechanism. This performs only 1 computed “goto” instead of sequence of branches (Refer to next slide). • put the most typical cases before • If compiler does not use jump-table, cases are evaluated in order of appearance; therefore, fewer comparisons are performed for more frequent cases. E-Consultancy FZC http://www.econsultancyfzc.com 2011

27. Sample “switch…case” to Pseudo Machine Code switch (i) { case 10: case 13: func_a(); break; case 11: func_b(); break; } // N.B.: This is not C++ code static address jump_table[] = { case_a, case_b, end, case_a }; unsigned int index = i - 10; if (index > 3) goto end; gotojump_table[index]; case_a: func_a(); goto end; case_b: func_b(); end: E-Consultancy FZC http://www.econsultancyfzc.com 2011

28. Making “for…” loop fast • for( i=0; i<10; i++){ ... } • Slowest • for( i=10; i--; ) { ... } • Fastest, if we don’t care for lower bound • for(i=10; i; i--){} • Faster as comparison is made with only zero value • Same as for(i=10; i!=0; i--){} E-Consultancy FZC http://www.econsultancyfzc.com 2011

29. Don’t prefer Loop Jamming Bad Way Good Way for(i=0; i<100; i++) { stuff(); morestuff(); } for(i=0; i<100; i++) { stuff(); } for(i=0; i<100; i++) { morestuff(); } E-Consultancy FZC http://www.econsultancyfzc.com 2011

30. Declare Variables in innermost scope • Better Performance • Less Overhead of creating / destroying object • Higher memory remains available • Objects not created unnecessarily when the conditions fail • Memory marked for deletion when scope is closed E-Consultancy FZC http://www.econsultancyfzc.com 2011

31. Other Ideas • Java – Use StringBuffer if multiple string manipulation operations are there • .NET – User StringBuilder (same as Java Case) • Stay away from Virtual Functions as far as possible as that involves lot of stack operations • Use Properties instead of fields if validation(s) needs to be performed. E.g. – Age • Use finally block wisely • Do not import unnecessary namespaces • Break large classes in multiple files (Partial Classes in .NET) • Use Double Buffer while updating UI through Threads E-Consultancy FZC http://www.econsultancyfzc.com 2011

32. Other Ideas • Use Threading to prevent blocking of application UI when performing large operations • Always handle Exceptions at the closest point of occurrence as they’ll bubble up if not handled properly • Exceptions always consume more resources • Try to stay away from traditional COM Objects E-Consultancy FZC http://www.econsultancyfzc.com 2011

33. Thanks for being here  Contact: +91 98793 56075 E-Mail: sanket.1985@gmail.com Download: http://econsultancyfzc.com/2011/10/material-download-c-c-optimization-techniques/ E-Consultancy FZC http://www.econsultancyfzc.com 2011