C++ Optimization Tips
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This document discusses various C++ optimization tips, including: 1. Using initialization lists to initialize member variables directly rather than assigning in the constructor body. 2. Avoiding unnecessary temporaries by modifying string concatenation to use addition assignment operators rather than creating multiple temporary strings. 3. Using memory pooling to allocate objects from a pre-allocated block of memory rather than individually, reducing memory overhead. 4. Inlining small, frequently called methods to remove method invocation overhead and allow better optimization by the compiler. Conditional inclusion and selective inlining techniques are presented.
C++ Optimization Tips
- 1. C++ Optimization TipsAbdelrahman Al-Ogail05-12-2010
- 2. AgendaHello WorldDoes it deserve?Initialization listTemporariesMemory poolingInlining
- 3. Hello WorldWhat we mean by “fast”?When to think about optimization?What to optimize?Fast path.
- 4. Does it deserve?//******************* VERSION 0 *******************\\voidMyFunc(){ string x = “1234568” ;}void main(){// Start timing here for (int i = 0; i < 1000000; i++) {MyFunc(); } // Stop timing here}
- 5. Does it deserve?//******************* VERSION 1 *******************\\voidMyFunc(){char* x = “1234568” ;}void main(){// Start timing here for (int i = 0; i < 1000000; i++) {MyFunc(); } // Stop timing here}
- 7. Initialization list//******************* VERSION 0 *******************\\classMyClass{public:MyClass(strings1, string s2, string s3) {_s1 = s1; _s2 = s2; _s3 = s3; }private:string _s1, _s2, _s3;};void main(){// Start timing here for (int i = 0; i < 1000000; i++) {MyClassinstance("123456", "123456", "123456"); } // Stop timing here}
- 8. Initialization list//******************* VERSION 1 *******************\\classMyClass{public:MyClass(strings1, string s2, string s3) : _s1(s1), _s2(s2), _s3(s3){}private:string _s1, _s2, _s3;};void main(){// Start timing here for (int i = 0; i < 1000000; i++) {MyClassinstance("123456", "123456", "123456"); } // Stop timing here}
- 10. Temporaries//******************* VERSION 0 *******************\\void main(){// Start timing here for (int i = 0; i < 1000000; i++) { string s1, s2, s3, s4, s5; s1 = s2 + s3 + s4 + s5; } // Stop timing here}
- 11. Temporaries//******************* VERSION 1 *******************\\void main(){// Start timing here for (int i = 0; i < 1000000; i++) { string s1, s2, s3, s4, s5; s1 = s2; s1 += s3; s1 += s4; s1 += s5; } // Stop timing here}
- 12. Temporaries
- 13. Memory PoolingSize of memory to allocate:Static.Dynamic.Environment:Single Threaded.Multi-Threaded.
- 14. Quote of Wisdom“Special circumstances allow for simplifying assumptions that in turn, provide opportunities for significant optimizations”
- 15. Memory Poolingvoid main(){ MyMemoryManager<MyClass> memManager; memManager.Reserve(10); MyClass* instance = memManager.Alloc();// Some code goes here… // Deallocate the allocated instancememMnagaer.DeAlloc(instance); // Some code goes here…memManager.Free();}
- 16. Memory Pooling
- 17. InliningWhy Inlining?Removes method invocation overhead.Let compiler able to optimize code.Why Not Inlining:Code expansion (page fault, exe image size).Linkage problems.Code recompilation.
- 18. InliningSome of method invocation overhead*:Passing method parametersCreative stack frame for new callBranching (IP)Passing return values.* architecture depended.
- 19. InliningWhat to inline:SingletonsTrivial methodsFast path most frequent method calls.
- 20. Inlining – Conditional Inlingx.hx.inlx.cpp#if !defined(_X_H_)#define_X_H_classX{...inty (inta );...};#ifdefined(INLINE)#includex.inl#endif#endif// _X_H_#if !defined(INLINE)#defineinline#endifinlineintX::y (inta){ ...}#if !defined(INLINE)#includex.inl#endif
- 21. Inlining – Selective InlingFile: x.inl:inlineintx::inline_y (inta){ ... // original implementation of y}File: x.h:classx {public: //assuming the original method "y" had public visibility ---intinline_y (inta);inty (inta); // this should already have been here ---};#include"x.inl"// this may already have been here alsoFile: x.c:intx::y (inta){returninline_y(a);}
- 22. Thanks!