星期六, 九月 30, 2006

C++ Code Optimizations - Mozilla Firefox

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C++ Optimizations

These optimizations are fairly easy to apply to existing code and in some cases can result in big speedups. Remember the all-important maxim though, the fastest code is code that isn't called.

Use Initialization Lists

Always use initialization lists in constructors. For example, use

    TMyClass::TMyClass(const TData &data) : m_Data(data)
    {
    }

rather than

    TMyClass::TMyClass(const TData &data)
    {
         m_Data = data;
    }

Without initialization lists, the variable's default constructor is invoked behind-the-scenes prior to the class's constructor, then its assignment operator is invoked. With initialization lists, only the copy constructor is invoked.



Optimize For Loops

Whereever possible, count down to zero rather than up to n. For example, use

    for (i = n-1; i >= 0; --i)

rather than

    for (i = 0; i < n; ++i)

The test is done every iteration and it's faster to test against zero than anything else. Note also that

    ++i

is faster than

    i++

when it appears in the third part of the for loop statement.



Use 'int'

Always use the int data type instead of char or short wherever possible. int is always the native type for the machine.



Make Local Functions Static

Always declare local functions as static, e.g.,

    static void foo()

This means they will not be visible to functions outside the .cpp file, and some C++ compilers can take advantage of this in their optimizations.



Optimize If Statements

Factor out jumps. For example, use

    bar();
    if (condition)
    {
         undoBar();
         foo();
    }

rather than

    if (condition)
    {
         foo();
    }
    else
    {
         bar();
    }

Use a profiler and good judgement to decide if undoing the bar() operation is faster than jumping.



Optimize Switch Statements

Put the most common cases first.



Avoid Expensive Operations

Addition is cheaper than multiplication and multiplication is cheaper than division. Factor out expensive operations whereever possible.



Initialize on Declaration

Whereever possible, initialize variables at the time they're declared. For example,

    TMyClass myClass = data;

is faster than

    TMyClass myClass;
    myClass = data;

Declaration then initialization invokes the object's default constructor then its assignment operator. Initializing in the declaration invokes only its copy constructor.



Pass By Reference

Always try to pass classes by reference rather than by value. For example, use

    void foo(TMyClass &myClass)

rather than

    void foo(TMyClass myClass)



Delay Variable Declarations

Leave variable declarations right until the point when they're needed. Remember that when a variable is declared its constructor is called. This is wasteful if the variable is not used in the current scope.



Use 'op='

Wherever possible, use 'op=' in favour of 'op'. For example, use

    myClass += value;

rather than

    myClass = myClass + value;

The first version is better than the second because it avoids creating a temporary object.



Inline Small Functions

Small, performance critical functions should be inlined using the inline keyword, e.g.,

    inline void foo()

This causes the compiler to duplicate the body of the function in the place it was called from. Inlining large functions can cause cache misses resulting in slower execution times.



Use Nameless Classes

Whereever possible, use nameless classes. For example,

    foo(TMyClass("abc"));

is faster than

    TMyClass myClass("abc");
    foo(myClass);

because, in the first case, the parameter and the class share memory.



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