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/Og (Global Optimizations)

Provides local and global optimizations, automatic-register allocation, and loop optimization. Deprecated.

/Og

The following optimizations are available:

  • Local and global common subexpression elimination

    In this optimization, the value of a common subexpression is calculated once. In the following example, if the values of b and c do not change between the three expressions, the compiler can assign the calculation of b + c to a temporary variable, and substitute the variable for b + c:

    a = b + c;
    d = b + c;
    e = b + c;
    

    For local common subexpression optimization, the compiler examines short sections of code for common subexpressions. For global common subexpression optimization, the compiler searches entire functions for common subexpressions.

  • Automatic register allocation

    This optimization allows the compiler to store frequently used variables and subexpressions in registers; the register keyword is ignored.

  • Loop optimization

    This optimization removes invariant subexpressions from the body of a loop. An optimal loop contains only expressions whose values change through each execution of the loop. In the following example, the expression x + y does not change in the loop body:

    i = -100;
    while( i < 0 ) {
        i += x + y;
    }
    

    After optimization, x + y is calculated once rather than every time the loop is executed:

    i = -100;
    t = x + y;
    while( i < 0 ) {
        i += t;
    }
    

    Loop optimization is much more effective when the compiler can assume no aliasing, which you set with __restrict, noalias, or restrict.

    NoteNote

    You can enable or disable global optimization on a function-by-function basis using the optimize pragma with the g option.

/Og also enables the Named Return Value optimization, which eliminates the copy constructor and destructor of a stack based return value. See /O1, /O2 (Minimize Size, Maximize Speed) for more information.

For related information, see Generate Intrinsic Functions (/Oi) and Full Optimization (/Ox).

/Og is deprecated; use either /O1 or /O2. For more information, see Deprecated Compiler Options.

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see How to: Open Project Property Pages.

  2. Click the C/C++ folder.

  3. Click the Command Line property page.

  4. Type the compiler option in the Additional Options box.

To set this compiler option programmatically

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