/Og (Global Optimizations)

 

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The latest version of this topic can be found at -Og (Global Optimizations).

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

Syntax

/Og  

Remarks

/Og is deprecated. These optimizations are now generally enabled by default. For more information on optimizations, see /O1, /O2 (Minimize Size, Maximize Speed) or /Ox (Full Optimization).

The following optimizations are available under /Og:

• 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.

  Note

  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).

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

• See AdditionalOptions.

See Also

/O Options (Optimize Code)
Compiler Options
Setting Compiler Options