og-global-optimizations.md

title	description	ms.date	f1_keywords	helpviewer_keywords	ms.assetid
/Og (Global Optimizations)	Describes the deprecated MSVC compiler option /Og, formerly used to enable global optimizations.	10/19/2021	VC.Project.VCCLCompilerTool.GlobalOptimizations /og	-Og compiler option [C++] global optimizations compiler option [C++] automatic register allocation /Og compiler option [C++] loop structures, optimizing common subexpression elimination Og compiler option [C++]	d10630cc-b9cf-4e97-bde3-8d7ee79e9435

/Og (Global Optimizations)

Deprecated. Provides local and global optimizations, automatic-register allocation, and loop optimization. We recommend you use either /O1 (Minimize Size) or /O2 (Maximize Speed) instead.

Syntax

/Og

Remarks

/Og is deprecated. These optimizations are now enabled by default when any optimizations are enabled. For more information on optimizations, see /O1, /O2 (Minimize Size, Maximize Speed), or /Ox (Enable Most Speed Optimizations).

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 don't change between the three expressions, the compiler can assign the calculation of b + c to a temporary variable, and use that 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 by default, and causes a diagnostic under /std:c++17 or later.

• 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 doesn't 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 together with the g option.

For related information, see /Oi (Generate intrinsic functions) and /Ox (Enable most speed optimizations).

To set this compiler option in the Visual Studio development environment

1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

2. Select the Configuration Properties > C/C++ > Command Line property page.

3. Enter the compiler option in the Additional Options box.

To set this compiler option programmatically

• See xref:Microsoft.VisualStudio.VCProjectEngine.VCCLCompilerTool.AdditionalOptions%2A.

See also

MSVC Compiler Command-Line Syntax