/Og (Global Optimizations) (original) (raw)

In this article

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

/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 AdditionalOptions.

See also

MSVC Compiler Command-Line Syntax

• Last updated on 2021-10-20