# /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.

/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 (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` 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 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. Click the C/C++ folder.

3. Click the Command Line property page.

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

## See also

MSVC Compiler Command-Line Syntax