/Og - Enable Global Optimization (Windows CE 5.0)

Windows CE 5.0
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This option provides local and global optimizations, automatic-register allocation, and loop optimization.

  • Local and global common subexpression elimination

    In this optimization, the value of a common subexpression is calculated once. In the following code 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 code 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 /Oa - Assume No Aliasing or /Ow - Assume Aliasing Across Function Calls.

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

See Also

Compiler Options | /Oa - Assume No Aliasing | /Oi - Generate Intrinsic Functions | /Op, /Op - Improve Floating-Point Consistency | /Ox - Use Maximum Optimization for Speed

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