/Og - Enable Global Optimization (Windows CE 5.0)

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:

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  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:

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  i = -100;
  while( i < 0 )
  {
      i += x + y;
  }
  

  After optimization, x + y is calculated once rather than every time the loop is executed:

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  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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