Optimizing expressions and program logic

Consider the following guidelines:

• If components of an expression are used in other expressions and they include function calls or there are function calls between the uses, assign the duplicated values to a local variable.
• Avoid forcing the compiler to convert numbers between integer and floating-point internal representations. For example:

  float array[10];
  float x = 1.0;
  int i;
  for (i = 0; i< 9; i++)  {     /* No conversions needed */
       array[i] = array[i]*x;
       x = x + 1.0;
      }
  for (i = 0; i< 9; i++)  {       /* Multiple conversions needed */
       array[i] = array[i]*i;
      }

  When you must use mixed-mode arithmetic, code the integer and floating-point arithmetic in separate computations whenever possible.

• Do not use global variables as loop indices or bounds.
• Avoid goto statements that jump into the middle of loops. Such statements inhibit certain optimizations.
• Improve the predictability of your code by making the fall-through path more probable. Code such as:

  if (error) {handle error} else {real code}

  should be written as:

  if (!error) {real code} else {error}

• If one or two cases of a switch statement are typically executed much more frequently than other cases, break out those cases by handling them separately before the switch statement. If possible, replace the switch statement by checking whether the value is in range to be obtained from an array.
• Use try blocks for exception handling only when necessary because they can inhibit optimization.
• Keep array index expressions as simple as possible.