Improving Other Optimization

This section contains coding hints that increase optimization for the other passes of the compiler.

C and Fortran Programs

• Use tables rather than if-then-else or switch statements. For example:

  typedef enum { BLUE, GREEN, RED, NCOLORS } COLOR;

  Instead of:

  switch ( c ) {

  case CASE0: x = 5; break;

  case CASE1: x = 10; break;

  case CASE2: x = 1; break;

  }

  Use:

  static int Mapping[NCOLORS] = { 5, 10, 1 };

  ...

  x = Mapping[c];

• As an optimizing technique, the compiler puts the first eight parameters of a parameter list into registers where they may remain during execution of the called routine. Therefore, always declare, as the first eight parameters, those variables that are most frequently manipulated in the called routine.

• Use word-size scalar variables instead of smaller ones. This practice can take more data space, but produces more efficient code.

C Programs Only

• Rely on libc.so functions (for example, strcpy, strlen, strcmp, bcopy, bzero, memset, and memcpy). These functions are carefully coded for efficiency.

• Use a signed data type, or cast to a signed data type, for any variable that does not require the full unsigned range and must be converted to floating-point. For example:

  double d;

  unsigned int u;

  int i;

  /* fast */ d = i;

  /* fast */ d = (int)u;

  /* slow */ d = u;

  Converting an unsigned type to floating-point takes significantly longer than converting signed types to floating-point; additional software support must be generated in the instruction stream for the former case.

• Use signed ints in 64-bit code if they may appear in mixed type expressions with long ints. Since the hardware automatically sign-extends the results of most 32-bit operations, this may avoid explicit zero-extension code. For example:

  unsigned int ui;

  signed int i;

  long int li;

  /* fast */ li += i;

  /* fast */ li += (int)ui;

  /* slow */ li += ui;