Extract from "Introduction to Program Optimizers"

scalar-replacement-of-aggregates.txt

Scalar Replacement of Aggregates
================================

Scalar replacement of aggregates makes other optimizations applicable to components of aggregates, such as C structures and Pascal records. It is a simple and effective optimization. It checks which components of the aggregate are simple scalars, and if they are not aliased (neither the components nor the whole aggregate), they are assigned to temporaries of the appropriate type. These temporaries become available to many optimizations.
The following example demonstrates how scalar replacement of Aggregates help other optimizations:

typedef enum { APPLE, BANANA, ORANGE } VARIETY;
typedef enum { LONG, ROUND } SHAPE;
typedef struct fruit {
VARIETY variety;
SHAPE shape; } FRUIT;
char* Red = "red" ;
char* Yellow = "yellow";
char* Orange = "orange";

char* color (CurrentFruit)
 FRUIT *CurrentFruit;
{   switch   (CurrentFruit->variety) {
      case APPLE:   return Red;
                    break;          
      case BANANA:  return Yellow;
                    break;         
      case ORANGE:  return Orange;
                    break;  
      }
} 

main()
{ Fruit snack;
  snack.variety = APPLE;
  snack.shape = ROUND;
  printf(“%s\n”,color(&snack));
}

Main procedure resulting from procedure integration and scalar replacement of aggregates (applied for the components of structure snack):

char* Red = "red" ;
char* Yellow = "yellow";
char* Orange = "orange";

main()
{ FRUIT snack
  VARIETY t1;
  SHAPE   t2;
  COLOR   t3;
  t1=APPLE;
  t2=ROUND;
  switch (t1) {
    case APPLE:  t3= Red;
                 break;  
    case BANANA: t3= Yellow;
                 break;          
    case ORANGE: t3= Orange;
                 break; 
  }
  printf(“%s\n”,t3);
}
And after constant propagation and dead-code elimination:

main()
{ printf(“%s\n” ,”red”);
}