Constant propagation

Constant propagation (cprop) is an optimization preformed by compilers. After cprop experssions that can be calculated at compiletime will be replaced by their value. Constant propagation is also able to change conditional branches to unconditional ones. The following code in C can be simplified using cprop:

 
 int a(){
   int b;
   int c;

   
   b=3;
   c=b*4;
   if(c>10){
     c=c-10;
   }
   return c;

}

A good compiler will reduce this to:

 
 int a(){
   return 2;
 }

Easily be implemented on SSA form as published by Wegman and Zadeck in 1991.

Do not confuse constant propagation with constant folding, which is implemented in the front-end.

See also: Control flow graph, Compiler optimization