Reaching definition

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In compiler theory, a reaching definition for a given instruction is an earlier instruction whose target variable can reach (be assigned to) the given one without an intervening assignment. For example, in the following code:

d1 : y := 3
d2 : x := y

d1 is a reaching definition for d2. In the following, example, however:

d1 : y := 3
d2 : y := 4
d3 : x := y

d1 is no longer a reaching definition for d3, because d2 kills its reach: the value defined in d1 is no longer available and cannot reach d3.

As analysis[edit]

The similarly named reaching definitions is a data-flow analysis which statically determines which definitions may reach a given point in the code. Because of its simplicity, it is often used as the canonical example of a data-flow analysis in textbooks. The data-flow confluence operator used is set union, and the analysis is forward flow. Reaching definitions are used to compute use-def chains.

The data-flow equations used for a given basic block in reaching definitions are:

In other words, the set of reaching definitions going into are all of the reaching definitions from 's predecessors, . consists of all of the basic blocks that come before in the control flow graph. The reaching definitions coming out of are all reaching definitions of its predecessors minus those reaching definitions whose variable is killed by plus any new definitions generated within .

For a generic instruction, we define the and sets as follows:

  • , a set of locally available definition in a basic block
  • , a set of definitions (not locally available, but in rest of program) killed by definitions in the basic block.

where is the set of all definitions that assign to the variable . Here is a unique label attached to the assigning instruction; thus, the domain of values in reaching definitions are these instruction labels.

Worklist Algorithm[edit]

Reaching definition is usually calculated using an iterative worklist algorithm.

Input: control flow graph CFG = (Nodes, Edges, Entry, Exit)

// Initialize
for all CFG nodes n in N,  
  OUT[n] = emptyset; // can optimize by OUT[n] = GEN[n];

// put all nodes into the changed set
// N is all nodes in graph, 
Changed = N; 

//Iterate 
while (Changed != emptyset)
{
  choose a node n in Changed;
  // remove it from the changed set
  Changed = Changed -{ n }; 

  // init IN[n] to be empty
  IN[n] = emptyset;  

  // calculate IN[n] from predecessors' OUT[p]
  for all nodes p in predecessors(n) 
     IN[n] = IN[n] Union OUT[p]; 

  oldout = OUT[n]; // save old OUT[n]
  
  // update OUT[n] using transfer function f_n ()
  OUT[n] = GEN[n] Union (IN[n] -KILL[n]);   

  // any change to OUT[n] compared to previous value?
  if (OUT[n] changed) // compare oldout vs. OUT[n]
  {  
    // if yes, put all successors of n into the changed set
    for all nodes s in successors(n) 
       Changed = Changed U { s };  
  }
}

Further reading[edit]

See also[edit]