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GO Classes DPP | COMPILER|DATA DEPENDENCY

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1 1 vote

Consider the following $C$ code segment:
 

A[i] = A[i] + 1; // Line 1
j = i + 1;       // Line 2
A[j] = A[i] * 2; // Line 3
k = i;           // Line 4
A[k] = A[k] + 1; // Line 5


In the context of data-flow analysis for code optimization, we are interested in Write-After-Read (WAR) dependencies.

Ignoring all loops and function calls, the number of distinct Write-After-Read (WAR) data dependencies in this code segment is $\_\_\_\_$

1 Answer

2 2 votes

Dependency $L_3 \rightarrow L_5$ (The potential WAR):
 

  • $L_3$ : Reads $\mathbf{A}[\mathbf{i}]$ (The location at index $i$)
     
  • $L_5$ : Writes $\mathbf{A}[\mathbf{k}]$ (The location at index $k$)
     


Since $L_4$ sets $\mathbf{k}=\mathbf{i}$, the locations are guaranteed to be the same: $\mathbf{A}[\mathbf{i}]$ is the same memory cell as $\mathbf{A}[\mathbf{k}]$.

  • $L_3$ reads $\mathbf{A}[\mathbf{i}]$.
     
  • $L_5$ writes $\mathbf{A}[\mathbf{i}]$.
     
  • Therefore, a WAR dependency $\mathbf{L}_3 \rightarrow \mathbf{L}_5$ exists.
     

Hence the correct answer is 1

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