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Constructing a directed acyclic graph for a basic block reveals

  • A.the syntax errors that the parser failed to detect
  • B.the number of passes the compiler will need to make
  • C.common subexpressions and values that are never used
  • D.the registers that the target machine has available

Answer: C

A node with no parent computes a value nobody consumes, and a shared node is a repeated computation. Both observations feed directly into local optimisation of the block. The graph is therefore built as much for analysis as for representation.

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