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A conservative approximation in data flow analysis is one that

  • A.computes the exact answer in every possible case
  • B.requires the program to contain no loops at all
  • C.may miss an optimisation but never enables an unsafe one
  • D.enables every optimisation but occasionally breaks the program

Answer: C

Correct answer: may miss an optimisation but never enables an unsafe one

Every data flow analysis errs deliberately, and the direction of the error is chosen so that uncertainty costs performance rather than correctness. Being told a variable might be live when it is not merely wastes a register; being told it is dead when it is not corrupts the program.

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