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Guarding each transformation with an analysis that establishes when it is safe is necessary because

  • A.the analysis makes the transformation run faster at compile time
  • B.the language standard requires every optimiser to be documented
  • C.without it the optimiser could not find any opportunities at all
  • D.an unsound transformation yields a program that is fast and wrong

Answer: D

Speed is worthless if the answer changes, so every pass carries with it the conditions under which it preserves meaning. Where the analysis cannot establish those conditions the transformation is simply not applied, which costs an opportunity rather than correctness.

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