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The phase ordering problem in optimisation refers to

  • A.the compiler running its phases in the wrong sequence
  • B.the sheer impossibility of parsing before the source has been scanned
  • C.the need to allocate registers before generating code
  • D.one transformation creating or destroying opportunities for another

Answer: D

Correct answer: one transformation creating or destroying opportunities for another

Inlining exposes constants that propagation can use; propagation exposes dead code; dead code elimination may make a loop invariant. But an ordering good for one program may be poor for another, and no single sequence is best in general - which is why compilers run some passes repeatedly rather than once each.

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