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A peephole pass is normally run after every other back end pass because

  • A.no other pass is permitted to alter the generated instructions
  • B.the target machine requires the code to be scanned once more
  • C.it is cheap and each earlier pass leaves new local redundancies
  • D.it is the only pass able to allocate the machine registers

Answer: C

Selection, allocation and scheduling each work on their own concerns and each leaves behind small local inefficiencies. The window is short enough that the pass costs almost nothing, so running it once more usually finds something worth removing.

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