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Sethi Ullman numbering assumes

  • A.an expression tree with no shared subexpressions
  • B.a machine with an unlimited number of registers
  • C.that every operation costs exactly one cycle
  • D.that the expression contains only unary operators

Answer: A

Sharing turns the tree into a graph and the optimality argument no longer holds. The method also assumes a uniform register set with no special purpose registers. Real machines often violate both assumptions, so the result is a useful guide rather than a rule.

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