Consider the grammar rule E → E1 – E2 for arithmetic expressions. The code generated is targeted to a CPU having a single user register. The subtraction operation requires the first operand to be in the register. If E1 and E2 do not have any common sub expression, in order to get the shortest possible code
(A) E1 should be evaluated first
(B) E2 should be evaluated first
(C) Evaluation of E1 and E2 should necessarily be interleaved
(D) Order of evaluation of E1 and E2 is of no consequence
E -> E1 – E2
Given that E1 and E2 don’t share any sub expression, most optimized usage of single user register for evaluation of this production rule would come only when E2 is evaluated before E1. This is because when we will have E1 evaluated in the register, E2 would have been already computed and stored at some memory location. Hence we could just use subtraction operation to take the user register as first operand, i.e. E1 and E2 value from its memory location referenced using some index register or some other form according to the instruction.
Hence correct answer should be (B) E2 should be evaluated first.
This Solution is contributed by Vineet Purswani.
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