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In a simplified computer, the instructions are: - `OP Rj, Ri` — performs `Rj OP Ri` and stores the result in register `Ri`. - `OP m, Ri` — performs `val OP Ri` and stores the result in register `Ri`, where `val` denotes the content of memory location `m`. - `MOV m, Ri` — moves the content of memory location `m` to register `Ri`. - `MOV Ri, m` — moves the content of register `Ri` to memory location `m`. The computer has only two registers, and `OP` is either `ADD` or `SUB`. Consider the following basic block: ```text t1 = a + b t2 = c + d t3 = e - t2 t4 = t1 - t3 ``` Assume that all operands are initially in memory and the final value of the computation should be in memory. What is the minimum number of `MOV` instructions in the code generated for this basic block?

1. 2
2. 3
3. 5
4. 6

Correct answer: 3

Solution

Because only two registers are available, some intermediate values must be kept in memory or recomputed. An efficient schedule can compute `t1`, then `t2`, then `t3`, and finally `t4` with only three `MOV` instructions. The minimum is 3.

Related GATE Technical questions

• Consider the following statements. S1: Every SLR(1) grammar is unambiguous, but there are certain unambiguous grammars that are not SLR(1). S2: For every grammar, there is a parser that takes at most linear time to parse a string of length $n$. Which of the following is correct?
• Which of the following is/are bottom-up parser(s)?
• Which ONE of the following statements is FALSE regarding the symbol table?
• Which ONE of the following techniques used in compiler code optimization uses live variable analysis?
• Which of the following statement(s) is/are TRUE while computing FIRST and FOLLOW during top-down parsing by a compiler?
• Which one of the following is a top-down parser?

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