Intermediate Code Generation Questions and Answers

An intermediate representation is generated rather than target code directly in order to

A.decouple the source language front end from the machine dependent parts
B.make the compiled program run faster than it otherwise would
C.satisfy some requirement of the language standard being implemented
D.cut down how many tokens the scanner is obliged to emit

Answer: A

Correct answer: decouple the source language front end from the machine dependent parts

With an intermediate form, m languages and n machines need m front ends and n back ends rather than m times n whole compilers. It also gives the optimiser a single representation to work on, independent of both ends - which is why the same optimisation passes serve every language the compiler accepts.

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A team adds an eighth target machine to a compiler suite that already supports 6 languages through a common intermediate representation. How many new components must be written?

A.14
B.1
C.6
D.8

Answer: B

A common intermediate representation decouples front ends from back ends, so adding a target costs exactly one component: \[ \text{new components} = 1 \ \text{(back end)} \] Without the shared representation the team would need one translator per language, \( 6 \times 1 = 6 \) new components, and the full matrix for \( m \) languages and \( n \) targets would be \( m \times n \) rather than \( m + n \). Avoiding exactly that multiplication is what the design buys.

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An instruction of the form x = y in three address code uses

A.four addresses, since the operator itself counts as one
B.no addresses at all, since nothing is being computed
C.fewer than three addresses, which the form permits
D.exactly three addresses with one of them left unnamed

Answer: C

The limit is an upper bound rather than a requirement. Copies use two addresses, unconditional jumps one, and only binary operations use the full three, so the name describes the widest instruction rather than every instruction in the set.

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A three address instruction of the form x = y + z is best described as

A.a copy instruction moving a value from one place to another
B.a conditional jump taken when the sum is non zero
C.a procedure call passing two arguments to a routine
D.a binary assignment naming a destination and two operands

Answer: D

Correct answer: a binary assignment naming a destination and two operands

The form is called three address because it names three: the result and the two operands. Every complex expression is broken down into a sequence of such instructions with temporaries linking them, which is what makes the representation simple enough to optimise and to translate.

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The following are all forms of three address instruction EXCEPT

A.a nested expression of the form x = (a + b) * (c + d)
B.a conditional jump of the form if x relop y goto L
C.an indexed assignment of the form x = y[i]
D.a copy of the form x = y

Answer: A

The first three each name at most three addresses. A nested expression names more, so it must be broken into several instructions with temporaries holding the intermediate results. The flattening is exactly what the intermediate code generator does.

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The expression (a + b) * (c - d) is translated into three address code with one operator per instruction. How many instructions are produced?

Answer: B

With one operator per instruction, the instruction count equals the operator count. The expression \( (a+b)*(c-d) \) contains three operators, so \[ n = 3 \] namely \( t_1 = a + b \), \( t_2 = c - d \), and \( t_3 = t_1 * t_2 \). Three temporaries are introduced, the last holding the value of the whole expression.

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An expression tree contains 7 binary operators and no unary ones. Translating it with one operator per instruction yields how many temporaries?

A.8
B.14
C.7
D.6

Answer: C

Each binary operator yields exactly one result, and each result needs one temporary, so the counts are equal: \[ \text{temporaries} = \text{operators} = 7 \] Optimisation may afterwards reduce how many actually occupy storage, since a temporary that is consumed immediately need not survive; register allocation is where that reduction really happens. The count of 7 is the number generated, not the number ultimately stored.

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A temporary name in three address code is

A.a user declared variable that lives only briefly
B.a machine register allocated during code generation
C.a label marking the destination of a jump instruction
D.a compiler generated name holding an intermediate result

Answer: D

Correct answer: a compiler generated name holding an intermediate result

Temporaries are invented by the compiler to hold the partial results of an expression, and they appear nowhere in the source. They are not registers - allocating registers to them is a later and separate problem, and there are usually far more temporaries than the machine has registers.

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A quadruple representation of three address code records

A.an operator, two operands and a result field
B.an operator and three operand fields with no result
C.four operands with the operator implied by position
D.the four machine instructions that implement the operation

Answer: A

Correct answer: an operator, two operands and a result field

The four fields give the quadruple its name, and the explicit result field is what makes it robust: each instruction names its own destination, so instructions can be moved, deleted or reordered without disturbing anything that refers to them.

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10.

A triple representation differs from a quadruple in that it

A.records which machine registers are used by each instruction
B.omits the result field and refers to instructions by position
C.adds a fourth operand field to each instruction
D.stores the operator as a string rather than a code

Answer: B

Correct answer: omits the result field and refers to instructions by position

A triple saves the result field by letting an instruction be referred to by its own index in the sequence. That is more compact but couples every reference to the instruction’s position, so any reordering breaks it - which is why indirect triples were devised, adding a level of indirection to restore the freedom.

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Intermediate Code Generation Questions and Answers

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