Syntax Directed Translation Questions and Answers

31.

Type checking is naturally expressed as a syntax directed translation because

A.the type of an expression depends on whichever target machine is chosen
B.type checking must be performed before the parse has begun
C.the type of an expression is synthesised from those of its operands
D.types are inherited from the enclosing statement in every case

Answer: C

Correct answer: the type of an expression is synthesised from those of its operands

The type of a sum follows from the types of its two operands, which follow from theirs, all the way down to the identifiers and constants. That is a purely synthesised computation, so type checking fits an S-attributed definition and can be done during a bottom up parse.

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32.

A coercion inserted where an integer meets a floating point operand is recorded during translation as

A.a change to the grammar production that matched the expression
B.an error reported to the user requiring an explicit cast
C.a remark recorded against the enclosing procedure in the name table
D.a conversion node added to the tree by the semantic rule

Answer: D

The rule detects the mismatch and inserts an explicit conversion so that later phases see consistent types. Whether a coercion is permitted at all is a decision of the language definition. Some languages forbid every implicit conversion and require an explicit cast.

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33.

Translating an expression into three address code by syntax directed translation typically gives each nonterminal

A.a place attribute naming the temporary that holds its value
B.a type attribute and nothing else besides that
C.an inherited attribute naming the target register to use
D.no attribute at all

Answer: A

The place attribute names where the value lives, usually a compiler generated temporary. The code itself is either another attribute or is emitted directly as a side effect. Which choice is made affects whether fragments can be reordered afterwards.

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34.

A new temporary name is created during expression translation by

A.the register allocator during the code generation phase
B.a function that returns a fresh name on each call
C.reusing the name of the leftmost operand of the expression
D.asking the symbol table for an unused user identifier

Answer: B

A counter guarantees a distinct name for each intermediate result. Reducing the number of temporaries is left to later optimisation rather than to the translation itself. Keeping the translation simple makes it far easier to verify as correct.

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35.

The expression a + b * c is translated into three address code with one operation per instruction. How many temporaries are introduced?

Answer: C

The multiplication produces one temporary and the addition a second, giving two in all. Each binary operator in the tree contributes exactly one temporary under this scheme. Counting operators is therefore enough to predict the number of temporaries.

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36.

An expression tree contains 5 binary operators. Translating it with one operation per three address instruction produces how many instructions?

A.6
B.4
C.10
D.5

Answer: D

Each operator becomes one instruction, so five are emitted. The number of temporaries introduced is also five, one holding the result of each operation. Later optimisation may eliminate several of them by reusing registers.

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37.

Short circuit evaluation of a boolean expression is expressed in a translation scheme using

A.inherited attributes holding the true and false jump targets
B.synthesised attributes holding the value of the expression
C.a single temporary holding the result of the whole expression
D.no attributes at all

Answer: A

Passing the destinations down the tree lets each subexpression emit a jump directly to the right place. This avoids computing a value that would immediately be tested and discarded. The saving is substantial in code full of compound conditions.

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38.

Backpatching in a syntax directed translation is used because

A.the parser must be able to reduce without consulting the lookahead
B.the destination of a jump is not known when the jump is emitted
C.the semantic rules are evaluated in an unpredictable order
D.the grammar cannot be made unambiguous by any transformation

Answer: B

Correct answer: the destination of a jump is not known when the jump is emitted

Translating a conditional or a loop in one pass means emitting the jump before the code it jumps over has been generated, so the destination cannot yet be known. The jump is emitted with the field blank, its position is recorded on a list, and the list is patched when the label is finally reached.

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39.

The lists maintained during backpatching hold

A.all of the productions that have been reduced during the parse
B.the temporaries that have been created and not yet used
C.the indices of instructions whose target field is still blank
D.the attribute values of every nonterminal seen so far

Answer: C

Merging two such lists is how the true exits of a subexpression are combined. When the destination becomes known every instruction on the list is patched in one sweep. The lists are merged as subexpressions combine, which keeps the cost linear.

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40.

Match each translation task with the attribute flow it mainly uses.
List I
A. computing the value of an expression
B. passing a declared type to a list of names
C. passing jump targets into a boolean expression
D. computing the width of a record type
List II
1. inherited
2. synthesised
3. synthesised
4. inherited

A.A-4, B-2, C-1, D-3
B.A-2, B-4, C-3, D-1
C.A-3, B-1, C-4, D-2
D.A-2, B-4, C-1, D-3

Answer: D

Values built from parts flow upward while context supplied from outside flows downward. Recognising which applies is what determines whether the definition can be S-attributed. A single downward flow anywhere forces the L-attributed treatment.

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