A syntax directed definition for computing the width of a record type uses
Answer: A
The total is built from the parts, so the flow is upward. Field offsets, by contrast, require a running total passed along the list and are naturally inherited. A record type therefore needs both directions of flow in the same definition.
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Computing the offset of each field within a record requires
Answer: B
Correct answer: an inherited attribute carrying the running total so far
Each field is placed at the current total and advances it, so the offset must be passed along the list from left to right. That is an inherited attribute, and it makes the record layout definition L-attributed - one of the standard illustrations of why inherited attributes are needed at all.
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A syntax directed definition in which an inherited attribute of a symbol depends on a symbol to its right is
Answer: C
The L condition forbids exactly this dependency. Such a definition may still be evaluable, but it needs multiple passes or an explicit topological sort of the dependency graph. Both are avoided in practice by restructuring the grammar instead.
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Multiple pass attribute evaluation is required when
Answer: D
Correct answer: the definition is neither S-attributed nor L-attributed
Both restricted classes can be evaluated in a single traversal. A definition outside them may have dependencies that no single left-to-right pass satisfies, so the tree must be traversed more than once, with a dependency analysis deciding what can be computed on each pass.
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Yacc supports inherited attributes only awkwardly because
Answer: A
Information cannot flow into a subtree that has already been parsed. Reaching down the value stack to a known offset is the usual trick, though it is fragile if the grammar changes. A marker nonterminal achieves the same effect far more robustly.
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Reaching into the Yacc value stack at a negative offset to read an earlier value is
Answer: B
Correct answer: a fragile technique that breaks if the grammar is changed
Reading below the current production’s entries on the value stack works only if the surrounding productions always place the same thing there, which is an assumption the grammar may violate the moment it is edited. Marker nonterminals achieve the same effect explicitly and survive change.
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An abstract syntax tree can itself be built as
Answer: C
Each rule builds a node from the nodes returned by its children, so the tree assembles itself bottom up. This is one of the most common S-attributed translations in any compiler. Each node holds pointers to the nodes its children returned.
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Building an abstract syntax tree during the parse rather than acting immediately allows
Answer: D
Optimisation and analysis often need repeated traversals. An explicit tree makes those possible, at the cost of the memory needed to hold the whole program's structure. Optimising compilers accept that cost without hesitation.
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A one pass compiler that emits code directly from semantic actions is restricted because
Answer: A
Correct answer: it cannot revisit earlier parts of the program after moving on
A one pass compiler emits code as it parses and never returns, so any decision requiring information from later in the program cannot be made properly - a forward reference, or an optimisation depending on how a variable is used further on. Backpatching handles the simplest of these; the rest require a second pass.
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The action associated with a production in a bottom up translation executes
Answer: B
Reduction is the only moment at which the parser knows the production was used. This is why every action other than one at the very end of a right side needs a marker nonterminal. The marker is inserted silently by the generator in most tools.
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