GATE CS 2017 Set 2 Q.5 (CS Q.5): Lexical Analysis

Match the following according to input (from the left column) to the compiler phase (in the right column) that processes it:

Input	Compiler Phase
(P) Syntax tree	(i) Code generator
(Q) Character stream	(ii) Syntax analyzer
(R) Intermediate representation	(iii) Semantic analyzer
(S) Token stream	(iv) Lexical analyzer

P $\to$ (ii), Q $\to$ (iii), R $\to$ (iv), S $\to$ (i)

P $\to$ (ii), Q $\to$ (i), R $\to$ (iii), S $\to$ (iv)

P $\to$ (iii), Q $\to$ (iv), R $\to$ (i), S $\to$ (ii)

P $\to$ (i), Q $\to$ (iv), R $\to$ (ii), S $\to$ (iii)

Official answer and solution

Solution

Identify the standard input consumed by each compiler phase in the classic front-end and back-end pipeline.

Trace the phases of a compiler:

Lexical analyzer: Reads the source program as a character stream and groups characters into tokens (token stream). Thus, input (Q) Character stream is processed by phase (iv) Lexical analyzer.
Syntax analyzer (parser): Takes the token stream from the lexical analyzer and constructs a parse tree or abstract syntax tree. Thus, input (S) Token stream is processed by phase (ii) Syntax analyzer.
Semantic analyzer: Takes the syntax tree and performs type checking, scope resolution, and semantic validations. Thus, input (P) Syntax tree is processed by phase (iii) Semantic analyzer.
Code generator: Takes intermediate representation (IR) code and produces target machine code. Thus, input (R) Intermediate representation is processed by phase (i) Code generator.

Combining the pairs: P $\to$ (iii), Q $\to$ (iv), R $\to$ (i), S $\to$ (ii), which corresponds to option C.

Final answer

Common trap

Confusing the input of a compiler phase with its output.

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