GATE CS 2015 Set 2 — Question 28
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Compiler Design → Code Optimization → Basic Blocks & Flow Graphs
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Question
In the context of abstract-syntax-tree (AST) and control-flow-graph (CFG), which one of the following is TRUE?
Correct answer
(C) The maximum number of successors of a node in an AST and a CFG depends on the input program
Solution
Let's evaluate each statement:
- (A) is FALSE. In a CFG, a back-edge (representing a loop) can have a successor node that corresponds to code appearing before the code for in the source program.
- (B) is FALSE. While an AST is a tree and thus acyclic, a CFG for a program with loops will contain cycles.
- (C) is TRUE. The number of successors of a node in a CFG can vary based on the program structure (e.g., a switch statement can have many successors). Similarly, in an AST, a node representing a function call or a complex expression can have a variable number of children depending on the input program.
- (D) is FALSE. In an AST, nodes often represent expressions or sub-expressions, which are parts of a statement, not necessarily a full statement. In a CFG, a basic block (node) can contain multiple statements.
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