GATE CS 2026 Set 1 — Question 42
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Compiler Design → Code Optimization → Basic Blocks & Flow Graphs
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Question
Consider the control flow graph shown in the figure.
Which one of the following options correctly lists the set of redundant expressions (common subexpressions) in the basic blocks B4 and B5?Note: All the variables are integers.
Correct answer
(D) B4: \ g k \ B5: \ \
Solution
An expression is redundant (available) at a point if it has been computed on every path reaching that point and its operands have not been modified since the computation.For Block B4:
- Predecessors are B2 and B3.
- Expression :
- Computed in B2 (). Operands not modified.
- Computed in B3 (). Operands not modified.
- Available on all paths to B4. Redundant.
- Expression :
- Computed in B1 ().
- Path B1 B2 B4: not modified in B2. Available.
- Path B1 B3 B4: B3 contains . This modifies . Not available.
- Not available on all paths. Not redundant.
- Predecessor is B4.
- Expression :
- Computed in B3 ().
- Path B1 B3 B4 B5: Available.
- Path B1 B2 B4 B5: B2 does not compute . Not available.
- Not available on all paths. Not redundant.
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