GATE CS 2015 Set 2 — Question 44
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Compiler Design → Code Optimization → Basic Blocks & Flow Graphs
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Question
Consider the intermediate code given below.
The number of nodes and edges in the control-flow-graph constructed for the above code, respectively, are
(1) i = 1
(2) j = 1
(3) t1 = 5 * i
(4) t2 = t1 + j
(5) t3 = 4 * t2
(6) t4 = t3
(7) a[t4] = -1
(8) j = j + 1
(9) if j<=5 goto (3)
(10) i=i+1
(11) if i<5 goto (2)
The number of nodes and edges in the control-flow-graph constructed for the above code, respectively, are
Correct answer
(B) 6 and 7
Solution
Step 1 – find the leaders (first statement of each basic block):
Edges: ENTRY→, →, →, →, →, →, →EXIT = 7.Answer: (B) 6 and 7
- (1): the first statement.
- (2): target of
goto (2). - (3): target of
goto (3). - (10): the statement immediately after the conditional jump (9).
- = {(1)}
- = {(2)}
- = {(3) … (9)}
- = {(10), (11)}
- (fall-through)
- (fall-through)
- (
if j<=5 goto (3), a self-loop) - (condition false, fall-through)
- (
if i<5 goto (2)) - EXIT (condition false)
Edges: ENTRY→, →, →, →, →, →, →EXIT = 7.Answer: (B) 6 and 7
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