GATE CS 2015 Set 1 — Question 46
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Compiler Design → Code Optimization → Basic Blocks & Flow Graphs
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Question
A variable is said to be live at a statement in a program if the following three conditions hold simultaneously:i. There exists a statement that uses
ii. There is a path from to in the flow graph corresponding to the program
iii. The path has no intervening assignment to including at and
The variables which are live both at the statement in basic block 2 and at the statement in basic block 3 of the above control flow graph are
ii. There is a path from to in the flow graph corresponding to the program
iii. The path has no intervening assignment to including at and
Correct answer
(C) r, u
Solution
To determine liveness, we check which variables are used along a path from the block without being redefined.At Block 2 ():
.Correct option is (C).
- Uses and . Since they are used immediately, they are live.
- Path : Block 4 is . Uses . Defines .
- is defined in Block 2, so it is not live-in at 2.
- is defined in Block 4, so dead.
- Path : Block 1 defines . So are dead at 2.
- Thus, live at 2: .
- Uses and . So are live.
- Path : Block 4 uses . Defines .
- is used in 4 and not defined in 3. So is live.
- is used in 4 and not defined in 3. So is live.
- is defined in 3, so dead before 3.
- Path : Block 1 defines . So is dead.
- Thus, live at 3: .
.Correct option is (C).
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