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Previous Year Question (PYQs)

Consider the program below which uses six temporary variables a, b, c, d, e, and f.
a = 10
b = 20
c = 30
d=a+c
f=c+c
b=c+e
e=b+f
d=5+e
return d+f

Assuming that all the above operations take their operands from registers, the minimum number of registers needed to execute this program without spilling is




Solution

Detailed Register Allocation Analysis

Objective: Determine the minimum number of registers needed to execute the program without spilling.

Live Range Analysis:

  • a: Line 1 → 4
  • b: Line 6 → 7
  • c: Line 3 → 6
  • d: Line 8 → 9
  • e: Line 7 → 8
  • f: Line 5 → 9

Max live variables: 3 (after lines 6 and 7)

✅ Final Answer: 3 registers are required to execute the program without spilling.


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