Compiler Design

a = 1 
b = 10
c = 20 
d = a + b
e = c + d 
f = c + e 
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

GROUP 1	GROUP 2
P. Intermediate representation	1. Activation records
Q. Top-down parsing	2. Code generation
R. Runtime environments	3. Leftmost derivation
S. Register allocation	4. Graph colouring