Question

Using either a register lifetime graph or colored graph, determine the minimum number of registers needed for the basic block below. Assume the following: • Variables do not need to persist between statements if they are not actively used. • Variables that are assigned to must persist to the end of the basic block. Code: C; k = i * t + b = o * k; a = b / i;

Answer 1

The answer is 3.

The variable i can be stored in r1 as, r1<- i. Similarly, r2<-t, r3<-c.

Now we can write the first line of code as,

r2=r1 *r2 + r3.

Now the register r3 is free and we can use it for future. So r3<-o;

The second line of code can be written as,

r3= r3 * r1. The value of b is now stored in register r3.

The third line can be now written as,

r3 = r3/r1; where r1, r2 and r3 are register variables.