To minimize spills to memory in the compiled code, we need to consider the limited number of registers available in the instruction set architecture. In this case, the processor has only two registers.
c = a + b;
d = c * a;
e = c + a;
x = c * c;
if (x > a) {
y = a * a;
} else {
d = d * d;
e = e * e;
}
The limited register constraint may lead to spills to memory when there are more than two live variables at the same time.
- Before
c = a + b;
, a
and b
are live.
- Before
d = c * a;
, a
, b
, and c
are live.
- Before
e = c + a;
, a
, b
, and c
are live.
- Before
x = c * c;
, a
, b
, and c
are live.
- Before the
if
statement, a
, b
, and c
are live.
- Inside the
if
branch, a
and y
are live.
- Inside the
else
branch, a
, b
, c
, d
, and e
are live.
Now, let's consider the limited registers (R1 and R2). We can observe that at most three variables (a
, b
, and c
) are live simultaneously before the if
statement. Therefore, we may need to spill one variable to memory.
The minimum number of spills to memory is:
1