Question

consider the basic block B.
q = 3
r = 10
s = q + r
t = 2 * r + s
t = q
u = q + r
v = q + t
w = 3 + x
Now consider the following basic block $B_2$.
q = 3
r = 10
s = q + r
t = q
u = q + r
v = q + t
w = 3 + x

State which optimization was performed on the basic block B to get $B_2$.

• A. Dead code elimination
• B. Copy photograph
• C. Constant propagation/folding
• D. Common sub-expression elimination

Answer 1

The correct option is A Dead code elimination

Dead code elimination is a compiler optimization to remove code which does not affect the program results. Removing such code has several benefits: it shrinks program size, an important consideration in some contexts and it allows the running program to avoid executing Irrelevant operations, which reduces its running time. It can also enable further optimizations by simplifying program structure. Dead code includes code that can never be executed (unreachable code) and code that only affects dead variables (written to, but never read again), that is, irrelevant to the program. In basic block B, we can replace t = $2\times r+s$ is a dead code because t is assigned q in the very next statement making t = 2 x r + s irrelevant to the program.