GATE CS 2014 Set 1 Q17 — Local Optimization

Let us analyze each statement to find the false one:

• Option A: By definition, a basic block is a sequence of consecutive instructions where the control flow enters only at the beginning and exits only at the end, without any branching or halting in between. Thus, this statement is TRUE.
• Option B: Available expression analysis determines which expressions have already been computed and remain valid at a given program point. This information is directly used to perform Common Subexpression Elimination (CSE). Thus, this statement is TRUE.
• Option C: Live variable analysis identifies whether the value of a variable at a specific point will be read along any path in the future. If a variable is not live (i.e., dead) and its assignment has no side effects, the assignment can be safely removed. This is used for Dead Code Elimination. Thus, this statement is TRUE.
• Option D: The transformation $x = 4 * 5 \Rightarrow x = 20$ evaluates a constant expression at compile-time rather than run-time. This optimization is known as Constant Folding, not common subexpression elimination. Thus, this statement is FALSE.

Therefore, the false statement is D.