Solution

The correct answer is A, C and D

Key Points

• A. The first three-address instruction in the intermediate code is a leader.
  • Leaders, in programming, are the first instructions of a ‘basic block’ in the control-flow graph.
  • A basic block is a straight-line code sequence with no branches in except to the entry and no branches out except at the exit.
  • The first instruction of a basic block is known as a leader. So, in context, the first three-address instruction would be a leader as it's the first instruction in the block. This statement is correct.
• B. The instruction which is exactly at the middle of the given three address code is a leader.
  • A leader doesn't get determined by its position in the middle of the given intermediate code. It becomes a leader if it's the first instruction in the block, or if it's the target of a conditional or unconditional jump, or if it immediately follows a jump. Just being at the "middle" doesn't qualify an instruction as a leader. Therefore, this statement is incorrect.
• C. Any instruction that is the target of a conditional or unconditional jump is a leader.
  • This is correct. An instruction becomes a leader if it becomes the target of a jump - whether that jump is conditional or unconditional. That's because it marks a new possible entry point into a block of code, and thus starts a new basic block.
• D. Any instruction that immediately follows a conditional or unconditional jump is a leader.
  • This statement is also correct. The instruction immediately following a conditional or unconditional jump also starts a new basic block. This is because the jump could transfer control elsewhere, and so the next instruction potentially starts a new sequence of code with no prior branches in.

So, summarizing, the correct statements here are A, C, and D, option 3.