Basic blocks and flow graphs in Compiler Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the start of a basic block in a flow graph.

Compiler Design

if instruction.is_leader():
    start_block = [1]

Drag options to blanks, or click blank then click option'

Ainstruction

BFalse

CNone

DTrue

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning a boolean value instead of the instruction.

Using None which does not represent a block start.

The start of a basic block is marked by a leader instruction, so we assign the current instruction as the start.

2fill in blank

medium

Complete the code to create edges between basic blocks in a flow graph.

Compiler Design

for block in basic_blocks:
    for successor in block.[1]:
        graph.add_edge(block, successor)

Drag options to blanks, or click blank then click option'

Apredecessors

Bleaders

Csuccessors

Dinstructions

Attempts:

3 left

💡 Hint

Common Mistakes

Using predecessors which point backward.

Using leaders or instructions which are not related to edges.

Edges in a flow graph connect a block to its successors, representing possible next blocks in execution.

3fill in blank

hard

Fix the error in the code that checks if a block ends with a jump instruction.

Compiler Design

if block.instructions[-1].[1] == 'jump':
    handle_jump(block)

Drag options to blanks, or click blank then click option'

Aopcode

Boperand

Clabel

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Checking operand or label which are not operation types.

Using address which is unrelated here.

The opcode field of an instruction tells what operation it performs, such as 'jump'.

4fill in blank

hard

Fill both blanks to build a dictionary mapping block leaders to their successors.

Compiler Design

block_map = {leader: block[1] for leader, block in blocks.items() if block[2]

Drag options to blanks, or click blank then click option'

A.successors

B.predecessors

Cis not None

Dis None

Attempts:

3 left

💡 Hint

Common Mistakes

Using predecessors instead of successors.

Checking if block is None instead of not None.

We map each leader to its block's successors only if the block exists (is not None).

5fill in blank

hard

Fill all three blanks to create a flow graph with nodes and edges from basic blocks.

Compiler Design

flow_graph = FlowGraph()
for block in [1]:
    flow_graph.add_node(block.[2])
    for succ in block.[3]:
        flow_graph.add_edge(block.[2], succ.[2])

Drag options to blanks, or click blank then click option'

Abasic_blocks

Bleader

Csuccessors

Dinstructions

Attempts:

3 left

💡 Hint

Common Mistakes

Using instructions instead of basic blocks.

Using instructions or predecessors instead of successors.

Using block instead of leader for node identity.

We iterate over basic_blocks, add nodes using each block's leader, and add edges to their successors.

Practice

1. What is a basic block in compiler design?

easy

A. A single instruction that causes a jump

B. A sequence of instructions with no jumps or jump targets inside

C. A graph showing all possible paths in a program

D. A data structure used to store variables

Solution

1. Step 1: Understand the definition of a basic block

   A basic block is a group of instructions executed sequentially without any jumps or jump targets inside it.

2. Step 2: Compare options with the definition

   A sequence of instructions with no jumps or jump targets inside matches this definition exactly, while others describe different concepts.

3. Final Answer:

   A sequence of instructions with no jumps or jump targets inside -> Option B

4. Quick Check:

   Basic block = sequence without jumps [OK]

Hint: Basic block = straight-line code with no jumps inside [OK]

Common Mistakes:

• Confusing basic block with flow graph
• Thinking a basic block is a single instruction
• Mixing up jumps inside and at block boundaries

2. Which of the following correctly represents a flow graph in compiler design?

easy

A. A list of variables used in a program

B. A table of function names and their parameters

C. A set of instructions executed sequentially

D. A directed graph where nodes are basic blocks and edges show possible control flow

Solution

1. Step 1: Define flow graph

   A flow graph is a directed graph where each node is a basic block and edges represent possible paths the program can take.

2. Step 2: Match options to definition

   A directed graph where nodes are basic blocks and edges show possible control flow describes this exactly. Other options describe unrelated concepts.

3. Final Answer:

   A directed graph where nodes are basic blocks and edges show possible control flow -> Option D

4. Quick Check:

   Flow graph = graph of basic blocks and edges [OK]

Hint: Flow graph = nodes as blocks, edges as jumps [OK]

Common Mistakes:

• Confusing flow graph with a list or table
• Thinking flow graph is just a sequence of instructions
• Ignoring direction of edges in the graph

3. Consider a program with three basic blocks: B1, B2, and B3. B1 jumps to B2 and B3; B2 jumps to B3; B3 has no jumps. Which edges appear in the flow graph?

medium

A. Edges: B1->B2, B1->B3, B2->B3

B. Edges: B1->B2, B2->B3 only

C. Edges: B1->B3, B3->B2

D. Edges: B2->B1, B3->B1

Solution

1. Step 1: Identify jumps from each block

   B1 jumps to B2 and B3, so edges B1->B2 and B1->B3 exist. B2 jumps to B3, so edge B2->B3 exists. B3 has no jumps, so no outgoing edges.

2. Step 2: List all edges in the flow graph

   Edges are B1->B2, B1->B3, and B2->B3, matching Edges: B1->B2, B1->B3, B2->B3.

3. Final Answer:

   Edges: B1->B2, B1->B3, B2->B3 -> Option A

4. Quick Check:

   Edges match jumps from blocks [OK]

Hint: Edges = jumps from each block [OK]

Common Mistakes:

• Missing one of the edges from B1
• Adding edges that don't exist
• Confusing direction of edges

4. A flow graph is constructed but one edge is missing between two basic blocks that should be connected. What is the most likely cause?

medium

A. The basic blocks were not identified correctly, missing a jump instruction

B. The program has no jumps at all

C. The flow graph always excludes edges between blocks

D. Basic blocks are not connected by edges in flow graphs

Solution

1. Step 1: Understand flow graph construction

   Edges in a flow graph represent jumps between basic blocks. Missing an edge usually means a jump was missed when identifying blocks.

2. Step 2: Analyze options

   The basic blocks were not identified correctly, missing a jump instruction correctly identifies the cause as incorrect block identification. Other options are false statements about flow graphs.

3. Final Answer:

   The basic blocks were not identified correctly, missing a jump instruction -> Option A

4. Quick Check:

   Missing edge = missed jump in blocks [OK]

Hint: Missing edge means missed jump in block detection [OK]

Common Mistakes:

• Assuming flow graphs exclude edges by design
• Thinking no jumps means missing edges
• Confusing basic blocks with edges

5. Given a program with nested if-else statements, how can flow graphs help optimize the program?

hard

A. By converting the program into machine code directly

B. By listing all variables used in the program

C. By showing all possible paths, they help identify unreachable code and optimize execution paths

D. By removing all loops automatically

Solution

1. Step 1: Understand the role of flow graphs in optimization

   Flow graphs display all possible execution paths, making it easier to spot unreachable code and redundant branches.

2. Step 2: Match options to optimization benefits

   By showing all possible paths, they help identify unreachable code and optimize execution paths correctly states that flow graphs help identify unreachable code and optimize paths. Other options describe unrelated or incorrect uses.

3. Final Answer:

   By showing all possible paths, they help identify unreachable code and optimize execution paths -> Option C

4. Quick Check:

   Flow graphs reveal paths for optimization [OK]

Hint: Flow graphs reveal unreachable code and optimize paths [OK]

Common Mistakes:

• Thinking flow graphs list variables
• Confusing flow graphs with code generation
• Assuming flow graphs remove loops automatically