Basic blocks and flow graphs in Compiler Design - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Basic Blocks

Which of the following best describes a basic block in compiler design?

A A group of functions called together in a program

B A set of instructions that can be executed in parallel

C A block of code that contains only conditional branches

D A sequence of instructions with no branches except at the entry and exit points

💡 Hint

Think about a block where control flow enters at the beginning and leaves at the end without interruption.

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.

📋 Factual

intermediate

Flow Graph Components

In a flow graph representing a program's control flow, what do the nodes and edges represent?

A Nodes represent variables; edges represent data dependencies

B Nodes represent basic blocks; edges represent possible control flow between blocks

C Nodes represent instructions; edges represent memory access

D Nodes represent functions; edges represent function calls

💡 Hint

Consider what a flow graph models in terms of program execution paths.

In a flow graph, nodes are basic blocks and edges show possible paths control can take from one block to another.

🔍 Analysis

advanced

Identifying Leaders in Basic Blocks

Given the following sequence of instructions, which instructions are leaders that start a new basic block?

1: a = 5
2: if a > 0 goto 5
3: b = a + 1
4: goto 6
5: b = a - 1
6: print(b)

A Instructions 1, 3, 5, and 6

B Instructions 1, 3, and 5

C Instructions 1, 2, and 6

D Instructions 1, 2, and 5

💡 Hint

Leaders include the first instruction, targets of jumps, and instructions following jumps.

Instruction 1 is the first instruction (always a leader). Instruction 3 immediately follows the conditional jump at instruction 2. Instruction 5 is the target of the jump at instruction 2 and instruction 6 is the target of the jump at instruction 4.

❓ Comparison

advanced

Difference Between Basic Blocks and Flow Graphs

Which statement correctly compares basic blocks and flow graphs?

A Basic blocks represent function calls; flow graphs represent variable dependencies

B Flow graphs are sequences of instructions; basic blocks show possible paths between instructions

C Basic blocks are nodes in a flow graph; flow graphs show control flow between these blocks

D Flow graphs are used only for data flow analysis; basic blocks are used only for syntax checking

💡 Hint

Think about how basic blocks and flow graphs relate to each other structurally.

Basic blocks form the nodes of a flow graph, which illustrates how control moves between these blocks.

❓ Reasoning

expert

Effect of Removing an Edge in a Flow Graph

In a flow graph of a program, what is the most likely effect of removing an edge that represents a conditional jump?

A It may cause some basic blocks to become unreachable, changing the program's behavior

B It will have no effect since edges do not affect execution

C It will convert the flow graph into a tree structure

D It will merge two basic blocks into one larger block

💡 Hint

Consider what happens if a path of execution is removed from the control flow.

Removing an edge that represents a conditional jump can make some blocks unreachable, potentially changing how the program runs.

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