Control Flow Abstraction Generator

Generate abstract Control Flow Graph representations of programs.

Overview

This skill analyzes program code and generates Control Flow Graphs (CFGs) that abstract the program's control flow structure. CFGs show how execution flows through the program via nodes (statements, conditions) and edges (control transfers), making them suitable for static analysis, formal verification, and program understanding.

How to Use

Provide:

1. Program code: Function or program to analyze
2. Analysis scope: Function-level or program-level
3. Output format (optional): Textual, DOT, JSON, or multiple

The skill will generate:

• CFG with nodes and edges
• Node types (entry, exit, statement, condition, merge)
• Edge types (sequential, true/false branches, back edges, calls)
• Optional: DOT format for visualization, JSON for tool integration

CFG Generation Workflow

Step 1: Parse Program Structure

Identify program constructs:

• Sequential statements: Assignments, expressions, declarations
• Conditional statements: if-then-else, switch-case
• Loop statements: while, for, do-while
• Function calls: Direct calls, recursive calls
• Control transfers: break, continue, return, goto
• Exception handling: try-catch-finally

Step 2: Create CFG Nodes

Generate nodes for each construct:

Entry Node: Function/program start

• Label: ENTRY or function name
• Type: entry
• Successors: First statement

Exit Node: Function/program end

• Label: EXIT or return
• Type: exit
• Predecessors: All return points

Statement Node: Regular statement

• Label: Statement text or line number
• Type: statement
• Represents: Assignment, call, expression

Condition Node: Branch decision

• Label: Boolean expression
• Type: condition
• Successors: True branch, false branch

Merge Node: Branch join point

• Label: MERGE or empty
• Type: merge
• Predecessors: Multiple branches

Step 3: Create CFG Edges

Connect nodes with appropriate edges:

Sequential Edge: Normal flow

• From: Statement/node
• To: Next statement/node
• Label: None or →

True Edge: Condition true branch

• From: Condition node
• To: True branch first statement
• Label: T or true

False Edge: Condition false branch

• From: Condition node
• To: False branch first statement
• Label: F or false

Back Edge: Loop iteration

• From: Loop body end
• To: Loop header
• Label: ↶ or back

Call Edge: Function invocation (interprocedural)

• From: Call site
• To: Called function entry
• Label: ⇒ or call

Return Edge: Function return (interprocedural)

• From: Called function exit
• To: Call site return point
• Label: ⇐ or return

Step 4: Handle Special Constructs

Loops: Create back edges from body to header

Break: Create edge from break statement to loop exit

Continue: Create back edge from continue to loop header

Return: Create edge from return to EXIT node

Exceptions: Create exception edges from try block to catch handlers

Step 5: Generate Output

Produce CFG in requested format(s):

• Textual representation
• DOT format for Graphviz
• JSON for tool integration

Example: Simple Conditional

Code:

def max_value(x, y):
    if x > y:
        result = x
    else:
        result = y
    return result

CFG (Textual):

Node 1 (ENTRY):
  Label: max_value
  Successors: [2]

Node 2 (x > y):
  Type: condition
  Predecessors: [1]
  Successors: [3 (true), 4 (false)]

Node 3 (result = x):
  Type: statement
  Predecessors: [2]
  Successors: [5]

Node 4 (result = y):
  Type: statement
  Predecessors: [2]
  Successors: [5]

Node 5 (MERGE):
  Type: merge
  Predecessors: [3, 4]
  Successors: [6]

Node 6 (return result):
  Type: statement
  Predecessors: [5]
  Successors: [7]

Node 7 (EXIT):
  Predecessors: [6]

CFG (Visual):

    ENTRY
      ↓
   [x > y]
    ↓   ↓
   T↓   ↓F
    ↓   ↓
[result=x] [result=y]
    ↓       ↓
    └→MERGE←┘
        ↓
  [return result]
        ↓
      EXIT

CFG (DOT):

digraph CFG {
  node [shape=box];

  n1 [label="ENTRY", shape=ellipse];
  n2 [label="x > y", shape=diamond];
  n3 [label="result = x"];
  n4 [label="result = y"];
  n5 [label="MERGE", shape=circle];
  n6 [label="return result"];
  n7 [label="EXIT", shape=ellipse];

  n1 -> n2;
  n2 -> n3 [label="T", color=green];
  n2 -> n4 [label="F", color=red];
  n3 -> n5;
  n4 -> n5;
  n5 -> n6;
  n6 -> n7;
}

Example: While Loop

Code:

def sum_to_n(n):
    sum = 0
    i = 0
    while i < n:
        sum += i
        i += 1
    return sum

CFG (Visual):

    ENTRY
      ↓
   [sum = 0]
      ↓
   [i = 0]
      ↓
      ┌─────────┐
      ↓         ↑
   [i < n]      ↑ (back edge)
    ↓   ↓       ↑
   T↓   ↓F      ↑
    ↓   ↓       ↑
[sum += i]      ↑
      ↓         ↑
  [i += 1]──────┘
      ↓F
[return sum]
      ↓
    EXIT

Key Features:

• Loop header: [i < n]
• Back edge: From [i += 1] to [i < n]
• Exit edge: False branch from condition to return

CFG (Textual):

Node 1 (ENTRY)
  → Node 2

Node 2 (sum = 0)
  → Node 3

Node 3 (i = 0)
  → Node 4

Node 4 (i < n) [LOOP HEADER]
  →T Node 5
  →F Node 7

Node 5 (sum += i)
  → Node 6

Node 6 (i += 1)
  → Node 4 [BACK EDGE]

Node 7 (return sum)
  → Node 8

Node 8 (EXIT)

Example: Nested Control Flow

Code:

def process(arr, threshold):
    result = []
    for item in arr:
        if item > threshold:
            result.append(item * 2)
        else:
            if item < 0:
                continue
            result.append(item)
    return result

CFG (Visual):

ENTRY
  ↓
[result = []]
  ↓
[i = 0]
  ↓
  ┌──────────────────────┐
  ↓                      ↑
[i < len(arr)]           ↑
  ↓T                     ↑
[item = arr[i]]          ↑
  ↓                      ↑
[item > threshold]       ↑
  ↓T            ↓F       ↑
[result.append  [item<0] ↑
 (item*2)]        ↓T     ↑
  ↓               └──────┘ (continue)
  ↓              ↓F
  ↓         [result.append(item)]
  ↓               ↓
  └──→MERGE←──────┘
       ↓
   [i += 1]───────┘
       ↓F
  [return result]
       ↓
     EXIT

Key Features:

• Outer loop: for loop over array
• Inner conditional: if-else with nested if
• Continue statement: back edge to loop header
• Multiple merge points

Example: Function Calls (Interprocedural)

Code:

def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n-1)

def compute(x):
    result = factorial(x)
    return result + 1

Intraprocedural CFG (factorial only):

factorial:ENTRY
      ↓
   [n <= 1]
    ↓     ↓
   T↓     ↓F
    ↓     ↓
[return 1] [call factorial(n-1)]
    ↓           ↓
    ↓      [return n * result]
    ↓           ↓
    └──→EXIT←───┘

Interprocedural CFG (with call edges):

compute:ENTRY
      ↓
[call factorial(x)] ⇒ factorial:ENTRY
      ↓                     ↓
      ↓                [n <= 1]
      ↓                  ↓   ↓
      ↓                 ...  ...
      ↓                     ↓
[result = ...] ⇐ factorial:EXIT
      ↓
[return result + 1]
      ↓
compute:EXIT

Key Features:

• Call edge: From call site to callee entry
• Return edge: From callee exit to call site
• Recursive call: Edge back to same function

Output Formats

Textual Format

Human-readable node and edge listing:

Node <id> (<label>):
  Type: <type>
  Predecessors: [<ids>]
  Successors: [<ids>]

DOT Format (Graphviz)

Graph visualization format:

digraph CFG {
  node [shape=box];
  n1 [label="...", shape=...];
  n1 -> n2 [label="...", color=...];
}

Generate PNG/SVG with: dot -Tpng cfg.dot -o cfg.png

JSON Format

Machine-readable for tool integration:

{
  "nodes": [
    {"id": 1, "label": "...", "type": "..."}
  ],
  "edges": [
    {"from": 1, "to": 2, "type": "..."}
  ]
}

Analysis Levels

Function-Level (Intraprocedural)