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Design and Debugging

约 363 个字 17 行代码 预计阅读时间 2 分钟 共被读过 次

15-213/15-513: Introduction to Computer Systems
8th Lecture, Sept. 19, 2024
Instructors: Brian Railing, Mohamed Farag

📚 After This Lecture, You Will Be Able To:

  1. Describe the steps to debug complex code failures.
  2. Identify ways to manage complexity when programming.
  3. State guidelines for communicating the intention of the code.

📝 Outline

Debugging

  • Defects and Failures
  • Scientific Debugging
  • Tools

Design

  • Managing Complexity
  • Communication (Naming, Comments)

🛠 Debugging

Defects, Errors, & Failures

  1. Defect: Programmer creates a fault.
  2. Error: Defect causes wrong results in data/control signals.
  3. Error Propagation: Erroneous state spreads.
  4. Failure: System/component fails to produce intended result.

🔍 Why errors ≠ failures?
Errors can be masked (e.g., ECC memory) or detected.

🧪 Scientific Debugging

  1. Hypothesis: Propose a defect explanation.
  2. Prediction: What happens if the hypothesis is true?
  3. Experiment: Test under controlled conditions.
  4. Observation: Collect data to confirm/refute the hypothesis.
  5. Fix & Confirm: Apply fix and validate.

Example: Atlas-Centaur Rocket Failure

  • First Failure: Clogged turbopumps due to plastic remnants.
  • Fix: Bake off plastic.
  • Second Failure: Leaking valve pushed to failure by efficiency demands.
  • Lesson: Reproduce the failure to diagnose.

🐛 Code Example: Buggy Fibonacci

C
int fib(int n) {
    int $f, f 0=1, f 1=1$;
    while $(n>1)$ {
        $$n=n-1;$$
        $f=f 0+f 1$;
        $f 0=f 1$;
        f1=f;
    }
    return $f;
}

Failure: fib(1) returns garbage (e.g., 134513905).

🔍 Hypothesis Table

Code Snippet Hypothesis
while(n>1) Loop condition incorrect for n=1
int f; f is uninitialized

🛠 Fix: Initialize f

C
int fib(int n) {
    int f = 1; // Initialize f
    ...
}

🧪 Experiment Results

  • Without Fix: fib(1) = 0 (uninitialized value).
  • With Fix: fib(1) = 1 (correct).

🛠 Tools for Debugging

  1. Compiler Flags:
    Bash
    gcc -Wall -Werror -O3 -o fib fib.c  # Catch uninitialized variables
  2. Valgrind: Detect memory leaks/uninitialized accesses.
  3. GDB: Step-through execution to trace state.

🎨 Design

Managing Complexity

  • Techniques:
  • Separation of Concerns
  • Modularity
  • Abstraction
  • DRY (Don’t Repeat Yourself)

Example: Cache Access Steps

  1. Convert address → tag, set index, block offset.
  2. Look up set.
  3. Check tag match.
  4. Handle hit/miss (evict LRU, load new line).

📛 Naming Guidelines

  1. Avoid meaningless names:
    - ❌ tmp, data, foo
    - ✅ employeeSalary, cacheLine
  2. Use domain terms:
    - In a cache lab: line, tag, setIndex.
  3. Limit word count:
    - ❌ arraysOfSetsOfLinesOfBlocks
    - ✅ cache

💬 Comments

  • Don’ts:
  • ❌ Explain what code does (i++ // increment i).
  • ❌ Apologize for bad code.
  • Dos:
  • ✅ Explain why (e.g., magic numbers):
    C
    // MAX_ADDRESS_LENGTH = 17 for 64-bit addresses (16 hex chars + null)
const int MAX_ADDRESS_LENGTH = 17;

📊 Summary

  1. Debugging: Systematic hypothesis-driven process. ✅
  2. Design: Manage complexity via modularity, naming, and clear communication. 📦
  3. Tools: Use -Wall, Valgrind, and debuggers to catch issues early. 🛠

"Testing shows the presence, not the absence, of defects." – Dijkstra

🔗 Resources:
- The Space Review: Atlas-Centaur Failure
- Computer Systems: A Programmer’s Perspective, 3rd Ed.