[This post is based on data gathered by my student Peng Li. He also wrote the undefined behavior checker.] The other day I posted about undefined integer behaviors in code written by students in a class I used to teach. This post is more of the same, this time from CS 5785, my advanced embedded […]

# Category Archives: Education

## Differential Whitebox Testing Is Good

[This post is based on data gathered by Chad Brubaker and Peng Li, respectively an undergrad and grad student in CS at Utah.] Two courses I’ve taught at Utah, CS 4400 and CS 5785, have assignments where students write short integer functions for which we — the instructors — have automatic graders. In 4400 the […]

## Visualizing A Few More Math Bugs

As a followup to last night’s post, here are some saturating signed subtraction functions I’ve received. Hopefully the correct one is obvious.

## Visualizing Math Bugs

After getting too tired to work tonight, I realized I had no Netflix and that I’m bored with the books I’m reading. Therefore, I present visualizations of several solutions I’ve received to my saturating arithmetic homework over the years. This is what a correct saturating signed add looks like, where the z-axis is the output: […]

## A Few Thoughts About Path Coverage

Klee isn’t the only tool in its class, nor was it the first, but it’s open source and well-engineered and it works. Klee’s goal is to generate a set of test inputs that collectively induce path coverage in a system under test. One of the scenarios I was curious about is the one where Klee […]

## Undefined Integer Behaviors in Student Code, Part 1

[This post is based on material from Chad Brubaker, a really smart CS undergrad at Utah who did all the work getting these data. The integer undefined behavior checker was created by my student Peng Li.] Integer undefined behaviors in C/C++, such as INT_MAX+1 or 1<<-1, create interesting opportunities for compiler optimizations and they also […]

## More Saturating Arithmetic

In a previous post I guessed that 91 bytes was close to the minimum size for implementing signed and unsigned saturating 32-bit addition and subtraction on x86. A commenter rightly pointed out that it should be possible to do better. Attached is my best shot: 83 bytes. Given the crappy x86 calling convention I’m going […]

## Fun With Saturating Arithmetic

An assignment that I often give my Advanced Embedded Systems class early in the semester is to implement saturating versions of signed and unsigned addition and subtraction. Saturating operations “stick” at the maximum or minimum value. (INT_MAX +sat 1) therefore evaluates to INT_MAX. The only wrinkle in the assignment is that solutions must be capable […]

## How to Debug

One of the painful parts of teaching a lab-based embedded systems course is that over and over I have to watch a team with a relatively simple bug in their code, but who is trying to fix it by repeatedly making random changes. Generally they start with code that’s pretty close to working and break […]

## The Truth About the Life of the Mind

[This piece is a followup to The Big Lie About the Life of the Mind.] Being a professor, like any other job, has its pros and cons. You’d hope that one of the advantages would be that the job encourages a person to live a life of the mind. Otherwise what’s the point, right? I […]