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Fun at the UNIX Terminal Part 1
This post is aimed at kids, like the 6th graders who I was recently teaching about programming in Python. It is more about having fun than about learning, but I hope that if you enjoy playing around at the UNIX terminal, you’ll eventually learn to use this kind of system for real. Keep in mind…
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Elk and Arch
I wanted to share a few pictures from a long, very cold snowshoe/hike I did in January. The goal was to reach a natural arch that I had previously spotted in upper City Creek Canyon. This was fun to find: I hadn’t realized there was an arch large enough to stand inside within walking distance…
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Do Expressive Programming Languages Always Have Undefined Behavior?
In the Hacker News comments on one of my previous posts about undefined behavior, someone said this: AFAIK Gödel’s incompleteness theorems imply that _any_ language will have at least some undefined behaviour. Let’s take a quick look at this statement, keeping in mind that incompleteness and undecidability can be remarkably tricky topics. Some years ago…
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Mapping Mountains on Exoplanets
Snow in the mountains evolves in predictable ways that are strongly influenced by the terrain. For example, warmish snowstorms leave a characteristic snow line that looks very much like a contour on a topographic map: (source) (source) On the other hand, when snow melts the effect is very different, and is often dominated by aspect.…
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Teaching Python Informally to Kids
For the last few months I’ve been running a “coding club” for my son’s sixth-grade class. Once a week, the interested students (about 2/3 of the class) stick around for an hour after school and I help them learn to program. The structure is basically like the lab part of a programming class, without the…
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Undefined Behavior != Unsafe Programming
Undefined behavior (UB) in C and C++ is a clear and present danger to developers, especially when they are writing code that will execute near a trust boundary. A less well-known kind of undefined behavior exists in the intermediate representation (IR) for most optimizing, ahead-of-time compilers. For example, LLVM IR has undef and poison in…
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Nine Mile Canyon
One of my boys and I spent Sunday exploring Nine Mile Canyon, in the remote Book Cliffs a few hours drive from Salt Lake City. This canyon is known for its dense collection of rock art and ruins, a lot of which can be seen from the paved road that follows the canyon bottom. This…
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Detecting Strict Aliasing Violations in the Wild
Type-based alias analysis, where pointers to different types are assumed to point to distinct objects, gives compilers a simple and effective way to disambiguate memory references in order to generate better code. Unfortunately, C and C++ make it easy for programmers to violate the assumptions upon which type-based alias analysis is built. “Strict aliasing” refers…
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A Quick Look at the SoftIron OverDrive 1000
ARM processors are ubiquitous in mobile, but you won’t find a lot of developers using ARM-based boxes for their day-to-day work. Recently I needed some ARM machines for compiler work and got a few Raspberry Pi 3 boards, which seemed reasonable since these have four cores at 1.2 GHz. However, while these boards are far…
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Introduction to Precision Farming
[My father, David Regehr, encouraged me to write this piece, provided some of its content, edited it, and agreed to let me use data from his farm.] [For readers outside the USA: Alas, we do not farm in metric here. In case you’re not familiar with the notation, 10″ is ten inches (25.4 cm) and…