When Josh, my older son’s best friend’s dad, suggested that we take our combined kids through the Left Fork of North Creek in Zion National Park (more commonly called The Subway), I wasn’t immediately excited. For one thing, it’s a somewhat technical canyon, and for another the permit that we got was for mid-May, towards […]
Author Archives: regehr
Pointer Overflow Checking is in LLVM
Production-grade memory safety for legacy C and C++ code has proven to be a frustratingly elusive goal: plenty of research solutions exist but none of them appear to be deployable as-is. So instead, we have a patchwork of partial solutions such as CFI, ASLR, stack canaries, hardened allocators, and NX. Today’s quick post is about […]
Compiler Optimizations are Awesome
This piece, which I hadn’t gotten around to writing until now since I thought it was all pretty obvious, explains why Daniel J. Bernstein’s talk, The death of optimizing compilers (audio) is wrong, and in fact compiler optimizations are extremely wonderful and aren’t going anywhere. First, the thesis of the talk is that almost all […]
Translation Validation of Bounded Exhaustive Test Cases
This piece is jointly authored by Nuno Lopes and John Regehr. Compilers should be correct, but it is not straightforward to formally verify a production-quality compiler implementation. It is just too difficult to recover the high-level algorithms by looking at an enormous mess of arithmetic, loops, and memory side effects. One solution is to write […]
Spring 2017
The hills above Salt Lake City are finally turning green. Earlier in the year my family took a short trip to southeast Utah but it rained so much one day that I didn’t think the dirt roads would be passable, so we visited Ratio, a land art installation near Green River UT. The next day […]
Taming Undefined Behavior in LLVM
Earlier I wrote that Undefined Behavior != Unsafe Programming, a piece intended to convince you that there’s nothing inherently wrong with undefined behavior as long as it isn’t in developer-facing parts of the system. Today I want to talk about a new paper about undefined behavior in LLVM that’s going to be presented in June […]
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 […]
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 […]
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 […]
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. […]