[This piece is posted in parallel on the IEEE Software blog. Karim Ali copyedited.] Since 2008, my colleagues and I have developed and maintained C-Reduce, a tool for programmatically reducing the size of C and C++ files that trigger compiler bugs. C-Reduce also usually does a credible job reducing test cases in languages other than […]
Author Archives: regehr
It’s Time for a Modern Synthesis Kernel
Alexia Massalin’s 1992 PhD thesis has long been one of my favorites. It promotes the view that operating systems can be much more efficient than then-current operating systems via runtime code generation, lock-free synchronization, and fine-grained scheduling. In this piece we’ll only look at runtime code generation, which can be cleanly separated from the other […]
Floating the Dirty Devil River
Packrafts are tough, light, individual-sized inflatable boats that people use to put together really amazing wilderness trips by combining rafting, hiking, and sometimes even biking. I’ve had sort of a low-grade obsession with packrafting since 2009 when I ran into some people in Alaska who were on their way to hike up and over the […]
Verifying Popcount
Popcount is the function that returns the number of set bits in its argument. Showing that a popcount implementation does what it claims to do has become one of my favorite examples to use when I need to quickly show students how we can reason about programs mathematically. Something like a selection sort is probably […]
Explaining Code using ASCII Art
People tend to be visual: we use pictures to understand problems. Mainstream programming languages, on the other hand, operate in an almost completely different kind of abstract space, leaving a big gap between programs and pictures. This piece is about pictures drawn using a text character set and then embedded in source code. I love […]
Walking or Biking to NSF
Since the National Science Foundation funds a large fraction of academic computer science research in the USA, we often end up traveling to Washington to visit the NSF. This post is just to say that if you are traveling light, if you need some exercise, if you have a bit of free time, and if the […]
Synthesizing Constants
(See this blog post for a short introduction to synthesis, or this paper for a long one.) In this piece I want to discuss an aspect of program synthesis that sounds like it should be easy, but isn’t: synthesizing constant values. For example, consider trying to synthesize an optimized x86-64 implementation of this code: The […]
Learning When Values are Changed by Implicit Integer Casts
C and C++ perform implicit casts when, for example, you pass an integer-typed variable to a function that expects a different type. When the target type is wider, there’s no problem, but when the target type is narrower or when it is the same size and the other signedness, integer values may silently change when […]
What’s the difference between an integer and a pointer?
(This piece is an alternate introduction and advertisement for a soon-to-be-published research paper.) In an assembly language we typically don’t have to worry very much about the distinction between pointers and integers. Some instructions happen to generate addresses whereas others behave arithmetically, but underneath there’s a single data type: bitvectors. At the opposite end of […]
Future Directions for Optimizing Compilers
I wanted to write a manifesto-ish sort of piece about what compilers are supposed to look like in the future. Nuno was the obvious coauthor since I’ve talked to him about this topic so much that I’m overall not really sure which parts started out as his ideas and which were mine. The article didn’t […]