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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…
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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…
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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…
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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…
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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…
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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…
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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…
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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…
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How LLVM Optimizes a Function
An optimizing, ahead-of-time compiler is usually structured as: A frontend that converts source code into an intermediate representation (IR). A target-independent optimization pipeline: a sequence of passes that successively rewrite the IR to eliminate inefficiencies and forms that cannot be readily translated into machine code. Sometimes called the “middle end.” A target-dependent backend that generates…
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How Clang Compiles a Function
I’ve been planning on writing a post about how LLVM optimizes a function, but it seems necessary to first write about how Clang translates C or C++ into LLVM. This is going to be fairly high-level: rather than looking at Clang’s internals, I’m going to focus on how Clang’s output relates to its input we’re…