Category: Compilers

  • Formal-Methods-Based Bugfinding for LLVM’s AArch64 Backend

    [This piece is co-authored by Ryan Berger and Stefan Mada (both Utah CS undergrads), by Nader Boushehri, and by John Regehr.] An optimizing compiler traditionally has three main parts: a frontend that translates a source language into an intermediate representation (IR), a “middle end” that rewrites IR into better IR, and then a backend that […]

  • Alive2 Part 3: Things You Can and Can’t Do with Undef in LLVM

    [Also see Part 1 and Part 2 in this series.] Let’s talk about these functions: unsigned add(unsigned x) { return x + x; } unsigned shift(unsigned x) { return x << 1; } From the point of view of the C and C++ abstract machines, their behavior is equivalent: in a program you’re writing, you […]

  • Alive2 Part 2: Tracking miscompilations in LLVM using its own unit tests

    [This piece is co-authored by Nuno P. Lopes and John Regehr.] Alive2 is a formal verification framework for LLVM optimizations. It includes multiple tools, including a plugin for `opt’ to verify whether the optimizations just run are correct or not. We gave an introduction to Alive2 in a previous post. A few years ago we […]

  • Alive2 Part 1: Introduction

    [This piece is co-authored by Nuno P. Lopes and John Regehr.] Compiler bugs threaten the correctness of almost any computer system that uses compiled code. Translation validation is a path towards reliably correct compilation that works by checking that an individual execution of the compiler did the right thing. We created a tool, Alive2, that […]

  • Precision Opportunities for Demanded Bits in LLVM

    [Although this post was written to stand by itself, it builds on the previous one. It is authored by Jubi Taneja, Zhengyang Liu, and John Regehr.] When designing computer systems, it can be useful to avoid specifying behaviors too tightly. For example, we might specify that a math library function only needs to return a […]

  • Testing Dataflow Analyses for Precision and Soundness

    [This piece is co-authored by Jubi Taneja, Zhengyang Liu, and John Regehr; it’s a summary of some of the findings from a paper that we just recently completed the camera ready copy for, that is going to be published at CGO (Code Generation and Optimization) 2020.] Update from Jan 12 2020: Looks like there’s a […]

  • Design and Evolution of C-Reduce (Part 2)

    Part 1 of this series introduced C-Reduce and showed how it combines a domain-independent core with a large collection of domain-specific passes in order to create a highly effective test-case reducer for C and C++ code. This part tells the rest of the story and concludes. Parallel Test-Case Reduction C-Reduce’s second research contribution is to […]

  • Design and Evolution of C-Reduce (Part 1)

    [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 […]

  • 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 […]

  • 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 […]