PLDI 2020
41st ACM SIGPLAN International Conference on Programming Language Design and Implementation (PLDI 2020)
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41st ACM SIGPLAN International Conference on Programming Language Design and Implementation (PLDI 2020), June 15–20, 2020, London, UK

PLDI 2020 – Proceedings

Contents - Abstracts - Authors

Frontmatter

Title Page


Message from the Chairs
Welcome to PLDI 2020, the 41st ACM SIGPLAN Conference on Programming Language Design and Implementation. We originally planned to hold the conference in London, United Kingdom, June 15-20. But due to the worldwide outbreak of COVID-19, this year's meeting is being held virtually instead. The table of contents in the proceedings reflects our original organization of papers into sessions, according to topic.

PLDI 2020 Organization
Committee Listings

Sponsors
Sponsors and partners

Synthesis I

Data-Driven Inference of Representation Invariants
Anders Miltner, Saswat Padhi, Todd Millstein, and David Walker
(Princeton University, USA; University of California at Los Angeles, USA)
A representation invariant is a property that holds of all values of abstract type produced by a module. Representation invariants play important roles in software engineering and program verification. In this paper, we develop a counterexample-driven algorithm for inferring a representation invariant that is sufficient to imply a desired specification for a module. The key novelty is a type-directed notion of visible inductiveness, which ensures that the algorithm makes progress toward its goal as it alternates between weakening and strengthening candidate invariants. The algorithm is parameterized by an example-based synthesis engine and a verifier, and we prove that it is sound and complete for first-order modules over finite types, assuming that the synthesizer and verifier are as well. We implement these ideas in a tool called Hanoi, which synthesizes representation invariants for recursive data types. Hanoi not only handles invariants for first-order code, but higher-order code as well. In its back end, Hanoi uses an enumerative synthesizer called Myth and an enumerative testing tool as a verifier. Because Hanoi uses testing for verification, it is not sound, though our empirical evaluation shows that it is successful on the benchmarks we investigated.

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Type Error Feedback via Analytic Program Repair
Georgios Sakkas, Madeline Endres, Benjamin Cosman, Westley Weimer, and Ranjit Jhala
(University of California at San Diego, USA; University of Michigan, USA)
We introduce Analytic Program Repair, a data-driven strategy for providing feedback for type-errors via repairs for the erroneous program. Our strategy is based on insight that similar errors have similar repairs. Thus, we show how to use a training dataset of pairs of ill-typed programs and their fixed versions to: (1) learn a collection of candidate repair templates by abstracting and partitioning the edits made in the training set into a representative set of templates; (2) predict the appropriate template from a given error, by training multi-class classifiers on the repair templates used in the training set; (3) synthesize a concrete repair from the template by enumerating and ranking correct (e.g. well-typed) terms matching the predicted template. We have implemented our approach in Rite: a type error reporting tool for OCaml programs. We present an evaluation of the accuracy and efficiency of Rite on a corpus of 4,500 ill-typed Ocaml programs drawn from two instances of an introductory programming course, and a user-study of the quality of the generated error messages that shows the locations and final repair quality to be better than the state-of-the-art tool in a statistically-significant manner.

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Synthesizing Structured CAD Models with Equality Saturation and Inverse Transformations
Chandrakana Nandi, Max Willsey, Adam Anderson, James R. Wilcox, Eva Darulova, Dan Grossman, and Zachary Tatlock
(University of Washington, USA; Certora, USA; MPI-SWS, Germany)
Recent program synthesis techniques help users customize CAD models(e.g., for 3D printing) by decompiling low-level triangle meshes to Constructive Solid Geometry (CSG) expressions. Without loops or functions, editing CSG can require many coordinated changes, and existing mesh decompilers use heuristics that can obfuscate high-level structure.
This paper proposes a second decompilation stage to robustly "shrink" unstructured CSG expressions into more editable programs with map and fold operators. We present Szalinski, a tool that uses Equality Saturation with semantics-preserving CAD rewrites to efficiently search for smaller equivalent programs. Szalinski relies on inverse transformations, a novel way for solvers to speculatively add equivalences to an E-graph. We qualitatively evaluate Szalinski in case studies, show how it composes with an existing mesh decompiler, and demonstrate that Szalinski can shrink large models in seconds.

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Language Implementation

Compiler and Runtime Support for Continuation Marks
Matthew Flatt and R. Kent Dybvig
(University of Utah, USA; Cisco Systems, USA)
Continuation marks enable dynamic binding and context inspection in a language with proper handling of tail calls and first-class, multi-prompt, delimited continuations. The simplest and most direct use of continuation marks is to implement dynamically scoped variables, such as the current output stream or the current exception handler. Other uses include stack inspection for debugging or security checks, serialization of an in-progress computation, and run-time elision of redundant checks. By exposing continuation marks to users of a programming language, more kinds of language extensions can be implemented as libraries without further changes to the compiler. At the same time, the compiler and runtime system must provide an efficient implementation of continuation marks to ensure that library-implemented language extensions are as effective as changing the compiler. Our implementation of continuation marks for Chez Scheme (in support of Racket) makes dynamic binding and lookup constant-time and fast, preserves the performance of Chez Scheme's first-class continuations, and imposes negligible overhead on program fragments that do not use first-class continuations or marks.

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Crafty: Efficient, HTM-Compatible Persistent Transactions
Kaan Genç, Michael D. Bond, and Guoqing Harry Xu
(Ohio State University, USA; University of California at Los Angeles, USA)
Byte-addressable persistent memory, such as Intel/Micron 3D XPoint, is an emerging technology that bridges the gap between volatile memory and persistent storage. Data in persistent memory survives crashes and restarts; however, it is challenging to ensure that this data is consistent after failures. Existing approaches incur significant performance costs to ensure crash consistency.
This paper introduces Crafty, a new approach for ensuring consistency and atomicity on persistent memory operations using commodity hardware with existing hardware transactional memory (HTM) capabilities, while incurring low overhead. Crafty employs a novel technique called nondestructive undo logging that leverages commodity HTM to control persist ordering. Our evaluation shows that Crafty outperforms state-of-the-art prior work under low contention, and performs competitively under high contention.

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From Folklore to Fact: Comparing Implementations of Stacks and Continuations
Kavon Farvardin and John Reppy
(University of Chicago, USA)
The efficient implementation of function calls and non-local control transfers is a critical part of modern language implementations and is important in the implementation of everything from recursion, higher-order functions, concurrency and coroutines, to task-based parallelism. In a compiler, these features can be supported by a variety of mechanisms, including call stacks, segmented stacks, and heap-allocated continuation closures.
An implementor of a high-level language with advanced control features might ask the question ``what is the best choice for my implementation?'' Unfortunately, the current literature does not provide much guidance, since previous studies suffer from various flaws in methodology and are outdated for modern hardware. In the absence of recent, well-normalized measurements and a holistic overview of their implementation specifics, the path of least resistance when choosing a strategy is to trust folklore, but the folklore is also suspect.
This paper attempts to remedy this situation by providing an ``apples-to-apples'' comparison of six different approaches to implementing call stacks and continuations. This comparison uses the same source language, compiler pipeline, LLVM-backend, and runtime system, with the only differences being those required by the differences in implementation strategy. We compare the implementation challenges of the different approaches, their sequential performance, and their suitability to support advanced control mechanisms, including supporting heavily threaded code. In addition to the comparison of implementation strategies, the paper's contributions also include a number of useful implementation techniques that we discovered along the way.

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Machine Learning I

Typilus: Neural Type Hints
Miltiadis Allamanis, Earl T. Barr, Soline Ducousso, and Zheng Gao
(Microsoft Research, UK; University College London, UK; ENSTA Paris, France)
Type inference over partial contexts in dynamically typed languages is challenging. In this work, we present a graph neural network model that predicts types by probabilistically reasoning over a program’s structure, names, and patterns. The network uses deep similarity learning to learn a TypeSpace — a continuous relaxation of the discrete space of types — and how to embed the type properties of a symbol (i.e. identifier) into it. Importantly, our model can employ one-shot learning to predict an open vocabulary of types, including rare and user-defined ones. We realise our approach in Typilus for Python that combines the TypeSpace with an optional type checker. We show that Typilus accurately predicts types. Typilus confidently predicts types for 70% of all annotatable symbols; when it predicts a type, that type optionally type checks 95% of the time. Typilus can also find incorrect type annotations; two important and popular open source libraries, fairseq and allennlp, accepted our pull requests that fixed the annotation errors Typilus discovered.

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Learning Nonlinear Loop Invariants with Gated Continuous Logic Networks
Jianan Yao, Gabriel Ryan, Justin Wong, Suman Jana, and Ronghui Gu
(Columbia University, USA)
Verifying real-world programs often requires inferring loop invariants with nonlinear constraints. This is especially true in programs that perform many numerical operations, such as control systems for avionics or industrial plants. Recently, data-driven methods for loop invariant inference have shown promise, especially on linear loop invariants. However, applying data-driven inference to nonlinear loop invariants is challenging due to the large numbers of and large magnitudes of high-order terms, the potential for overfitting on a small number of samples, and the large space of possible nonlinear inequality bounds.
In this paper, we introduce a new neural architecture for general SMT learning, the Gated Continuous Logic Network (G-CLN), and apply it to nonlinear loop invariant learning. G-CLNs extend the Continuous Logic Network (CLN) architecture with gating units and dropout, which allow the model to robustly learn general invariants over large numbers of terms. To address overfitting that arises from finite program sampling, we introduce fractional sampling—a sound relaxation of loop semantics to continuous functions that facilitates unbounded sampling on the real domain. We additionally design a new CLN activation function, the Piecewise Biased Quadratic Unit (PBQU), for naturally learning tight inequality bounds.
We incorporate these methods into a nonlinear loop invariant inference system that can learn general nonlinear loop invariants. We evaluate our system on a benchmark of nonlinear loop invariants and show it solves 26 out of 27 problems, 3 more than prior work, with an average runtime of 53.3 seconds. We further demonstrate the generic learning ability of G-CLNs by solving all 124 problems in the linear Code2Inv benchmark. We also perform a quantitative stability evaluation and show G-CLNs have a convergence rate of 97.5% on quadratic problems, a 39.2% improvement over CLN models.

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Blended, Precise Semantic Program Embeddings
Ke Wang and Zhendong Su
(Visa Research, USA; ETH Zurich, Switzerland)
Learning neural program embeddings is key to utilizing deep neural networks in program languages research --- precise and efficient program representations enable the application of deep models to a wide range of program analysis tasks. Existing approaches predominately learn to embed programs from their source code, and, as a result, they do not capture deep, precise program semantics. On the other hand, models learned from runtime information critically depend on the quality of program executions, thus leading to trained models with highly variant quality. This paper tackles these inherent weaknesses of prior approaches by introducing a new deep neural network, Liger, which learns program representations from a mixture of symbolic and concrete execution traces. We have evaluated Liger on two tasks: method name prediction and semantics classification. Results show that Liger is significantly more accurate than the state-of-the-art static model code2seq in predicting method names, and requires on average around 10x fewer executions covering nearly 4x fewer paths than the state-of-the-art dynamic model DYPRO in both tasks. Liger offers a new, interesting design point in the space of neural program embeddings and opens up this new direction for exploration.

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Security

Towards a Verified Range Analysis for JavaScript JITs
Fraser Brown, John Renner, Andres Nötzli, Sorin Lerner, Hovav Shacham, and Deian Stefan
(Stanford University, USA; University of California at San Diego, USA; University of Texas at Austin, USA)
We present VeRA, a system for verifying the range analysis pass in browser just-in-time (JIT) compilers. Browser developers write range analysis routines in a subset of C++, and verification developers write infrastructure to verify custom analysis properties. Then, VeRA automatically verifies the range analysis routines, which browser developers can integrate directly into the JIT. We use VeRA to translate and verify Firefox range analysis routines, and it detects a new, confirmed bug that has existed in the browser for six years.

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Binary Rewriting without Control Flow Recovery
Gregory J. Duck, Xiang Gao, and Abhik Roychoudhury
(National University of Singapore, Singapore)
Static binary rewriting has many important applications in software security and systems, such as hardening, repair, patching, instrumentation, and debugging. While many different static binary rewriting tools have been proposed, most rely on recovering control flow information from the input binary. The recovery step is necessary since the rewriting process may move instructions, meaning that the set of jump targets in the rewritten binary needs to be adjusted accordingly. Since the static recovery of control flow information is a hard problem in general, most tools rely on a set of simplifying heuristics or assumptions, such as specific compilers, specific source languages, or binary file meta information. However, the reliance on assumptions or heuristics tends to scale poorly in practice, and most state-of-the-art static binary rewriting tools cannot handle very large/complex programs such as web browsers.
In this paper we present E9Patch, a tool that can statically rewrite x86_64 binaries without any knowledge of control flow information. To do so, E9Patch develops a suite of binary rewriting methodologies---such as instruction punning, padding, and eviction---that can insert jumps to trampolines without the need to move other instructions. Since this preserves the set of jump targets, the need for control flow recovery and related heuristics is eliminated. As such, E9Patch is robust by design, and can scale to very large (>100MB) stripped binaries including the Google Chrome and FireFox web browsers. We also evaluate the effectiveness of E9Patch against realistic applications such as binary instrumentation, hardening and repair.

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BlankIt Library Debloating: Getting What You Want Instead of Cutting What You Don’t
Chris Porter, Girish Mururu, Prithayan Barua, and Santosh Pande
(Georgia Institute of Technology, USA)
Modern software systems make extensive use of libraries derived from C and C++. Because of the lack of memory safety in these languages, however, the libraries may suffer from vulnerabilities, which can expose the applications to potential attacks. For example, a very large number of return-oriented programming gadgets exist in glibc that allow stitching together semantically valid but malicious Turing-complete and -incomplete programs. While CVEs get discovered and often patched and remedied, such gadgets serve as building blocks of future undiscovered attacks, opening an ever-growing set of possibilities for generating malicious programs. Thus, significant reduction in the quantity and expressiveness (utility) of such gadgets for libraries is an important problem.
In this work, we propose a new approach for handling an application’s library functions that focuses on the principle of “getting only what you want.” This is a significant departure from the current approaches that focus on “cutting what is unwanted.” Our approach focuses on activating/deactivating library functions on demand in order to reduce the dynamically linked code surface, so that the possibilities of constructing malicious programs diminishes substantially. The key idea is to load only the set of library functions that will be used at each library call site within the application at runtime. This approach of demand-driven loading relies on an input-aware oracle that predicts a near-exact set of library functions needed at a given call site during the execution. The predicted functions are loaded just in time and unloaded on return.
We present a decision-tree based predictor, which acts as an oracle, and an optimized runtime system, which works directly with library binaries like GNU libc and libstdc++. We show that on average, the proposed scheme cuts the exposed code surface of libraries by 97.2%, reduces ROP gadgets present in linked libraries by 97.9%, achieves a prediction accuracy in most cases of at least 97%, and adds a runtime overhead of 18% on all libraries (16% for glibc, 2% for others) across all benchmarks of SPEC 2006. Further, we demonstrate BlankIt on two real-world applications, sshd and nginx, with a high amount of debloating and low overheads.

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Verification I

Verifying Concurrent Search Structure Templates
Siddharth Krishna, Nisarg Patel, Dennis Shasha, and Thomas Wies
(Microsoft Research, UK; New York University, USA)
Concurrent separation logics have had great success reasoning about concurrent data structures. This success stems from their application of modularity on multiple levels, leading to proofs that are decomposed according to program structure, program state, and individual threads. Despite these advances, it remains difficult to achieve proof reuse across different data structure implementations. For the large class of search structures, we demonstrate how one can achieve further proof modularity by decoupling the proof of thread safety from the proof of structural integrity. We base our work on the template algorithms of Shasha and Goodman that dictate how threads interact but abstract from the concrete layout of nodes in memory. Building on the recently proposed flow framework of compositional abstractions and the separation logic Iris, we show how to prove correctness of template algorithms, and how to instantiate them to obtain multiple verified implementations.
We demonstrate our approach by mechanizing the proofs of three concurrent search structure templates, based on link, give-up, and lock-coupling synchronization, and deriving verified implementations based on B-trees, hash tables, and linked lists. These case studies include algorithms used in real-world file systems and databases, which have been beyond the capability of prior automated or mechanized verification techniques. In addition, our approach reduces proof complexity and is able to achieve significant proof reuse.

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Armada: Low-Effort Verification of High-Performance Concurrent Programs
Jacob R. Lorch, Yixuan Chen, Manos Kapritsos, Bryan Parno, Shaz Qadeer, Upamanyu Sharma, James R. Wilcox, and Xueyuan Zhao
(Microsoft Research, USA; University of Michigan, USA; Yale University, USA; Carnegie Mellon University, USA; Calibra, USA; Certora, USA)
Safely writing high-performance concurrent programs is notoriously difficult. To aid developers, we introduce Armada, a language and tool designed to formally verify such programs with relatively little effort. Via a C-like language and a small-step, state-machine-based semantics, Armada gives developers the flexibility to choose arbitrary memory layout and synchronization primitives so they are never constrained in their pursuit of performance. To reduce developer effort, Armada leverages SMT-powered automation and a library of powerful reasoning techniques, including rely-guarantee, TSO elimination, reduction, and alias analysis. All these techniques are proven sound, and Armada can be soundly extended with additional strategies over time. Using Armada, we verify four concurrent case studies and show that we can achieve performance equivalent to that of unverified code.

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Decidable Verification under a Causally Consistent Shared Memory
Ori Lahav and Udi Boker
(Tel Aviv University, Israel; IDC Herzliya, Israel)
Causal consistency is one of the most fundamental and widely used consistency models weaker than sequential consistency. In this paper, we study the verification of safety properties for finite-state concurrent programs running under a causally consistent shared memory model. We establish the decidability of this problem for a standard model of causal consistency (called also "Causal Convergence" and "Strong-Release-Acquire"). Our proof proceeds by developing an alternative operational semantics, based on the notion of a thread potential, that is equivalent to the existing declarative semantics and constitutes a well-structured transition system. In particular, our result allows for the verification of a large family of programs in the Release/Acquire fragment of C/C++11 (RA). Indeed, while verification under RA was recently shown to be undecidable for general programs, since RA coincides with the model we study here for write/write-race-free programs, the decidability of verification under RA for this widely used class of programs follows from our result. The novel operational semantics may also be of independent use in the investigation of weakly consistent shared memory models and their verification.

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Inductive Sequentialization of Asynchronous Programs
Bernhard Kragl, Constantin Enea, Thomas A. Henzinger, Suha Orhun Mutluergil, and Shaz Qadeer
(IST Austria, Austria; IRIF, France; University of Paris, France; CNRS, France; Calibra, USA)
Asynchronous programs are notoriously difficult to reason about because they spawn computation tasks which take effect asynchronously in a nondeterministic way. Devising inductive invariants for such programs requires understanding and stating complex relationships between an unbounded number of computation tasks in arbitrarily long executions. In this paper, we introduce inductive sequentialization, a new proof rule that sidesteps this complexity via a sequential reduction, a sequential program that captures every behavior of the original program up to reordering of coarse-grained commutative actions. A sequential reduction of a concurrent program is easy to reason about since it corresponds to a simple execution of the program in an idealized synchronous environment, where processes act in a fixed order and at the same speed. We have implemented and integrated our proof rule in the CIVL verifier, allowing us to provably derive fine-grained implementations of asynchronous programs. We have successfully applied our proof rule to a diverse set of message-passing protocols, including leader election protocols, two-phase commit, and Paxos.

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Language Design I

The Essence of Bluespec: A Core Language for Rule-Based Hardware Design
Thomas Bourgeat, Clément Pit-Claudel, Adam Chlipala, and Arvind
(Massachusetts Institute of Technology, USA)
The Bluespec hardware-description language presents a significantly higher-level view than hardware engineers are used to, exposing a simpler concurrency model that promotes formal proof, without compromising on performance of compiled circuits. Unfortunately, the cost model of Bluespec has been unclear, with performance details depending on a mix of user hints and opaque static analysis of potential concurrency conflicts within a design. In this paper we present Koika, a derivative of Bluespec that preserves its desirable properties and yet gives direct control over the scheduling decisions that determine performance. Koika has a novel and deterministic operational semantics that uses dynamic analysis to avoid concurrency anomalies. Our implementation includes Coq definitions of syntax, semantics, key metatheorems, and a verified compiler to circuits. We argue that most of the extra circuitry required for dynamic analysis can be eliminated by compile-time BSV-style static analysis.

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LLHD: A Multi-level Intermediate Representation for Hardware Description Languages
Fabian Schuiki, Andreas Kurth, Tobias Grosser, and Luca Benini
(ETH Zurich, Switzerland)
Modern Hardware Description Languages (HDLs) such as SystemVerilog or VHDL are, due to their sheer complexity, insufficient to transport designs through modern circuit design flows. Instead, each design automation tool lowers HDLs to its own Intermediate Representation (IR). These tools are monolithic and mostly proprietary, disagree in their implementation of HDLs, and while many redundant IRs exists, no IR today can be used through the entire circuit design flow. To solve this problem, we propose the LLHD multi-level IR. LLHD is designed as simple, unambiguous reference description of a digital circuit, yet fully captures existing HDLs. We show this with our reference compiler on designs as complex as full CPU cores. LLHD comes with lowering passes to a hardware-near structural IR, which readily integrates with existing tools. LLHD establishes the basis for innovation in HDLs and tools without redundant compilers or disjoint IRs. For instance, we implement an LLHD simulator that runs up to 2.4× faster than commercial simulators but produces equivalent, cycle-accurate results. An initial vertically-integrated research prototype is capable of representing all levels of the IR, implements lowering from the behavioural to the structural IR, and covers a sufficient subset of SystemVerilog to support a full CPU design.

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On the Principles of Differentiable Quantum Programming Languages
Shaopeng Zhu, Shih-Han Hung, Shouvanik Chakrabarti, and Xiaodi Wu
(University of Maryland, USA)
Variational Quantum Circuits (VQCs), or the so-called quantum neural-networks, are predicted to be one of the most important near-term quantum applications, not only because of their similar promises as classical neural-networks, but also because of their feasibility on near-term noisy intermediate-size quantum (NISQ) machines. The need for gradient information in the training procedure of VQC applications has stimulated the development of auto-differentiation techniques for quantum circuits. We propose the first formalization of this technique, not only in the context of quantum circuits but also for imperative quantum programs (e.g., with controls), inspired by the success of differentiable programming languages in classical machine learning. In particular, we overcome a few unique difficulties caused by exotic quantum features (such as quantum no-cloning) and provide a rigorous formulation of differentiation applied to bounded-loop imperative quantum programs, its code-transformation rules, as well as a sound logic to reason about their correctness. Moreover, we have implemented our code transformation in OCaml and demonstrated the resource-efficiency of our scheme both analytically and empirically. We also conduct a case study of training a VQC instance with controls, which shows the advantage of our scheme over existing auto-differentiation for quantum circuits without controls.

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Silq: A High-Level Quantum Language with Safe Uncomputation and Intuitive Semantics
Benjamin Bichsel, Maximilian Baader, Timon Gehr, and Martin Vechev
(ETH Zurich, Switzerland)
Existing quantum languages force the programmer to work at a low level of abstraction leading to unintuitive and cluttered code. A fundamental reason is that dropping temporary values from the program state requires explicitly applying quantum operations that safely uncompute these values.
We present Silq, the first quantum language that addresses this challenge by supporting safe, automatic uncomputation. This enables an intuitive semantics that implicitly drops temporary values, as in classical computation. To ensure physicality of Silq's semantics, its type system leverages novel annotations to reject unphysical programs.
Our experimental evaluation demonstrates that Silq programs are not only easier to read and write, but also significantly shorter than equivalent programs in other quantum languages (on average -46% for Q#, -38% for Quipper), while using only half the number of quantum primitives.

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Memory Management

Improving Program Locality in the GC using Hotness
Albert Mingkun Yang, Erik Österlund, and Tobias Wrigstad
(Uppsala University, Sweden; Oracle, Sweden)
The hierarchical memory system with increasingly small and increasingly fast memory closer to the CPU has for long been at the heart of hiding, or mitigating the performance gap between memories and processors. To utilise this hardware, programs must be written to exhibit good object locality. In languages like C/C++, programmers can carefully plan how objects should be laid out (albeit time consuming and error-prone); for managed languages, especially ones with moving garbage collectors, a manually created optimal layout may be destroyed in the process of object relocation. For managed languages that present an abstract view of memory, the solution lies in making the garbage collector aware of object locality, and strive to achieve and maintain good locality, even in the face of multi-phased programs that exhibit different behaviour across different phases.
This paper presents a GC design that dynamically reorganises objects in the order mutators access them, and additionally strives to separate frequently and infrequently used objects in memory. This improves locality and the efficiency of hardware prefetching. Identifying frequently used objects is done at run-time, with small overhead. HCSGC also offers tunability, for shifting relocation work towards mutators, or for more or less aggressive object relocation.
The ideas are evaluated in the context of the ZGC collector on OpenJDK and yields performance improvements of 5% (tradebeans), 9% (h2) and an impressive 25–45% (JGraphT), all with 95% confidence. For SPECjbb, results are inconclusive due to a fluctuating baseline.

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A Marriage of Pointer- and Epoch-Based Reclamation
Jeehoon Kang and Jaehwang Jung
(KAIST, South Korea)
All pointer-based nonblocking concurrent data structures should deal with the problem of safe memory reclamation: before reclaiming a memory block, a thread should ensure no other threads hold a local pointer to the block that may later be dereferenced. Various safe memory reclamation schemes have been proposed in the literature, but none of them satisfy the following desired properties at the same time: (i) robust: a non-cooperative thread does not prevent the other threads from reclaiming an unbounded number of blocks; (ii) fast: it does not incur significant time overhead; (iii) compact: it does not incur significant space overhead; (iv) self-contained: it neither relies on special hardware/OS supports nor intrusively affects execution environments; and (v) widely applicable: it supports many data structures.
We introduce PEBR, which we believe is the first scheme that satisfies all the properties above. PEBR is inspired by Snowflake’s hybrid design of pointer- and epoch-based reclamation schemes (PBR and EBR, resp.) that is mostly robust, fast, and compact but neither self-contained nor widely applicable. To achieve self-containedness, we design algorithms using only the standard C/C++ concurrency features and process-wide memory fence. To achieve wide applicability, we characterize PEBR’s requirement for safe reclamation that is satisfied by a variety of data structures, including Harris’s and Harris-Herlihy-Shavit’s lists that are not supported by PBR. We experimentally evaluate whether PEBR is fast and robust using microbenchmarks, for which PEBR performs comparably to the state-of-the-art schemes.

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CARAT: A Case for Virtual Memory through Compiler- and Runtime-Based Address Translation
Brian Suchy, Simone Campanoni, Nikos Hardavellas, and Peter Dinda
(Northwestern University, USA)
Virtual memory is a critical abstraction in modern computer systems. Its common model, paging, is currently seeing considerable innovation, yet its implementations continue to be co-designs between power-hungry/latency-adding hardware (e.g., TLBs, pagewalk caches, pagewalkers, etc) and software (the OS kernel). We make a case for a new model for virtual memory, compiler- and runtime-based address translation (CARAT), which instead is a co-design between the compiler and the OS kernel. CARAT can operate without any hardware support, although it could also be retrofitted into a traditional paging model, and could leverage simpler hardware support. CARAT uses compile-time transformations and optimizations combined with tightly-coupled runtime/kernel interaction to generate programs that run efficiently in a physical address space, but nonetheless allow the kernel to maintain protection and dynamically manage physical memory similar to what is possible using traditional virtual memory. We argue for the feasibility of CARAT through an empirical study of application characteristics and kernel behavior, as well as through the design, implementation, and performance evaluation of a CARAT prototype. Because our prototype works at the IR level (in particular, via LLVM bitcode), it can be applied to most C and C++ programs with minimal or no restrictions.

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Concurrency

Repairing and Mechanising the JavaScript Relaxed Memory Model
Conrad Watt, Christopher Pulte, Anton Podkopaev, Guillaume Barbier, Stephen Dolan, Shaked Flur, Jean Pichon-Pharabod, and Shu-yu Guo
(University of Cambridge, UK; National Research University Higher School of Economics, Russia; MPI-SWS, Germany; ENS Rennes, France; Bloomberg, USA)
Modern JavaScript includes the SharedArrayBuffer feature, which provides access to true shared memory concurrency. SharedArrayBuffers are simple linear buffers of bytes, and the JavaScript specification defines an axiomatic relaxed memory model to describe their behaviour. While this model is heavily based on the C/C++11 model, it diverges in some key areas. JavaScript chooses to give a well-defined semantics to data-races, unlike the "undefined behaviour" of C/C++11. Moreover, the JavaScript model is mixed-size. This means that its accesses are not to discrete locations, but to (possibly overlapping) ranges of bytes.
We show that the model, in violation of the design intention, does not support a compilation scheme to ARMv8 which is used in practice. We propose a correction, which also incorporates a previously proposed fix for a failure of the model to provide Sequential Consistency of Data-Race-Free programs (SC-DRF), an important correctness condition. We use model checking, in Alloy, to generate small counter-examples for these deficiencies, and investigate our correction. To accomplish this, we also develop a mixed-size extension to the existing ARMv8 axiomatic model.
Guided by our Alloy experimentation, we mechanise (in Coq) the JavaScript model (corrected and uncorrected), our ARMv8 model, and, for the corrected JavaScript model, a "model-internal" SC-DRF proof and a compilation scheme correctness proof to ARMv8. In addition, we investigate a non-mixed-size subset of the corrected JavaScript model, and give proofs of compilation correctness for this subset to x86-TSO, Power, RISC-V, ARMv7, and (again) ARMv8, via the Intermediate Memory Model (IMM).
As a result of our work, the JavaScript standards body (ECMA TC39) will include fixes for both issues in an upcoming edition of the specification.

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Promising 2.0: Global Optimizations in Relaxed Memory Concurrency
Sung-Hwan Lee, Minki Cho, Anton Podkopaev, Soham Chakraborty, Chung-Kil Hur, Ori Lahav, and Viktor Vafeiadis
(Seoul National University, South Korea; National Research University Higher School of Economics, Russia; MPI-SWS, Germany; IIT Delhi, India; Tel Aviv University, Israel)
For more than fifteen years, researchers have tried to support global optimizations in a usable semantics for a concurrent programming language, yet this task has been proven to be very difficult because of (1) the infamous “out of thin air” problem, and (2) the subtle interaction between global and thread-local optimizations.
In this paper, we present a solution to this problem by redesigning a key component of the promising semantics (PS) of Kang et al. Our updated PS 2.0 model supports all the results known about the original PS model (i.e., thread-local optimizations, hardware mappings, DRF theorems), but additionally enables transformations based on global value-range analysis as well as register promotion (i.e., making accesses to a shared location local if the location is accessed by only one thread). PS 2.0 also resolves a problem with the compilation of relaxed RMWs to ARMv8, which required an unintended extra fence.

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NVTraverse: In NVRAM Data Structures, the Destination Is More Important Than the Journey
Michal Friedman, Naama Ben-David, Yuanhao Wei, Guy E. Blelloch, and Erez Petrank
(Technion, Israel; Carnegie Mellon University, USA)
The recent availability of fast, dense, byte-addressable non-volatile memory has led to increasing interest in the problem of designing durable data structures that can recover from system crashes. However, designing durable concurrent data structures that are correct and efficient has proven to be very difficult, leading to many inefficient or incorrect algorithms. In this paper, we present a general transformation that takes a lock-free data structure from a general class called traversal data structure (that we formally define) and automatically transforms it into an implementation of the data structure for the NVRAM setting that is provably durably linearizable and highly efficient. The transformation hinges on the observation that many data structure operations begin with a traversal phase that does not need to be persisted, and thus we only begin persisting when the traversal reaches its destination. We demonstrate the transformation's efficiency through extensive measurements on a system with Intel's recently released Optane DC persistent memory, showing that it can outperform competitors on many workloads.

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Type Systems

Predictable Accelerator Design with Time-Sensitive Affine Types
Rachit Nigam, Sachille Atapattu, Samuel Thomas, Zhijing Li, Theodore Bauer, Yuwei Ye, Apurva Koti, Adrian Sampson, and Zhiru Zhang
(Cornell University, USA)
Field-programmable gate arrays (FPGAs) provide an opportunity to co-design applications with hardware accelerators, yet they remain difficult to program. High-level synthesis (HLS) tools promise to raise the level of abstraction by compiling C or C++ to accelerator designs. Repurposing legacy software languages, however, requires complex heuristics to map imperative code onto hardware structures. We find that the black-box heuristics in HLS can be unpredictable: changing parameters in the program that should improve performance can counterintuitively yield slower and larger designs. This paper proposes a type system that restricts HLS to programs that can predictably compile to hardware accelerators. The key idea is to model consumable hardware resources with a time-sensitive affine type system that prevents simultaneous uses of the same hardware structure. We implement the type system in Dahlia, a language that compiles to HLS C++, and show that it can reduce the size of HLS parameter spaces while accepting Pareto-optimal designs.

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Type-Directed Scheduling of Streaming Accelerators
David Durst, Matthew Feldman, Dillon Huff, David Akeley, Ross Daly, Gilbert Louis Bernstein, Marco Patrignani, Kayvon Fatahalian, and Pat Hanrahan
(Stanford University, USA; University of California at Los Angeles, USA; University of California at Berkeley, USA; CISPA, Germany)
Designing efficient, application-specialized hardware accelerators requires assessing trade-offs between a hardware module’s performance and resource requirements. To facilitate hardware design space exploration, we describe Aetherling, a system for automatically compiling data-parallel programs into statically scheduled, streaming hardware circuits. Aetherling contributes a space- and time-aware intermediate language featuring data-parallel operators that represent parallel or sequential hardware modules, and sequence data types that encode a module’s throughput by specifying when sequence elements are produced or consumed. As a result, well-typed operator composition in the space-time language corresponds to connecting hardware modules via statically scheduled, streaming interfaces.
We provide rules for transforming programs written in a standard data-parallel language (that carries no information about hardware implementation) into equivalent space-time language programs. We then provide a scheduling algorithm that searches over the space of transformations to quickly generate area-efficient hardware designs that achieve a programmer-specified throughput. Using benchmarks from the image processing domain, we demonstrate that Aetherling enables rapid exploration of hardware designs with different throughput and area characteristics, and yields results that require 1.8-7.9× fewer FPGA slices than those of prior hardware generation systems.

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FreezeML: Complete and Easy Type Inference for First-Class Polymorphism
Frank Emrich, Sam Lindley, Jan Stolarek, James Cheney, and Jonathan Coates
(University of Edinburgh, UK; Imperial College London, UK; Lodz University of Technology, Poland; Alan Turing Institute, UK)
ML is remarkable in providing statically typed polymorphism without the programmer ever having to write any type annotations. The cost of this parsimony is that the programmer is limited to a form of polymorphism in which quantifiers can occur only at the outermost level of a type and type variables can be instantiated only with monomorphic types.
Type inference for unrestricted System F-style polymorphism is undecidable in general. Nevertheless, the literature abounds with a range of proposals to bridge the gap between ML and System F.
We put forth a new proposal, FreezeML, a conservative extension of ML with two new features. First, let- and lambda-binders may be annotated with arbitrary System F types. Second, variable occurrences may be frozen, explicitly disabling instantiation. FreezeML is equipped with type-preserving translations back and forth between System F and admits a type inference algorithm, an extension of algorithm W, that is sound and complete and which yields principal types.

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Smart Contracts

Securing Smart Contract with Runtime Validation
Ao Li, Jemin Andrew Choi, and Fan Long
(University of Toronto, Canada)
We present Solythesis, a source to source Solidity compiler which takes a smart contract code and a user specified invariant as the input and produces an instrumented contract that rejects all transactions that violate the invariant. The design of Solythesis is driven by our observation that the consensus protocol and the storage layer are the primary and the secondary performance bottlenecks of Ethereum, respectively. Solythesis operates with our novel delta update and delta check techniques to minimize the overhead caused by the instrumented storage access statements. Our experimental results validate our hypothesis that the overhead of runtime validation, which is often too expensive for other domains, is in fact negligible for smart contracts. The CPU overhead of Solythesis is only 0.1% on average for our 23 benchmark contracts.

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Ethainter: A Smart Contract Security Analyzer for Composite Vulnerabilities
Lexi Brent, Neville Grech, Sifis Lagouvardos, Bernhard Scholz, and Yannis Smaragdakis
(International Computer Science Institute, USA; University of Sydney, Australia; University of Athens, Greece)
Smart contracts on permissionless blockchains are exposed to inherent security risks due to interactions with untrusted entities. Static analyzers are essential for identifying security risks and avoiding millions of dollars worth of damage.
We introduce Ethainter, a security analyzer checking information flow with data sanitization in smart contracts. Ethainter identifies composite attacks that involve an escalation of tainted information, through multiple transactions, leading to severe violations. The analysis scales to the entire blockchain, consisting of hundreds of thousands of unique smart contracts, deployed over millions of accounts. Ethainter is more precise than previous approaches, as we confirm by automatic exploit generation (e.g., destroying over 800 contracts on the Ropsten network) and by manual inspection, showing a very high precision of 82.5% valid warnings for end-to-end vulnerabilities. Ethainter’s balance of precision and completeness offers significant advantages over other tools such as Securify, Securify2, and teEther.

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Behavioral Simulation for Smart Contracts
Sidi Mohamed Beillahi, Gabriela Ciocarlie, Michael Emmi, and Constantin Enea
(University of Paris Diderot, France; IRIF, France; CNRS, France; SRI International, USA; IUF, France)
While smart contracts have the potential to revolutionize many important applications like banking, trade, and supply-chain, their reliable deployment begs for rigorous formal verification. Since most smart contracts are not annotated with formal specifications, general verification of functional properties is impeded.
In this work, we propose an automated approach to verify unannotated smart contracts against specifications ascribed to a few manually-annotated contracts. In particular, we propose a notion of behavioral refinement, which implies inheritance of functional properties. Furthermore, we propose an automated approach to inductive proof, by synthesizing simulation relations on the states of related contracts. Empirically, we demonstrate that behavioral simulations can be synthesized automatically for several ubiquitous classes like tokens, auctions, and escrow, thus enabling the verification of unannotated contracts against functional specifications.

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Synthesis II

Multi-modal Synthesis of Regular Expressions
Qiaochu Chen, Xinyu Wang, Xi Ye, Greg Durrett, and Isil Dillig
(University of Texas at Austin, USA; University of Michigan at Ann Arbor, USA)
In this paper, we propose a multi-modal synthesis technique for automatically constructing regular expressions (regexes) from a combination of examples and natural language. Using multiple modalities is useful in this context because natural language alone is often highly ambiguous, whereas examples in isolation are often not sufficient for conveying user intent. Our proposed technique first parses the English description into a so-called hierarchical sketch that guides our programming-by-example (PBE) engine. Since the hierarchical sketch captures crucial hints, the PBE engine can leverage this information to both prioritize the search as well as make useful deductions for pruning the search space.
We have implemented the proposed technique in a tool called Regel and evaluate it on over three hundred regexes. Our evaluation shows that Regel achieves 80 % accuracy whereas the NLP-only and PBE-only baselines achieve 43 % and 26 % respectively. We also compare our proposed PBE engine against an adaptation of AlphaRegex, a state-of-the-art regex synthesis tool, and show that our proposed PBE engine is an order of magnitude faster, even if we adapt the search algorithm of AlphaRegex to leverage the sketch. Finally, we conduct a user study involving 20 participants and show that users are twice as likely to successfully come up with the desired regex using Regel compared to without it.

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Optimizing Homomorphic Evaluation Circuits by Program Synthesis and Term Rewriting
DongKwon Lee, Woosuk Lee, Hakjoo Oh, and Kwangkeun Yi
(Seoul National University, South Korea; Hanyang University, South Korea; Korea University, South Korea)
We present a new and general method for optimizing homomorphic evaluation circuits. Although fully homomorphic encryption (FHE) holds the promise of enabling safe and secure third party computation, building FHE applications has been challenging due to their high computational costs. Domain-specific optimizations require a great deal of expertise on the underlying FHE schemes, and FHE compilers that aims to lower the hurdle, generate outcomes that are typically sub-optimal as they rely on manually-developed optimization rules. In this paper, based on the prior work of FHE compilers, we propose a method for automatically learning and using optimization rules for FHE circuits. Our method focuses on reducing the maximum multiplicative depth, the decisive performance bottleneck, of FHE circuits by combining program synthesis and term rewriting. It first uses program synthesis to learn equivalences of small circuits as rewrite rules from a set of training circuits. Then, we perform term rewriting on the input circuit to obtain a new circuit that has lower multiplicative depth. Our rewriting method maximally generalizes the learned rules based on the equational matching and its soundness and termination properties are formally proven. Experimental results show that our method generates circuits that can be homomorphically evaluated 1.18x – 3.71x faster (with the geometric mean of 2.05x) than the state-of-the-art method. Our method is also orthogonal to existing domain-specific optimizations.

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CacheQuery: Learning Replacement Policies from Hardware Caches
Pepe Vila, Pierre Ganty, Marco Guarnieri, and Boris Köpf
(IMDEA Software Institute, Spain; Universidad Politécnica de Madrid, Spain; Microsoft Research, UK)
We show how to infer deterministic cache replacement policies using off-the-shelf automata learning and program synthesis techniques. For this, we construct and chain two abstractions that expose the cache replacement policy of any set in the cache hierarchy as a membership oracle to the learning algorithm, based on timing measurements on a silicon CPU. Our experiments demonstrate an advantage in scope and scalability over prior art and uncover two previously undocumented cache replacement policies.

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Language Design II

HipHop.js: (A)Synchronous Reactive Web Programming
Gérard Berry and Manuel Serrano
(Collège de France, France; Inria, France)
We present HipHop.js, a synchronous reactive language that adds synchronous concurrency and preemption to JavaScript. Inspired from Esterel, HipHop.js simplifies the programming of non-trivial temporal behaviors as found in complex web interfaces or IoT controllers and the cooperation between synchronous and asynchronous activities. HipHop.js is compiled into plain sequential JavaScript and executes on unmodified runtime environments. We use three examples to present and discuss HipHop.js: a simple web login form to introduce the language and show how it differs from JavaScript, and two real life examples, a medical prescription pillbox and an interactive music system that show why concurrency and preemption help programming such temporal applications.

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EVA: An Encrypted Vector Arithmetic Language and Compiler for Efficient Homomorphic Computation
Roshan Dathathri, Blagovesta Kostova, Olli Saarikivi, Wei Dai, Kim Laine, and Madan Musuvathi
(University of Texas at Austin, USA; EPFL, Switzerland; Microsoft Research, USA)
Fully-Homomorphic Encryption (FHE) offers powerful capabilities by enabling secure offloading of both storage and computation, and recent innovations in schemes and implementations have made it all the more attractive. At the same time, FHE is notoriously hard to use with a very constrained programming model, a very unusual performance profile, and many cryptographic constraints. Existing compilers for FHE either target simpler but less efficient FHE schemes or only support specific domains where they can rely on expert-provided high-level runtimes to hide complications.
This paper presents a new FHE language called Encrypted Vector Arithmetic (EVA), which includes an optimizing compiler that generates correct and secure FHE programs, while hiding all the complexities of the target FHE scheme. Bolstered by our optimizing compiler, programmers can develop efficient general-purpose FHE applications directly in EVA. For example, we have developed image processing applications using EVA, with a very few lines of code.
EVA is designed to also work as an intermediate representation that can be a target for compiling higher-level domain-specific languages. To demonstrate this, we have re-targeted CHET, an existing domain-specific compiler for neural network inference, onto EVA. Due to the novel optimizations in EVA, its programs are on average 5.3× faster than those generated by CHET. We believe that EVA would enable a wider adoption of FHE by making it easier to develop FHE applications and domain-specific FHE compilers.

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Towards an API for the Real Numbers
Hans-J. Boehm
(Google, USA)
The real numbers are pervasive, both in daily life, and in mathematics. Students spend much time studying their properties. Yet computers and programming languages generally provide only an approximation geared towards performance, at the expense of many of the nice properties we were taught in high school.
Although this is entirely appropriate for many applications, particularly those that are sensitive to arithmetic performance in the usual sense, we argue that there are others where it is a poor choice. If arithmetic computations and result are directly exposed to human users who are not floating point experts, floating point approximations tend to be viewed as bugs. For applications such as calculators, spreadsheets, and various verification tasks, the cost of precision sacrifices is high, and the performance benefit is often not critical. We argue that previous attempts to provide accurate and understandable results for such applications using the recursive reals were great steps in the right direction, but they do not suffice. Comparing recursive reals diverges if they are equal. In many cases, comparison of numbers, including equal ones, is both important, particularly in simple cases, and intractable in the general case.
We propose an API for a real number type that explicitly provides decidable equality in the easy common cases, in which it is often unnatural not to. We describe a surprisingly compact and simple implementation in detail. The approach relies heavily on classical number theory results. We demonstrate the utility of such a facility in two applications: testing floating point functions, and to implement arithmetic in Google's Android calculator application.

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Responsive Parallelism with Futures and State
Stefan K. Muller, Kyle Singer, Noah Goldstein, Umut A. Acar, Kunal Agrawal, and I-Ting Angelina Lee
(Carnegie Mellon University, USA; Washington University in St. Louis, USA)
Motivated by the increasing shift to multicore computers, recent work has developed language support for responsive parallel applications that mix compute-intensive tasks with latency-sensitive, usually interactive, tasks. These developments include calculi that allow assigning priorities to threads, type systems that can rule out priority inversions, and accompanying cost models for predicting responsiveness. These advances share one important limitation: all of this work assumes purely functional programming. This is a significant restriction, because many realistic interactive applications, from games to robots to web servers, use mutable state, e.g., for communication between threads.
In this paper, we lift the restriction concerning the use of state. We present λi4, a calculus with implicit parallelism in the form of prioritized futures and mutable state in the form of references. Because both futures and references are first-class values, λi4 programs can exhibit complex dependencies, including interaction between threads and with the external world (users, network, etc). To reason about the responsiveness of λi4 programs, we extend traditional graph-based cost models for parallelism to account for dependencies created via mutable state, and we present a type system to outlaw priority inversions that can lead to unbounded blocking. We show that these techniques are practical by implementing them in C++ and present an empirical evaluation.

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Performance

SympleGraph: Distributed Graph Processing with Precise Loop-Carried Dependency Guarantee
Youwei Zhuo, Jingji Chen, Qinyi Luo, Yanzhi Wang, Hailong Yang, Depei Qian, and Xuehai Qian
(University of Southern California, USA; Northeastern University, USA; Beihang University, China)
Graph analytics is an important way to understand relationships in real-world applications. At the age of big data, graphs have grown to billions of edges. This motivates distributed graph processing. Graph processing frameworks ask programmers to specify graph computations in user- defined functions (UDFs) of graph-oriented programming model. Due to the nature of distributed execution, current frameworks cannot precisely enforce the semantics of UDFs, leading to unnecessary computation and communication. In essence, there exists a gap between programming model and runtime execution. This paper proposes SympleGraph, a novel distributed graph processing framework that precisely enforces loop-carried dependency, i.e., when a condition is satisfied by a neighbor, all following neighbors can be skipped. SympleGraph instruments the UDFs to express the loop-carried dependency, then the distributed execution framework enforces the precise semantics by performing dependency propagation dynamically. Enforcing loop-carried dependency requires the sequential processing of the neighbors of each vertex distributed in different nodes. Therefore, the major challenge is to enable sufficient parallelism to achieve high performance. We propose to use circulant scheduling in the framework to allow different machines to process disjoint sets of edges/vertices in parallel while satisfying the sequential requirement. It achieves a good trade-off between precise semantics and parallelism. The significant speedups in most graphs and algorithms indicate that the benefits of eliminating unnecessary computation and communication overshadow the reduced parallelism. Communication efficiency is further optimized by 1) selectively propagating dependency for large-degree vertices to increase net benefits; 2) double buffering to hide communication latency. In a 16-node cluster, SympleGraph outperforms the state-of-the-art system Gemini and D-Galois on average by 1.42× and 3.30×, and up to 2.30× and 7.76×, respectively. The communication reduction compared to Gemini is 40.95% on average and up to 67.48%.

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PMEvo: Portable Inference of Port Mappings for Out-of-Order Processors by Evolutionary Optimization
Fabian Ritter and Sebastian Hack
(Saarland University, Germany)
Achieving peak performance in a computer system requires optimizations in every layer of the system, be it hardware or software. A detailed understanding of the underlying hardware, and especially the processor, is crucial to optimize software. One key criterion for the performance of a processor is its ability to exploit instruction-level parallelism. This ability is determined by the port mapping of the processor, which describes the execution units of the processor for each instruction.
Processor manufacturers usually do not share the port mappings of their microarchitectures. While approaches to automatically infer port mappings from experiments exist, they are based on processor-specific hardware performance counters that are not available on every platform.
We present PMEvo, a framework to automatically infer port mappings solely based on the measurement of the execution time of short instruction sequences. PMEvo uses an evolutionary algorithm that evaluates the fitness of candidate mappings with an analytical throughput model formulated as a linear program. Our prototype implementation infers a port mapping for Intel's Skylake architecture that predicts measured instruction throughput with an accuracy that is competitive to existing work. Furthermore, it finds port mappings for AMD's Zen+ architecture and the ARM Cortex-A72 architecture, which are out of scope of existing techniques.

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PMThreads: Persistent Memory Threads Harnessing Versioned Shadow Copies
Zhenwei Wu, Kai Lu, Andrew Nisbet, Wenzhe Zhang, and Mikel Luján
(National University of Defense Technology, China; University of Manchester, UK)
Byte-addressable non-volatile memory (NVM) makes it possible to perform fast in-memory accesses to persistent data using standard load/store processor instructions. Some approaches for NVM are based on durable memory transactions and provide a persistent programming paradigm. However, they cannot be applied to existing multi-threaded applications without extensive source code modifications. Durable transactions typically rely on logging to enforce failure-atomic commits that include additional writes to NVM and considerable ordering overheads.
This paper presents PMThreads, a novel user-space runtime that provides transparent failure-atomicity for lock-based parallel programs. A shadow DRAM page is used to buffer application writes for efficient propagation to a dual-copy NVM persistent storage framework during a global quiescent state. In this state, the working NVM copy and the crash-consistent copy of each page are atomically updated, and their roles are switched. A global quiescent state is entered at timed intervals by intercepting pthread lock acquire and release operations to ensure that no thread holds a lock to persistent data.
Running on a dual-socket system with 20 cores, we show that PMThreads substantially outperforms the state-of-the-art Atlas, Mnemosyne and NVthreads systems for lock-based benchmarks (Phoenix, PARSEC benchmarks, and microbenchmark stress tests). Using Memcached, we also investigate the scalability of PMThreads and the effect of different time intervals for the quiescent state.

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SCAF: A Speculation-Aware Collaborative Dependence Analysis Framework
Sotiris Apostolakis, Ziyang Xu, Zujun Tan, Greg Chan, Simone Campanoni, and David I. August
(Princeton University, USA; Northwestern University, USA)
Program analysis determines the potential dataflow and control flow relationships among instructions so that compiler optimizations can respect these relationships to transform code correctly. Since many of these relationships rarely or never occur, speculative optimizations assert they do not exist while optimizing the code. To preserve correctness, speculative optimizations add validation checks to activate recovery code when these assertions prove untrue. This approach results in many missed opportunities because program analysis and thus other optimizations remain unaware of the full impact of these dynamically-enforced speculative assertions. To address this problem, this paper presents SCAF, a Speculation-aware Collaborative dependence Analysis Framework. SCAF learns of available speculative assertions via profiling, computes their full impact on memory dependence analysis, and makes this resulting information available for all code optimizations. SCAF is modular (adding new analysis modules is easy) and collaborative (modules cooperate to produce a result more precise than the confluence of all individual results). Relative to the best prior speculation-aware dependence analysis technique, by computing the full impact of speculation on memory dependence analysis, SCAF dramatically reduces the need for expensive-to-validate memory speculation in the hot loops of all 16 evaluated C/C++ SPEC benchmarks.

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Verification II

Scalable Validation of Binary Lifters
Sandeep Dasgupta, Sushant Dinesh, Deepan Venkatesh, Vikram S. Adve, and Christopher W. Fletcher
(University of Illinois at Urbana-Champaign, USA)
Validating the correctness of binary lifters is pivotal to gain trust in binary analysis, especially when used in scenarios where correctness is important. Existing approaches focus on validating the correctness of lifting instructions or basic blocks in isolation and do not scale to full programs. In this work, we show that formal translation validation of single instructions for a complex ISA like x86-64 is not only practical, but can be used as a building block for scalable full-program validation. Our work is the first to do translation validation of single instructions on an architecture as extensive as x86-64, uses the most precise formal semantics available, and has the widest coverage in terms of the number of instructions tested for correctness. Next, we develop a novel technique that uses validated instructions to enable program-level validation, without resorting to performance-heavy semantic equivalence checking. Specifically, we compose the validated IR sequences using a tool we develop called Compositional Lifter to create a reference standard. The semantic equivalence check between the reference and the lifter output is then reduced to a graph-isomorphism check through the use of semantic preserving transformations. The translation validation of instructions in isolation revealed 29 new bugs in McSema – a mature open-source lifter from x86-64 to LLVM IR. Towards the validation of full programs, our approach was able to prove the translational correctness of 2254/2348 functions taken from LLVM’s single-source benchmark test-suite.

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Polynomial Invariant Generation for Non-deterministic Recursive Programs
Krishnendu Chatterjee, Hongfei Fu, Amir Kafshdar Goharshady, and Ehsan Kafshdar Goharshady
(IST Austria, Austria; Shanghai Jiao Tong University, China; Ferdowsi University of Mashhad, Iran)
We consider the classical problem of invariant generation for programs with polynomial assignments and focus on synthesizing invariants that are a conjunction of strict polynomial inequalities. We present a sound and semi-complete method based on positivstellensaetze, i.e. theorems in semi-algebraic geometry that characterize positive polynomials over a semi-algebraic set.
On the theoretical side, the worst-case complexity of our approach is subexponential, whereas the worst-case complexity of the previous complete method (Kapur, ACA 2004) is doubly-exponential. Even when restricted to linear invariants, the best previous complexity for complete invariant generation is exponential (Colon et al, CAV 2003). On the practical side, we reduce the invariant generation problem to quadratic programming (QCLP), which is a classical optimization problem with many industrial solvers. We demonstrate the applicability of our approach by providing experimental results on several academic benchmarks. To the best of our knowledge, the only previous invariant generation method that provides completeness guarantees for invariants consisting of polynomial inequalities is (Kapur, ACA 2004), which relies on quantifier elimination and cannot even handle toy programs such as our running example.

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Templates and Recurrences: Better Together
Jason Breck, John Cyphert, Zachary Kincaid, and Thomas Reps
(University of Wisconsin-Madison, USA; Princeton University, USA)
This paper is the confluence of two streams of ideas in the literature on generating numerical invariants, namely: (1) template-based methods, and (2) recurrence-based methods.
A template-based method begins with a template that contains unknown quantities, and finds invariants that match the template by extracting and solving constraints on the unknowns. A disadvantage of template-based methods is that they require fixing the set of terms that may appear in an invariant in advance. This disadvantage is particularly prominent for non-linear invariant generation, because the user must supply maximum degrees on polynomials, bases for exponents, etc.
On the other hand, recurrence-based methods are able to find sophisticated non-linear mathematical relations, including polynomials, exponentials, and logarithms, because such relations arise as the solutions to recurrences. However, a disadvantage of past recurrence-based invariant-generation methods is that they are primarily loop-based analyses: they use recurrences to relate the pre-state and post-state of a loop, so it is not obvious how to apply them to a recursive procedure, especially if the procedure is non-linearly recursive (e.g., a tree-traversal algorithm).
In this paper, we combine these two approaches and obtain a technique that uses templates in which the unknowns are functions rather than numbers, and the constraints on the unknowns are recurrences. The technique synthesizes invariants involving polynomials, exponentials, and logarithms, even in the presence of arbitrary control-flow, including any combination of loops, branches, and (possibly non-linear) recursion. For instance, it is able to show that (i) the time taken by merge-sort is O(n log(n)), and (ii) the time taken by Strassen’s algorithm is O(nlog2(7)).

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First-Order Quantified Separators
Jason R. Koenig, Oded Padon, Neil Immerman, and Alex Aiken
(Stanford University, USA; University of Massachusetts at Amherst, USA)
Quantified first-order formulas, often with quantifier alternations, are increasingly used in the verification of complex systems. While automated theorem provers for first-order logic are becoming more robust, invariant inference tools that handle quantifiers are currently restricted to purely universal formulas. We define and analyze first-order quantified separators and their application to inferring quantified invariants with alternations. A separator for a given set of positively and negatively labeled structures is a formula that is true on positive structures and false on negative structures. We investigate the problem of finding a separator from the class of formulas in prenex normal form with a bounded number of quantifiers and show this problem is NP-complete by reduction to and from SAT. We also give a practical separation algorithm, which we use to demonstrate the first invariant inference procedure able to infer invariants with quantifier alternations.

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Bug Finding

Validating SMT Solvers via Semantic Fusion
Dominik Winterer, Chengyu Zhang, and Zhendong Su
(ETH Zurich, Switzerland; East China Normal University, China)
We introduce Semantic Fusion, a general, effective methodology for validating Satisfiability Modulo Theory (SMT) solvers. Our key idea is to fuse two existing equisatisfiable (i.e., both satisfiable or unsatisfiable) formulas into a new formula that combines the structures of its ancestors in a novel manner and preserves the satisfiability by construction. This fused formula is then used for validating SMT solvers.
We realized Semantic Fusion as YinYang, a practical SMT solver testing tool. During four months of extensive testing, YinYang has found 45 confirmed, unique bugs in the default arithmetic and string solvers of Z3 and CVC4, the two state-of-the-art SMT solvers. Among these, 41 have already been fixed by the developers. The majority (29/45) of these bugs expose critical soundness issues. Our bug reports and testing effort have been well-appreciated by SMT solver developers.

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Debugging and Detecting Numerical Errors in Computation with Posits
Sangeeta Chowdhary, Jay P. Lim, and Santosh Nagarakatte
(Rutgers University, USA)
Posit is a recently proposed alternative to the floating point representation (FP). It provides tapered accuracy. Given a fixed number of bits, the posit representation can provide better precision for some numbers compared to FP, which has generated significant interest in numerous domains. Being a representation with tapered accuracy, it can introduce high rounding errors for numbers outside the above golden zone. Programmers currently lack tools to detect and debug errors while programming with posits.
This paper presents PositDebug, a compile-time instrumentation that performs shadow execution with high precision values to detect various errors in computation using posits. To assist the programmer in debugging the reported error, PositDebug also provides directed acyclic graphs of instructions, which are likely responsible for the error. A contribution of this paper is the design of the metadata per memory location for shadow execution that enables productive debugging of errors with long-running programs. We have used PositDebug to detect and debug errors in various numerical applications written using posits. To demonstrate that these ideas are applicable even for FP programs, we have built a shadow execution framework for FP programs that is an order of magnitude faster than Herbgrind.

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SmartTrack: Efficient Predictive Race Detection
Jake Roemer, Kaan Genç, and Michael D. Bond
(Ohio State University, USA)
Widely used data race detectors, including the state-of-the-art FastTrack algorithm, incur performance costs that are acceptable for regular in-house testing, but miss races detectable from the analyzed execution. Predictive analyses detect more data races in an analyzed execution than FastTrack detects, but at significantly higher performance cost.
This paper presents SmartTrack, an algorithm that optimizes predictive race detection analyses, including two analyses from prior work and a new analysis introduced in this paper. SmartTrack incorporates two main optimizations: (1) epoch and ownership optimizations from prior work, applied to predictive analysis for the first time, and (2) novel conflicting critical section optimizations introduced by this paper. Our evaluation shows that SmartTrack achieves performance competitive with FastTrack—a qualitative improvement in the state of the art for data race detection.

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Understanding Memory and Thread Safety Practices and Issues in Real-World Rust Programs
Boqin Qin, Yilun Chen, Zeming Yu, Linhai Song, and Yiying Zhang
(Pennsylvania State University, USA; Purdue University, USA; University of California at San Diego, USA)
Rust is a young programming language designed for systems software development. It aims to provide safety guarantees like high-level languages and performance efficiency like low-level languages. The core design of Rust is a set of strict safety rules enforced by compile-time checking. To support more low-level controls, Rust allows programmers to bypass these compiler checks to write unsafe code.

It is important to understand what safety issues exist in real Rust programs and how Rust safety mechanisms impact programming practices. We performed the first empirical study of Rust by close, manual inspection of 850 unsafe code usages and 170 bugs in five open-source Rust projects, five widely-used Rust libraries, two online security databases, and the Rust standard library. Our study answers three important questions: how and why do programmers write unsafe code, what memory-safety issues real Rust programs have, and what concurrency bugs Rust programmers make. Our study reveals interesting real-world Rust program behaviors and new issues Rust programmers make. Based on our study results, we propose several directions of building Rust bug detectors and built two static bug detectors, both of which revealed previously unknown bugs.

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Static Analysis

Fast Graph Simplification for Interleaved Dyck-Reachability
Yuanbo Li, Qirun Zhang, and Thomas Reps
(Georgia Institute of Technology, USA; University of Wisconsin-Madison, USA)
Many program-analysis problems can be formulated as graph-reachability problems. Interleaved Dyck language reachability. Interleaved Dyck language reachability (InterDyck-reachability) is a fundamental framework to express a wide variety of program-analysis problems over edge-labeled graphs. The InterDyck language represents an intersection of multiple matched-parenthesis languages (i.e., Dyck languages). In practice, program analyses typically leverage one Dyck language to achieve context-sensitivity, and other Dyck languages to model data dependences, such as field-sensitivity and pointer references/dereferences. In the ideal case, an InterDyck-reachability framework should model multiple Dyck languages simultaneously.
Unfortunately, precise InterDyck-reachability is undecidable. Any practical solution must over-approximate the exact answer. In the literature, a lot of work has been proposed to over-approximate the InterDyck-reachability formulation. This paper offers a new perspective on improving both the precision and the scalability of InterDyck-reachability: we aim to simplify the underlying input graph G. Our key insight is based on the observation that if an edge is not contributing to any InterDyck-path, we can safely eliminate it from G. Our technique is orthogonal to the InterDyck-reachability formulation, and can serve as a pre-processing step with any over-approximating approaches for InterDyck-reachability. We have applied our graph simplification algorithm to pre-processing the graphs from a recent InterDyck-reachability-based taint analysis for Android. Our evaluation on three popular InterDyck-reachability algorithms yields promising results. In particular, our graph-simplification method improves both the scalability and precision of all three InterDyck-reachability algorithms, sometimes dramatically.

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Static Analysis of Java Enterprise Applications: Frameworks and Caches, the Elephants in the Room
Anastasios Antoniadis, Nikos Filippakis, Paddy Krishnan, Raghavendra Ramesh, Nicholas Allen, and Yannis Smaragdakis
(University of Athens, Greece; CERN, Switzerland; Oracle Labs, Australia; ConsenSys, Australia)
Enterprise applications are a major success domain of Java, and Java is the default setting for much modern static analysis research. It would stand to reason that high-quality static analysis of Java enterprise applications would be commonplace, but this is far from true. Major analysis frameworks feature virtually no support for enterprise applications and offer analyses that are woefully incomplete and vastly imprecise, when at all scalable.
In this work, we present two techniques for drastically enhancing the completeness and precision of static analysis for Java enterprise applications. The first technique identifies domain-specific concepts underlying all enterprise application frameworks, captures them in an extensible, declarative form, and achieves modeling of components and entry points in a largely framework-independent way. The second technique offers precision and scalability via a sound-modulo-analysis modeling of standard data structures.
In realistic enterprise applications (an order of magnitude larger than prior benchmarks in the literature) our techniques achieve high degrees of completeness (on average more than 4x higher than conventional techniques) and speedups of about 6x compared to the most precise conventional analysis, with higher precision on multiple metrics. The result is JackEE, an enterprise analysis framework that can offer precise, high-completeness static modeling of realistic enterprise applications.

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Automated Derivation of Parametric Data Movement Lower Bounds for Affine Programs
Auguste Olivry, Julien Langou, Louis-Noël Pouchet, P. Sadayappan, and Fabrice Rastello
(Grenoble Alps University, France; CNRS, France; Inria, France; Grenoble INP, France; LIG, France; University of Colorado at Denver, USA; Colorado State University, USA; University of Utah, USA)
Researchers and practitioners have for long worked on improving the computational complexity of algorithms, focusing on reducing the number of operations needed to perform a computation. However the hardware trend nowadays clearly shows a higher performance and energy cost for data movements than computations: quality algorithms have to minimize data movements as much as possible.
The theoretical operational complexity of an algorithm is a function of the total number of operations that must be executed, regardless of the order in which they will actually be executed. But theoretical data movement (or, I/O) complexity is fundamentally different: one must consider all possible legal schedules of the operations to determine the minimal number of data movements achievable, a major theoretical challenge. I/O complexity has been studied via complex manual proofs, e.g., refined from Ω(n3/√S) for matrix-multiply on a cache size S by Hong & Kung to 2n3/√S by Smith et al. While asymptotic complexity may be sufficient to compare I/O potential between broadly different algorithms, the accuracy of the reasoning depends on the tightness of these I/O lower bounds. Precisely, exposing constants is essential to enable precise comparison between different algorithms: for example the 2n3/√S lower bound allows to demonstrate the optimality of panel-panel tiling for matrix-multiplication.
We present the first static analysis to automatically derive non-asymptotic parametric expressions of data movement lower bounds with scaling constants, for arbitrary affine computations. Our approach is fully automatic, assisting algorithm designers to reason about I/O complexity and make educated decisions about algorithmic alternatives.

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Code Generation

Automatic Generation of Efficient Sparse Tensor Format Conversion Routines
Stephen Chou, Fredrik Kjolstad, and Saman Amarasinghe
(Massachusetts Institute of Technology, USA; Stanford University, USA)
This paper shows how to generate code that efficiently converts sparse tensors between disparate storage formats (data layouts) such as CSR, DIA, ELL, and many others. We decompose sparse tensor conversion into three logical phases: coordinate remapping, analysis, and assembly. We then develop a language that precisely describes how different formats group together and order a tensor’s nonzeros in memory. This lets a compiler emit code that performs complex remappings of nonzeros when converting between formats. We also develop a query language that can extract statistics about sparse tensors, and we show how to emit efficient analysis code that computes such queries. Finally, we define an abstract interface that captures how data structures for storing a tensor can be efficiently assembled given specific statistics about the tensor. Disparate formats can implement this common interface, thus letting a compiler emit optimized sparse tensor conversion code for arbitrary combinations of many formats without hard-coding for any specific combination.
Our evaluation shows that the technique generates sparse tensor conversion routines with performance between 1.00 and 2.01× that of hand-optimized versions in SPARSKIT and Intel MKL, two popular sparse linear algebra libraries. And by emitting code that avoids materializing temporaries, which both libraries need for many combinations of source and target formats, our technique outperforms those libraries by 1.78 to 4.01× for CSC/COO to DIA/ELL conversion.

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OOElala: Order-of-Evaluation Based Alias Analysis for Compiler Optimization
Ankush Phulia, Vaibhav Bhagee, and Sorav Bansal
(IIT Delhi, India)
In C, the order of evaluation of expressions is unspecified; further for expressions that do not involve function calls, C semantics ensure that there cannot be a data race between two evaluations that can proceed in either order (or concurrently). We explore the optimization opportunity enabled by these non-deterministic expression evaluation semantics in C, and provide a sound compile-time alias analysis to realize the same. Our algorithm is implemented as a part of the Clang/LLVM infrastructure, in a tool called OOElala. Our experimental results demonstrate that the untapped optimization opportunity is significant: code patterns that enable such optimizations are common; the enabled transformations can range from vectorization to improved instruction selection and register allocation; and the resulting speedups can be as high as 2.6x on already-optimized code.

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Effective Function Merging in the SSA Form
Rodrigo C. O. Rocha, Pavlos Petoumenos, Zheng Wang, Murray Cole, and Hugh Leather
(University of Edinburgh, UK; University of Manchester, UK; University of Leeds, UK)
Function merging is an important optimization for reducing code size. This technique eliminates redundant code across functions by merging them into a single function. While initially limited to identical or trivially similar functions, the most recent approach can identify all merging opportunities in arbitrary pairs of functions. However, this approach has a serious limitation which prevents it from reaching its full potential. Because it cannot handle phi-nodes, the state-of-the-art applies register demotion to eliminate them before applying its core algorithm. While a superficially minor workaround, this has a three-fold negative effect: by artificially lengthening the instruction sequences to be aligned, it hinders the identification of mergeable instruction; it prevents a vast number of functions from being profitably merged; it increases compilation overheads, both in terms of compile-time and memory usage.
We present SalSSA, a novel approach that fully supports the SSA form, removing any need for register demotion. By doing so, we notably increase the number of profitably merged functions. We implement SalSSA in LLVM and apply it to the SPEC 2006 and 2017 suites. Experimental results show that our approach delivers on average, 7.9% to 9.7% reduction on the final size of the compiled code. This translates to around 2x more code size reduction over the state-of-the-art. Moreover, as a result of aligning shorter sequences of instructions and reducing the number of wasteful merge operations, our new approach incurs an average compile-time overhead of only 5%, 3x less than the state-of-the-art, while also reducing memory usage by over 2x.

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Probabilistic Programming

Proving Almost-Sure Termination by Omega-Regular Decomposition
Jianhui Chen and Fei He
(Tsinghua University, China)
Almost-sure termination is the most basic liveness property of probabilistic programs. We present a novel decomposition-based approach for proving almost-sure termination of probabilistic programs with complex control-flow structure and non-determinism. Our approach automatically decomposes the runs of the probabilistic program into a finite union of ω-regular subsets and then proves almost-sure termination of each subset based on the notion of localized ranking supermartingales. Compared to the lexicographic methods and the compositional methods, our approach does not require a lexicographic order over the ranking supermartingales as well as the so-called unaffecting condition. Thus it has high generality. We present the algorithm of our approach and prove its soundness, as well as its relative completeness. We show that our approach can be applied to some hard cases and the evaluation on the benchmarks of previous works shows the significant efficiency of our approach.

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λPSI: Exact Inference for Higher-Order Probabilistic Programs
Timon Gehr, Samuel Steffen, and Martin Vechev
(ETH Zurich, Switzerland)
We present λPSI, the first probabilistic programming language and system that supports higher-order exact inference for probabilistic programs with first-class functions, nested inference and discrete, continuous and mixed random variables. λPSI’s solver is based on symbolic reasoning and computes the exact distribution represented by a program.
We show that λPSI is practically effective—it automatically computes exact distributions for a number of interesting applications, from rational agents to information theory, many of which could so far only be handled approximately.

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Reactive Probabilistic Programming
Guillaume Baudart, Louis Mandel, Eric Atkinson, Benjamin Sherman, Marc Pouzet, and Michael Carbin
(IBM Research, USA; Massachusetts Institute of Technology, USA; ENS, France; PSL University, France)
Synchronous modeling is at the heart of programming languages like Lustre, Esterel, or Scade used routinely for implementing safety critical control software, e.g., fly-by-wire and engine control in planes. However, to date these languages have had limited modern support for modeling uncertainty --- probabilistic aspects of the software's environment or behavior --- even though modeling uncertainty is a primary activity when designing a control system.
In this paper we present ProbZelus the first synchronous probabilistic programming language. ProbZelus conservatively provides the facilities of a synchronous language to write control software, with probabilistic constructs to model uncertainties and perform inference-in-the-loop.
We present the design and implementation of the language. We propose a measure-theoretic semantics of probabilistic stream functions and a simple type discipline to separate deterministic and probabilistic expressions. We demonstrate a semantics-preserving compilation into a first-order functional language that lends itself to a simple presentation of inference algorithms for streaming models. We also redesign the delayed sampling inference algorithm to provide efficient streaming inference. Together with an evaluation on several reactive applications, our results demonstrate that ProbZelus enables the design of reactive probabilistic applications and efficient, bounded memory inference.

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Symbolic Execution

Constant-Time Foundations for the New Spectre Era
Sunjay Cauligi, Craig Disselkoen, Klaus v. Gleissenthall, Dean Tullsen, Deian Stefan, Tamara Rezk, and Gilles Barthe
(University of California at San Diego, USA; Inria, France; MPI-SP, Germany; IMDEA Software Institute, Spain)
The constant-time discipline is a software-based countermeasure used for protecting high assurance cryptographic implementations against timing side-channel attacks. Constant-time is effective (it protects against many known attacks), rigorous (it can be formalized using program semantics), and amenable to automated verification. Yet, the advent of micro-architectural attacks makes constant-time as it exists today far less useful.
This paper lays foundations for constant-time programming in the presence of speculative and out-of-order execution. We present an operational semantics and a formal definition of constant-time programs in this extended setting. Our semantics eschews formalization of microarchitectural features (that are instead assumed under adversary control), and yields a notion of constant-time that retains the elegance and tractability of the usual notion. We demonstrate the relevance of our semantics in two ways: First, by contrasting existing Spectre-like attacks with our definition of constant-time. Second, by implementing a static analysis tool, Pitchfork, which detects violations of our extended constant-time property in real world cryptographic libraries.

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Gillian, Part I: A Multi-language Platform for Symbolic Execution
José Fragoso Santos, Petar Maksimović, Sacha-Élie Ayoun, and Philippa Gardner
(INESC-ID, Portugal; Instituto Superior Técnico, University of Lisbon, Portugal; Imperial College London, UK)
We introduce Gillian, a platform for developing symbolic analysis tools for programming languages. Here, we focus on the symbolic execution engine at the heart of Gillian, which is parametric on the memory model of the target language. We give a formal description of the symbolic analysis and a modular implementation that closely follows this description. We prove a parametric soundness result, introducing restriction on abstract states, which generalises path conditions used in classical symbolic execution. We instantiate to obtain trusted symbolic testing tools for JavaScript and C, and use these tools to find bugs in real-world code, thus demonstrating the viability of our parametric approach.

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Efficient Handling of String-Number Conversion
Parosh Aziz Abdulla, Mohamed Faouzi Atig, Yu-Fang Chen, Bui Phi Diep, Julian Dolby, Petr Janků, Hsin-Hung Lin, Lukáš Holík, and Wei-Cheng Wu
(Uppsala University, Sweden; Academia Sinica, Taiwan; IBM Research, USA; Brno University of Technology, Czechia; University of Southern California, USA)
String-number conversion is an important class of constraints needed for the symbolic execution of string-manipulating programs. In particular solving string constraints with string-number conversion is necessary for the analysis of scripting languages such as JavaScript and Python, where string-number conversion is a part of the definition of the core semantics of these languages. However, solving this type of constraint is very challenging for the state-of-the-art solvers. We propose in this paper an approach that can efficiently support both string-number conversion and other common types of string constraints. Experimental results show that it significantly outperforms other state-of-the-art tools on benchmarks that involves string-number conversion.

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Networking and Hardware

NV: An Intermediate Language for Verification of Network Control Planes
Nick Giannarakis, Devon Loehr, Ryan Beckett, and David Walker
(Princeton University, USA; Microsoft Research, USA)
Network misconfiguration has caused a raft of high-profile outages over the past decade, spurring researchers to develop a variety of network analysis and verification tools. Unfortunately, developing and maintaining such tools is an enormous challenge due to the complexity of network configuration languages. Inspired by work on intermediate languages for verification such as Boogie and Why3, we develop NV, an intermediate language for verification of network control planes. NV carefully walks the line between expressiveness and tractability, making it possible to build models for a practical subset of real protocols and their configurations, and also facilitate rapid development of tools that outperform state-of-the-art simulators (seconds vs minutes) and verifiers (often 10x faster). Furthermore, we show that it is possible to develop novel analyses just by writing new NV programs. In particular, we implement a new fault-tolerance analysis that scales to far larger networks than existing tools.

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Detecting Network Load Violations for Distributed Control Planes
Kausik Subramanian, Anubhavnidhi Abhashkumar, Loris D'Antoni, and Aditya Akella
(University of Wisconsin-Madison, USA)
One of the major challenges faced by network operators pertains to whether their network can meet input traffic demand, avoid overload, and satisfy service-level agreements. Automatically verifying if no network links are overloaded is complicated---requires modeling frequent network failures, complex routing and load-balancing technologies, and evolving traffic requirements. We present QARC, a distributed control plane abstraction that can automatically verify whether a control plane may cause link-load violations under failures. QARC is fully automatic and can help operators program networks that are more resilient to failures and upgrade the network to avoid violations. We apply QARC to real datacenter and ISP networks and find interesting cases of load violations. QARC can detect violations in under an hour.

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Compiler-Directed Soft Error Resilience for Lightweight GPU Register File Protection
Hongjune Kim, Jianping Zeng, Qingrui Liu, Mohammad Abdel-Majeed, Jaejin Lee, and Changhee Jung
(Seoul National University, South Korea; Purdue University, USA; Annapurna Labs, USA; University of Jordan, Jordan)
This paper presents Penny, a compiler-directed resilience scheme for protecting GPU register files (RF) against soft errors. Penny replaces the conventional error correction code (ECC) based RF protection by using less expensive error detection code (EDC) along with idempotence based recovery. Compared to the ECC protection, Penny can achieve either the same level of RF resilience yet with significantly lower hardware costs or stronger resilience using the same ECC due to its ability to detect multi-bit errors when it is used solely for detection. In particular, to address the lack of store buffers in GPUs, which causes both checkpoint storage overwriting and the high cost of checkpointing stores, Penny provides several compiler optimizations such as storage coloring and checkpoint pruning. Across 25 benchmarks, Penny causes only ≈3% run-time overhead on average.

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Adaptive Low-Overhead Scheduling for Periodic and Reactive Intermittent Execution
Kiwan Maeng and Brandon Lucia
(Carnegie Mellon University, USA)
Batteryless energy-harvesting devices eliminate the need in batteries for deployed sensor systems, enabling longer lifetime and easier maintenance. However, such devices cannot support an event-driven execution model (e.g., periodic or reactive execution), restricting the use cases and hampering real-world deployment. Without knowing exactly how much energy can be harvested in the future, robustly scheduling periodic and reactive workloads is challenging. We introduce CatNap, an event-driven energy-harvesting system with a new programming model that asks the programmer to express a subset of the code that is time-critical. CatNap isolates and reserves energy for the time-critical code, reliably executing it on schedule while deferring execution of the rest of the code. CatNap degrades execution quality when a decrease in the incoming power renders it impossible to maintain its schedule. Our evaluation on a real energy-harvesting setup shows that CatNap works well with end-to-end, real-world deployment settings. CatNap reliably runs periodic events when a prior system misses the deadline by 7.3x and supports reactive applications with a 100% success rate when a prior work shows less than a 2% success rate.

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Parsing, Debugging, and Code Search

Faster General Parsing through Context-Free Memoization
Grzegorz Herman
(Jagiellonian University, Poland)
We present a novel parsing algorithm for all context-free languages. The algorithm features a clean mathematical formulation: parsing is expressed as a series of standard operations on regular languages and relations. Parsing complexity w.r.t. input length matches the state of the art: it is worst-case cubic, quadratic for unambiguous grammars, and linear for LR-regular grammars. What distinguishes our approach is that parsing can be implemented using only immutable, acyclic data structures. We also propose a parsing optimization technique called context-free memoization. It allows handling an overwhelming majority of input symbols using a simple stack and a lookup table, similarly to the operation of a deterministic LR(1) parser. This allows our proof-of-concept implementation to outperform the best current implementations of common generalized parsing algorithms (Earley, GLR, and GLL). Tested on a large Java source corpus, parsing is 3–5 times faster, while recognition—35 times faster.

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Zippy LL(1) Parsing with Derivatives
Romain Edelmann, Jad Hamza, and Viktor Kunčak
(EPFL, Switzerland)
In this paper, we present an efficient, functional, and formally verified parsing algorithm for LL(1) context-free expressions based on the concept of derivatives of formal languages. Parsing with derivatives is an elegant parsing technique, which, in the general case, suffers from cubic worst-case time complexity and slow performance in practice. We specialise the parsing with derivatives algorithm to LL(1) context-free expressions, where alternatives can be chosen given a single token of lookahead. We formalise the notion of LL(1) expressions and show how to efficiently check the LL(1) property. Next, we present a novel linear-time parsing with derivatives algorithm for LL(1) expressions operating on a zipper-inspired data structure. We prove the algorithm correct in Coq and present an implementation as a part of Scallion, a parser combinators framework in Scala with enumeration and pretty printing capabilities.

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Debug Information Validation for Optimized Code
Yuanbo Li, Shuo Ding, Qirun Zhang, and Davide Italiano
(Georgia Institute of Technology, USA; Apple, USA)
Almost all modern production software is compiled with optimization. Debugging optimized code is a desirable functionality. For example, developers usually perform post-mortem debugging on the coredumps produced by software crashes. Designing reliable debugging techniques for optimized code has been well-studied in the past. However, little is known about the correctness of the debug information generated by optimizing compilers when debugging optimized code.
Optimizing compilers emit debug information (e.g., DWARF information) to support source code debuggers. Wrong debug information causes debuggers to either crash or to display wrong variable values. Existing debugger validation techniques only focus on testing the interactive aspect of debuggers for dynamic languages (i.e., with unoptimized code). Validating debug information for optimized code raises some unique challenges: (1) many breakpoints cannot be reached by debuggers due to code optimization; and (2) inspecting some arbitrary variables such as uninitialized variables introduces undefined behaviors.
This paper presents the first generic framework for systematically testing debug information with optimized code. We introduce a novel concept called actionable program. An actionable program Ps, v contains a program location s and a variable v to inspect. Our key insight is that in both the unoptimized program Ps,v and the optimized program Ps,v, debuggers should be able to stop at the program location s and inspect the value of the variable v without any undefined behaviors. Our framework generates actionable programs and does systematic testing by comparing the debugger output of Ps, v and the actual value of v at line s in Ps, v. We have applied our framework to two mainstream optimizing C compilers (i.e., GCC and LLVM). Our framework has led to 47 confirmed bug reports, 11 of which have already been fixed. Moreover, in three days, our technique has found 2 confirmed bugs in the Rust compiler. The results have demonstrated the effectiveness and generality of our framework.

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Semantic Code Search via Equational Reasoning
Varot Premtoon, James Koppel, and Armando Solar-Lezama
(Massachusetts Institute of Technology, USA)
We present a new approach to semantic code search based on equational reasoning, and the Yogo tool implementing this approach. Our approach works by considering not only the dataflow graph of a function, but also the dataflow graphs of all equivalent functions reachable via a set of rewrite rules. In doing so, it can recognize an operation even if it uses alternate APIs, is in a different but mathematically-equivalent form, is split apart with temporary variables, or is interleaved with other code. Furthermore, it can recognize when code is an instance of some higher-level concept such as iterating through a file. Because of this, from a single query, Yogo can find equivalent code in multiple languages. Our evaluation further shows the utility of Yogo beyond code search: encoding a buggy pattern as a Yogo query, we found a bug in Oracle’s Graal compiler which had been missed by a hand-written static analyzer designed for that exact kind of bug. Yogo is built on the Cubix multi-language infrastructure, and currently supports Java and Python.

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Machine Learning II

Proving Data-Poisoning Robustness in Decision Trees
Samuel Drews, Aws Albarghouthi, and Loris D'Antoni
(University of Wisconsin-Madison, USA)
Machine learning models are brittle, and small changes in the training data can result in different predictions. We study the problem of proving that a prediction is robust to data poisoning, where an attacker can inject a number of malicious elements into the training set to influence the learned model. We target decision-tree models, a popular and simple class of machine learning models that underlies many complex learning techniques. We present a sound verification technique based on abstract interpretation and implement it in a tool called Antidote. Antidote abstractly trains decision trees for an intractably large space of possible poisoned datasets. Due to the soundness of our abstraction, Antidote can produce proofs that, for a given input, the corresponding prediction would not have changed had the training set been tampered with or not. We demonstrate the effectiveness of Antidote on a number of popular datasets.

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A Study of the Learnability of Relational Properties: Model Counting Meets Machine Learning (MCML)
Muhammad Usman, Wenxi Wang, Marko Vasic, Kaiyuan Wang, Haris Vikalo, and Sarfraz Khurshid
(University of Texas at Austin, USA; Google, USA)
This paper introduces the MCML approach for empirically studying the learnability of relational properties that can be expressed in the well-known software design language Alloy. A key novelty of MCML is quantification of the performance of and semantic differences among trained machine learning (ML) models, specifically decision trees, with respect to entire (bounded) input spaces, and not just for given training and test datasets (as is the common practice). MCML reduces the quantification problems to the classic complexity theory problem of model counting, and employs state-of-the-art model counters. The results show that relatively simple ML models can achieve surprisingly high performance (accuracy and F1-score) when evaluated in the common setting of using training and test datasets -- even when the training dataset is much smaller than the test dataset -- indicating the seeming simplicity of learning relational properties. However, MCML metrics based on model counting show that the performance can degrade substantially when tested against the entire (bounded) input space, indicating the high complexity of precisely learning these properties, and the usefulness of model counting in quantifying the true performance.

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Learning Fast and Precise Numerical Analysis
Jingxuan He, Gagandeep Singh, Markus Püschel, and Martin Vechev
(ETH Zurich, Switzerland)
Numerical abstract domains are a key component of modern static analyzers. Despite recent advances, precise analysis with highly expressive domains remains too costly for many real-world programs. To address this challenge, we introduce a new data-driven method, called LAIT, that produces a faster and more scalable numerical analysis without significant loss of precision. Our approach is based on the key insight that sequences of abstract elements produced by the analyzer contain redundancy which can be exploited to increase performance without compromising precision significantly. Concretely, we present an iterative learning algorithm that learns a neural policy that identifies and removes redundant constraints at various points in the sequence. We believe that our method is generic and can be applied to various numerical domains.
We instantiate LAIT for the widely used Polyhedra and Octagon domains. Our evaluation of LAIT on a range of real-world applications with both domains shows that while the approach is designed to be generic, it is orders of magnitude faster on the most costly benchmarks than a state-of-the-art numerical library while maintaining close-to-original analysis precision. Further, LAIT outperforms hand-crafted heuristics and a domain-specific learning approach in terms of both precision and speed.

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Synthesis III

Exact and Approximate Methods for Proving Unrealizability of Syntax-Guided Synthesis Problems
Qinheping Hu, John Cyphert, Loris D'Antoni, and Thomas Reps
(University of Wisconsin-Madison, USA)
We consider the problem of automatically establishing that a given syntax-guided-synthesis (SyGuS) problem is unrealizable (i.e., has no solution). We formulate the problem of proving that a SyGuS problem is unrealizable over a finite set of examples as one of solving a set of equations: the solution yields an overapproximation of the set of possible outputs that any term in the search space can produce on the given examples. If none of the possible outputs agrees with all of the examples, our technique has proven that the given SyGuS problem is unrealizable. We then present an algorithm for exactly solving the set of equations that result from SyGuS problems over linear integer arithmetic (LIA) and LIA with conditionals (CLIA), thereby showing that LIA and CLIA SyGuS problems over finitely many examples are decidable. We implement the proposed technique and algorithms in a tool called Nay. Nay can prove unrealizability for 70/132 existing SyGuS benchmarks, with running times comparable to those of the state-of-the-art tool Nope. Moreover, Nay can solve 11 benchmarks that Nope cannot solve.

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Question Selection for Interactive Program Synthesis
Ruyi Ji, Jingjing Liang, Yingfei Xiong, Lu Zhang, and Zhenjiang Hu
(Peking University, China)
Interactive program synthesis aims to solve the ambiguity in specifications, and selecting the proper question to minimize the rounds of interactions is critical to the performance of interactive program synthesis. In this paper we address this question selection problem and propose two algorithms. SampleSy approximates a state-of-the-art strategy proposed for optimal decision tree and has a short response time to enable interaction. EpsSy further reduces the rounds of interactions by approximating SampleSy with a bounded error rate. To implement the two algorithms, we further propose VSampler, an approach to sampling programs from a probabilistic context-free grammar based on version space algebra. The evaluation shows the effectiveness of both algorithms.

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Reconciling Enumerative and Deductive Program Synthesis
Kangjing Huang, Xiaokang Qiu, Peiyuan Shen, and Yanjun Wang
(Purdue University, USA)
Syntax-guided synthesis (SyGuS) aims to find a program satisfying semantic specification as well as user-provided structural hypotheses. There are two main synthesis approaches: enumerative synthesis, which repeatedly enumerates possible candidate programs and checks their correctness, and deductive synthesis, which leverages a symbolic procedure to construct implementations from specifications. Neither approach is strictly better than the other: automated deductive synthesis is usually very efficient but only works for special grammars or applications; enumerative synthesis is very generally applicable but limited in scalability.
In this paper, we propose a cooperative synthesis technique for SyGuS problems with the conditional linear integer arithmetic (CLIA) background theory, as a novel integration of the two approaches, combining the best of the two worlds. The technique exploits several novel divide-and-conquer strategies to split a large synthesis problem to smaller subproblems. The subproblems are solved separately and their solutions are combined to form a final solution. The technique integrates two synthesis engines: a pure deductive component that can efficiently solve some problems, and a height-based enumeration algorithm that can handle arbitrary grammar. We implemented the cooperative synthesis technique, and evaluated it on a wide range of benchmarks. Experiments showed that our technique can solve many challenging synthesis problems not possible before, and tends to be more scalable than state-of-the-art synthesis algorithms.

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