36th European Conference on Object-Oriented Programming (ECOOP 2022)

Document

Complete Volume

DOI: 10.4230/LIPIcs.ECOOP.2022

LIPIcs, Volume 222, ECOOP 2022, Complete Volume

Authors: Karim Ali and Jan Vitek

Abstract

LIPIcs, Volume 222, ECOOP 2022, Complete Volume

Cite as

36th European Conference on Object-Oriented Programming (ECOOP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 222, pp. 1-940, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@Proceedings{ali_et_al:LIPIcs.ECOOP.2022,
  title =	{{LIPIcs, Volume 222, ECOOP 2022, Complete Volume}},
  booktitle =	{36th European Conference on Object-Oriented Programming (ECOOP 2022)},
  pages =	{1--940},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-225-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{222},
  editor =	{Ali, Karim and Vitek, Jan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2022},
  URN =		{urn:nbn:de:0030-drops-162276},
  doi =		{10.4230/LIPIcs.ECOOP.2022},
  annote =	{Keywords: LIPIcs, Volume 222, ECOOP 2022, Complete Volume}
}

@InProceedings{holik_et_al:LIPIcs.ECOOP.2022.19,
  author =	{Hol{\'\i}k, Luk\'{a}\v{s} and Peringer, Petr and Rogalewicz, Adam and \v{S}okov\'{a}, Veronika and Vojnar, Tom\'{a}\v{s} and Zuleger, Florian},
  title =	{{Low-Level Bi-Abduction}},
  booktitle =	{36th European Conference on Object-Oriented Programming (ECOOP 2022)},
  pages =	{19:1--19:30},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-225-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{222},
  editor =	{Ali, Karim and Vitek, Jan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2022.19},
  URN =		{urn:nbn:de:0030-drops-162477},
  doi =		{10.4230/LIPIcs.ECOOP.2022.19},
  annote =	{Keywords: programs with dynamic linked data structures, programs with pointers, low-level pointer operations, static analysis, shape analysis, separation logic, bi-abduction}
}

@InProceedings{lumsdaine_et_al:LIPIcs.ECOOP.2022.31,
  author =	{Lumsdaine, Andrew and D'Alessandro, Luke and Deweese, Kevin and Firoz, Jesun and Liu, Xu Tony and McMillan, Scott and Ratzloff, John Phillip and Zalewski, Marcin},
  title =	{{NWGraph: A Library of Generic Graph Algorithms and Data Structures in C++20}},
  booktitle =	{36th European Conference on Object-Oriented Programming (ECOOP 2022)},
  pages =	{31:1--31:28},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-225-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{222},
  editor =	{Ali, Karim and Vitek, Jan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2022.31},
  URN =		{urn:nbn:de:0030-drops-162592},
  doi =		{10.4230/LIPIcs.ECOOP.2022.31},
  annote =	{Keywords: Graph library, generic programming, graph algorithms}
}

Document

Extended Abstract

DOI: 10.4230/LIPIcs.ECOOP.2022.32

Vincent: Green Hot Methods in the JVM (Extended Abstract)

Authors: Kenan Liu, Khaled Mahmoud, Joonhwan Yoo, and Yu David Liu

Abstract

In this paper, we show the energy efficiency of Java applications can be improved by applying Dynamic Voltage and Frequency Scaling (DVFS) inside the Java Virtual Machine (JVM). We augment the JVM to record the energy consumption of hot methods as the underlying CPU is run at different clock frequencies; after all the frequency possibilities for a method have been explored, the execution of the method in an optimized run is set to the CPU frequency that leads to the most energy-efficient execution for that method. We introduce a new sampling methodology to overcome the dual challenges in our design: both the underlying measurement mechanism for energy profiling and the DVFS for energy optimization are overhead-prone. We extend JikesRVM with our approach and benchmark it over the DaCapo suite on a server-class Linux machine. Experiments show we are able to use 14.9% less energy than built-in power management in Linux, and improve energy efficiency by 21.1% w.r.t. the metric of Energy-Delay Product (EDP).

Cite as

Kenan Liu, Khaled Mahmoud, Joonhwan Yoo, and Yu David Liu. Vincent: Green Hot Methods in the JVM (Extended Abstract). In 36th European Conference on Object-Oriented Programming (ECOOP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 222, pp. 32:1-32:30, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{liu_et_al:LIPIcs.ECOOP.2022.32,
  author =	{Liu, Kenan and Mahmoud, Khaled and Yoo, Joonhwan and Liu, Yu David},
  title =	{{Vincent: Green Hot Methods in the JVM}},
  booktitle =	{36th European Conference on Object-Oriented Programming (ECOOP 2022)},
  pages =	{32:1--32:30},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-225-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{222},
  editor =	{Ali, Karim and Vitek, Jan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2022.32},
  URN =		{urn:nbn:de:0030-drops-162605},
  doi =		{10.4230/LIPIcs.ECOOP.2022.32},
  annote =	{Keywords: energy efficiency, JVM, just-in-time compilation}
}

Document

Extended Abstract

DOI: 10.4230/LIPIcs.ECOOP.2022.33

Hinted Dictionaries: Efficient Functional Ordered Sets and Maps (Extended Abstract)

Authors: Amir Shaikhha, Mahdi Ghorbani, and Hesam Shahrokhi

Abstract

Sets and maps are two essential collection types for programming used widely in data analytics [Shaikhha et al., 2022]. The underlying implementation for both are normally based on 1) hash tables or 2) ordered data structures. The former provides (average-case) constant-time lookup, insertion, and deletion operations, while the latter performs these operations in a logarithmic time. The trade-off between these two approaches has been heavily investigated in systems communities [Kim et al., 2009]. An important class of operations are those dealing with two collection types, such as set-set-union or the merge of two maps. One of the main advantages of hash-based implementations is a straightforward implementation for such operations with a linear computational complexity. However, naïvely using ordered dictionaries results in an implementation with a computational complexity of O(n log(n)). Motivating Example. The following C++ code computes the intersection of two sets, implemented by std::unordered_set, a hash-table-based set: std::unordered_set<K> result; for(auto& e : set1) { if(set2.count(e)) result.emplace(e); } However, the same fact is not true for ordered data structures; changing the dictionary type to std::set, an ordered implementation, results in a program with O(n log(n)) computational complexity. This is because both the count (lookup) and emplace (insertion) methods have logarithmic computational complexity. As a partial remedy, the standard library of C++ provides an alternative insertion method that can take linear time, if used appropriately. The emplace_hint method takes a hint for the position that the element will be inserted. If the hint correctly specifies the insertion point, the computational complexity will be amortized to constant time. std::set<K> result; auto hint = result.begin(); for(auto& e : set1) { if(set2.count(e)) hint = result.emplace_hint(hint, e); } However, the above implementation still suffers from an O(n log(n)) computational complexity, due to the logarithmic computational complexity of the lookup operation (count) of the second set. Thanks to the orderedness of the second set, one can observe that once an element is looked up, there is no longer any need to search its preceding elements at the next iterations. By leveraging this feature, we can provide a hinted lookup method with an amortized constant run-time. Hinted Data Structures. The following code, shows an alternative implementation for set intersection that uses such hinted lookup operations: hinted_set<K> result; hinted_set<K>::hint_t hint = result.begin(); for(auto& e : set1) { hinted_set<K>::hint_t hint2 = set2.seek(e); if(hint2.found) hint = result.insert_hint(hint, e); set2.after(hint2); } The above hinted set data-structure enables faster insertion and lookup by providing a cursor through a hint object (of type hint_t). The seek method returns the hint object hint2 pointing to element e. Thanks to the invocation of set2.after(hint2), the irrelevant elements of set2 (which are smaller than e) are no longer considered in the next iterations. The expression hint2.found specifies if the element exists in set2 or not. Finally, if an element exists in the second set (specified by hint2.found), it is inserted into its correct position using insert_hint. The existing work on efficient ordered dictionaries can be divided into two categories. First, in the imperative world, there are C++ ordered dictionaries (e.g., std::map) with limited hinting capabilities only for insertion through emplace_hint, but not for deletion and lookup, as observed previously. Second, from the functional world, Adams' sets [Adams, 1993] provide efficient implementations for set-set operators. Functional languages such as Haskell have implemented ordered sets and maps based on them for more than twenty years [Straka, 2010]. Furthermore, it has been shown [Blelloch et al., 2016] that Adams' maps can be used to provide a parallel implementation for balanced trees such as AVL, Red-Black, Weight-Balanced, and Treaps. However, Adams' maps do not expose any hint-based operations to the programmer. At first glance, these two approaches seem completely irrelevant to each other. The key contribution of this paper is hinted dictionaries, an ordered data structure that unifies the techniques from both imperative and functional worlds. The essential building block of hinted dictionaries are hint objects, that enable faster operations (than the traditional O(log n) complexity) by maintaining a pointer into the data structure. The underlying representation for hinted dictionaries can be sorted arrays, unbalanced trees, and balanced trees by sharing the same interface. In our running example, alternative data structures can be provided by simply changing the type signature of the hinted set from hinted_set to another implementation, without modifying anything else.

Cite as

Amir Shaikhha, Mahdi Ghorbani, and Hesam Shahrokhi. Hinted Dictionaries: Efficient Functional Ordered Sets and Maps (Extended Abstract). In 36th European Conference on Object-Oriented Programming (ECOOP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 222, pp. 33:1-33:3, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{shaikhha_et_al:LIPIcs.ECOOP.2022.33,
  author =	{Shaikhha, Amir and Ghorbani, Mahdi and Shahrokhi, Hesam},
  title =	{{Hinted Dictionaries: Efficient Functional Ordered Sets and Maps}},
  booktitle =	{36th European Conference on Object-Oriented Programming (ECOOP 2022)},
  pages =	{33:1--33:3},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-225-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{222},
  editor =	{Ali, Karim and Vitek, Jan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2022.33},
  URN =		{urn:nbn:de:0030-drops-162610},
  doi =		{10.4230/LIPIcs.ECOOP.2022.33},
  annote =	{Keywords: Functional Collections, Ordered Dictionaries, Sparse Linear Algebra}
}

Document

Extended Abstract

DOI: 10.4230/LIPIcs.ECOOP.2022.34

Slicing of Probabilistic Programs Based on Specifications (Extended Abstract)

Authors: Marcelo Navarro and Federico Olmedo

Abstract

We present the first slicing approach for probabilistic programs based on specifications. Concretely, we show that when probabilistic programs are accompanied by their functional specifications in the form of pre- and post-condition, one can exploit this semantic information to produce specification-preserving slices strictly more precise than slices yielded by conventional techniques based on data/control dependency. To illustrate this, assume that Alice and Bob repeatedly flip a fair coin until observing a matching outcome, either both heads or both tails. However, Alice decides to "trick" Bob and switches the outcome of her coin, before comparing it to Bob’s. The game can be encoded by the program below, which is instrumented with a variable n that tracks the required number of rounds until observing the first match. The program terminates after K loop iterations with probability 1/(2^K) provided K > 0, and with probability 0 otherwise, satisfying the annotated specification. \\ pre: 1/(2^K) [K > 0] n := 0; a, b := 0, 1; while (a ̸= b) do n := n + 1; {a := 0} [1/2] {a := 1}; a := 1 − a; {b := 0} [1/2] {b := 1} \\ post: [n = K] Traditional slicing techniques based on data/control dependencies conclude that the only valid slice of the program (w.r.t. output variable n) is the very same program. However, our slicing approach allows removing the assignment a := 1-a from the loop body, while preserving the program specification. At the technical level, our slicing technique works by propagating post-conditions backward using the greatest pre-expectation transformer - the probabilistic counterpart of Dijkstra’s weakest pre-condition transformer. This endows programs with an axiomatic semantics, expressed in terms of a verification condition generator (VCGen) that yields quantitative proof obligations. In particular, we design (and prove sound) VCGens for both the partial (allowing divergence) and the total (requiring termination) correctness of probabilistic programs, making our slicing technique termination-sensitive. To handle iteration, we assume that program loops are annotated with invariants. To reason about (probabilistic) termination, we assume that loop annotations also include (probabilistic) variants. Another appealing property of our slicing technique is its modularity: It yields valid slices of a program from valid slices of its subprograms. Most importantly, this involves only local reasoning. Besides developing the theoretical foundations of our slicing approach, we also exhibit an algorithm for computing program slices. Interestingly, the algorithm computes the least slice that can be derived from the slicing approach, according to a proper notion of slice size, using, as main ingredient, a shortest-path algorithm. Finally, we demonstrate the applicability of our approach by means of a few illustrative examples and a case study from the probabilistic modeling field.

Cite as

Marcelo Navarro and Federico Olmedo. Slicing of Probabilistic Programs Based on Specifications (Extended Abstract). In 36th European Conference on Object-Oriented Programming (ECOOP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 222, pp. 34:1-34:2, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{navarro_et_al:LIPIcs.ECOOP.2022.34,
  author =	{Navarro, Marcelo and Olmedo, Federico},
  title =	{{Slicing of Probabilistic Programs Based on Specifications}},
  booktitle =	{36th European Conference on Object-Oriented Programming (ECOOP 2022)},
  pages =	{34:1--34:2},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-225-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{222},
  editor =	{Ali, Karim and Vitek, Jan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2022.34},
  URN =		{urn:nbn:de:0030-drops-162628},
  doi =		{10.4230/LIPIcs.ECOOP.2022.34},
  annote =	{Keywords: probabilistic programming, program slicing, expectation transformer semantics, verification condition generator}
}

Document

Extended Abstract

DOI: 10.4230/LIPIcs.ECOOP.2022.35

Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications (Extended Abstract)

Authors: David Richter, David Kretzler, Pascal Weisenburger, Guido Salvaneschi, Sebastian Faust, and Mira Mezini

Abstract

Decentralized applications (dApps) consist of smart contracts that run on blockchains and clients that model collaborating parties. dApps are used to model financial and legal business functionality. Today, contracts and clients are written as separate programs - in different programming languages - communicating via send and receive operations. This makes distributed program flow awkward to express and reason about, increasing the potential for mismatches in the client-contract interface, which can be exploited by malicious clients, potentially leading to huge financial losses. In this paper, we present Prisma, a language for tierless decentralized applications, where the contract and its clients are defined in one unit. Pairs of send and receive actions that "belong together" are encapsulated into a single direct-style operation, which is executed differently by sending and receiving parties. This enables expressing distributed program flow via standard control flow and renders mismatching communication impossible. We prove formally that our compiler preserves program behavior in presence of an attacker controlling the client code. We systematically compare Prisma with mainstream and advanced programming models for dApps and provide empirical evidence for its expressiveness and performance. The design space of dApp programming and other multi-party languages depends on one major choice: a local model versus a global model. In a local model, parties are defined in separate programs and their interactions are encoded via send and receive effects. In a global language, parties are defined within one shared program and interactions are encoded via combined send-and-receive operations with no effects visible to the outside world. The global model is followed by tierless [Christian Queinnec, 2000; Cooper et al., 2007; Choi and Chang, 2019; Fowler et al., 2019; Serrano et al., 2006; Serrano and Prunet, 2016; Radanne et al., 2016; Weisenburger et al., 2018] and choreographic [Kohei Honda et al., 2011; Fabrizio Montesi et al., 2014; Saverio Giallorenzo et al., 2020] languages. However, known approaches to dApp programming follow the local model, thus rely on explicitly specifying the client-contract interaction protocol. Moreover, the contract and clients are implemented in different languages, hence, developers have to master two technology stacks. The dominating approach in industry is Solidity [Mix, 2019] for the contract and JavaScript for clients. Solidity relies on expressing the protocol using assertions in the contract code, which are checked at run time [Solidity documentation - common patterns, 2020]. Failing to insert the correct assertions may give parties illegal access to monetary values to the detriment of others [Nikolić et al., 2018; Luu et al., 2016]. In research, contract languages [Ankush Das et al., 2019; Michael J. Coblenz, 2017; Franklin Schrans et al., 2018; Franklin Schrans et al., 2019; Michael J. Coblenz et al., 2019; Michael J. Coblenz et al., 2019; Reed Oei et al., 2020; Sam Blackshear et al., 2019] have been proposed that rely on advanced type systems such as session types, type states, and linear types. The global model has not been explored for dApp programming. This is unfortunate given the potential to get by with a standard typing discipline and to avoid intricacies and potential mismatches of a two-language stack. Our work fills this gap by proposing Prisma - the first language that features a global programming model for Ethereum dApps. While we focus on the Ethereum blockchain, we believe our techniques to be applicable to other smart contract platforms. Prisma enables interleaving contract and client logic within the same program and adopts a direct style (DS) notation for encoding send-and-receive operations (with our awaitCl language construct) akin to languages with async/await [Gavin M. Bierman et al., 2012; Scala async rfc]. DS addresses shortcomings with the currently dominant encoding of the protocol’s finite state machines (FSM) [Mix, 2019; Michael J. Coblenz, 2017; Franklin Schrans et al., 2018; Franklin Schrans et al., 2019; Michael J. Coblenz et al., 2019; Michael J. Coblenz et al., 2019]. We argue writing FSM style corresponds to a control-flow graph of basic blocks, which is low-level and more suited to be written by a compiler than by a human. With FSM style, the contract is a passive entity whose execution is driven by clients. whereas the DS encoding allows the contract to actively ask clients for input, fitting dApp execution where a dominant contract controls execution and diverts control to other parties when their input is needed. In the following Prisma snippet, the payout function is a function invoked by the contract when it is time to pay money to a client. In Prisma, variables, methods and classes are separated into two namespaces, one for the contract and one for the clients. The payout method is located on the contract via the annotation @co. The body of the method diverts the control to the client using awaitCl(...) { ... }, hence the contained readLine call is executed on the client. Note that no explicit send/receive operations are needed but the communication protocol is expressed through the program control flow. Only after the check client == toBePayed that the correct client replied, the current contact balance balance() is transferred to the client via transfer. @co def payout(toBePayed: Arr[Address]): Unit = { awaitCl(client => client == toBePayed) { readLine("Press enter for payout") } toBePayed.transfer(balance()) } Overall, Prisma relieves the developer from the responsibility of correctly managing distributed, asynchronous program flows and the heterogeneous technology stack. Instead, the burden is put on the compiler, which distributes the program flow by means of selective continuation-passing-style (CPS) translation and defunctionalisation and inserts guards against malicious client interactions. We needed to develop a CPS translation for the code that runs on the Ethereum Virtual Machine (EVM) since the EVM has no built-in support for concurrency primitives which could be used for asynchronous communication. While CPS translations are well-known, we cannot use them out-of-the-box because the control flow is interwoven with distribution in our case. A CPS translation that does not take distribution into account would allow malicious clients to force the contract to deviate from the intended control flow by sending a spoofed continuation. Thus, it was imperative to prove correctness of our distributed CPS translation to ensure control-flow integrity of the contract.

Cite as

David Richter, David Kretzler, Pascal Weisenburger, Guido Salvaneschi, Sebastian Faust, and Mira Mezini. Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications (Extended Abstract). In 36th European Conference on Object-Oriented Programming (ECOOP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 222, pp. 35:1-35:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{richter_et_al:LIPIcs.ECOOP.2022.35,
  author =	{Richter, David and Kretzler, David and Weisenburger, Pascal and Salvaneschi, Guido and Faust, Sebastian and Mezini, Mira},
  title =	{{Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications}},
  booktitle =	{36th European Conference on Object-Oriented Programming (ECOOP 2022)},
  pages =	{35:1--35:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-225-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{222},
  editor =	{Ali, Karim and Vitek, Jan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2022.35},
  URN =		{urn:nbn:de:0030-drops-162632},
  doi =		{10.4230/LIPIcs.ECOOP.2022.35},
  annote =	{Keywords: Domain Specific Languages, Smart Contracts, Scala}
}

LIPIcs, Volume 222

36th European Conference on Object-Oriented Programming (ECOOP 2022)

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