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Sun 20 - Fri 25 October 2019 Athens, Greece
Thu 24 Oct 2019 11:22 - 11:45 at Attica - Language Design Chair(s): Tiark Rompf

WebAssembly (Wasm) is a safe, portable virtual instruction set that can be hosted in a wide range of environments, such as a Web browser. It is a low-level language whose instructions are intended to compile directly to bare hardware. While the initial version of Wasm focussed on single-threaded computation, a recent proposal extends it with low-level support for multiple threads and atomic instructions for synchronised access to shared memory. To support the correct compilation of concurrent programs, it is necessary to give a suitable specification of its memory model.

Wasm's language definition is based on a fully formalised specification that carefully avoids undefined behaviour. We present a substantial extension to this semantics, incorporating a relaxed memory model, along with a few proposed extensions. Wasm's memory model is unique in that its linear address space can be dynamically grown during execution, while all accesses are bounds-checked. This leads to the novel problem of specifying how observations about the size of the memory can propagate between threads. We argue that, considering desirable compilation schemes, we cannot give a sequentially consistent semantics to memory growth.

We show that our model provides sequential consistency for data-race-free executions (SC-DRF). However, because Wasm is to run on the Web, we must also consider interoperability of its model with that of JavaScript. We show, by counter-example, that JavaScript's memory model is not SC-DRF, in contrast to what is claimed in its specification. We propose two axiomatic conditions that should be added to the JavaScript model to correct this difference.

We also describe a prototype SMT-based litmus tool which acts as an oracle for our axiomatic model, visualising its behaviours, including memory resizing.

Thu 24 Oct
Times are displayed in time zone: (GMT+03:00) Beirut change

11:00 - 12:30: OOPSLA - Language Design at Attica
Chair(s): Tiark RompfPurdue University
splash-2019-oopsla11:00 - 11:22
Vilhelm SjöbergYale University, Yuyang SangYale University, Shu-chun WengYale University, Zhong ShaoYale University
DOI Pre-print
splash-2019-oopsla11:22 - 11:45
Conrad WattUniversity of Cambridge, Andreas RossbergDfinity Stiftung, Jean Pichon-PharabodUniversity of Cambridge
splash-2019-oopsla11:45 - 12:07
Ilya SergeyYale-NUS College and National University of Singapore, Vaivaswatha NagarajZilliqa Research, Jacob JohannsenZilliqa Research, Amrit KumarZilliqa Research, Anton TrunovZilliqa Research, Ken ChanZilliqa Research
DOI Pre-print File Attached
splash-2019-oopsla12:07 - 12:30
Filip KrikavaCzech Technical University, Heather MillerCarnegie Mellon University, Jan VitekNortheastern University
DOI Pre-print