71 research outputs found

    The problem of programming language concurrency semantics

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    Despite decades of research, we do not have a satisfactory concurrency semantics for any general-purpose programming language that aims to support concurrent systems code. The Java Memory Model has been shown to be unsound with respect to standard compiler optimisations, while the C/C++11 model is too weak, admitting undesirable thin-air executions. Our goal in this paper is to articulate this major open problem as clearly as is currently possible, showing how it arises from the combination of multiprocessor relaxed-memory behaviour and the desire to accommodate current compiler optimisations. We make several novel contributions that each shed some light on the problem, constraining the possible solutions and identifying new difficulties. First we give a positive result, proving in HOL4 that the existing axiomatic model for C/C++11 guarantees sequentially consistent semantics for simple race-free programs that do not use low-level atomics (DRF-SC, one of the core design goals). We then describe the thin-air problem and show that it cannot be solved, without restricting current compiler optimisations, using any per-candidate-execution condition in the style of the C/C++11 model. Thin-air executions were thought to be confined to programs using relaxed atomics, but we further show that they recur when one attempts to integrate the concurrency model with more of C, mixing atomic and nonatomic accesses, and that also breaks the DRF-SC result. We then describe a semantics based on an explicit operational construction of out-of-order execution, giving the desired behaviour for thin-air examples but exposing further difficulties with accommodating existing compiler optimisations. Finally, we show that there are major difficulties integrating concurrency semantics with the C/C++ notion of undefined behaviour. We hope thereby to stimulate and enable research on this key issue

    Analytical model of asymmetrical Mixed-Mode Bending test of adhesively bonded GFRP joint

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    This paper presents new analytical model of asymmetric mixed-mode bending (MMB) specimen of adhesively bonded pultruded GFRP joints. An easily applicable relationship for the calculation of the strain energy release rate of the asymmetric MMB specimens is proposed based on the beam theory. The model is capable to analyze stacking sequence as well as various crack propagation paths. In the paper the effect of the various fiber bridging length and different crack propagation paths is analyzed analytically and supported by experimental results. The methodology and results presented in this paper could be utilized for the design of both joint geometry and lay-up of the laminates constituting the joint or for the prediction of the fracture behavior of such structures

    On partial order semantics for SAT/SMT-based symbolic encodings of weak memory concurrency

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    Concurrent systems are notoriously difficult to analyze, and technological advances such as weak memory architectures greatly compound this problem. This has renewed interest in partial order semantics as a theoretical foundation for formal verification techniques. Among these, symbolic techniques have been shown to be particularly effective at finding concurrency-related bugs because they can leverage highly optimized decision procedures such as SAT/SMT solvers. This paper gives new fundamental results on partial order semantics for SAT/SMT-based symbolic encodings of weak memory concurrency. In particular, we give the theoretical basis for a decision procedure that can handle a fragment of concurrent programs endowed with least fixed point operators. In addition, we show that a certain partial order semantics of relaxed sequential consistency is equivalent to the conjunction of three extensively studied weak memory axioms by Alglave et al. An important consequence of this equivalence is an asymptotically smaller symbolic encoding for bounded model checking which has only a quadratic number of partial order constraints compared to the state-of-the-art cubic-size encoding.Comment: 15 pages, 3 figure

    Local martingales measures

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    Convex analysis for sets of local martingales measures

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    The investigation of calcium carbonate formation using micro-Raman spectroscopy

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    Calcium carbonate (CaCO3) has got significant importance in nature and in several industrial processes. In its different mineral modifications, calcium carbonate is highly abundant in the earth crust, occurring as calcite, aragonite and vaterite (anhydrous crystalline polymorphs) or ikaite (calcium carbonate hexahydrate), calcium carbonate monohydrate, (hydrated crystal forms) and amorphous calcium carbonate (ACC). The CaCO3 can be formed during the process of biomineralization or synthetically with using gas diffusion method or mixing supersaturated solutions containing calcium cations and carbonate anions. The so-called carbonation reaction, plays a crucial role in hardening of lime mortars, a class of binders of relevance in culture heritage conservation. During this process, the CaCO3 is formed from Ca(OH)2 that uptake atmospheric CO2 in aqueous medium. This contribution is focused on the investigation with micro-Raman spectroscopy of CaCO3 modifications appeared during the synthesis from supersaturated aqueous calcium chloride and potassium carbonate solutions, under different reaction conditions [1], as well as during the carbonation reaction [2,3]
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