Beyond operator-precedence grammars and languages

Operator Precedence Languages (OPL) are deterministic context-free and have desirable properties. OPL are parallely parsable, and, when structurally compatible, are closed under Boolean operations, concatenation and star; they include the Input Driven languages. OPL use three relations between two terminal symbols, to assign syntax structure to words. We extend such relations to k-tuples of consecutive symbols, in agreement with strictly locally testable regular languages. For each k, the new corresponding class of Higher-order Operator Precedence languages properly includes the OPL and enjoy many of their properties. OPL are a strict hierarchy based on k, which contains maximal languages

APERIODICITY, STAR-FREENESS, AND FIRST-ORDER LOGIC DEFINABILITY OF OPERATOR PRECEDENCE LANGUAGES

A classic result in formal language theory is the equivalence among non-counting, or aperiodic, regular languages, and languages defined through star-free regular expressions, or first-order logic. Past attempts to extend this result beyond the realm of regular languages have met with difficulties: for instance it is known that star-free tree languages may violate the non-counting property and there are aperiodic tree languages that cannot be defined through first-order logic. We extend such classic equivalence results to a significant family of deterministic context-free languages, the operator-precedence languages (OPL), which strictly includes the widely investigated visibly pushdown, alias input-driven, family and other structured context-free languages. The OP model originated in the ’60s for defining programming languages and is still used by high performance compilers; its rich algebraic properties have been investigated initially in connection with grammar learning and recently completed with further closure properties and with monadic second order logic definition. We introduce an extension of regular expressions, the OP-expressions (OPE) which define the OPLs and, under the star-free hypothesis, define first-order definable and non-counting OPLs. Then, we prove, through a fairly articulated grammar transformation, that aperiodic OPLs are first-order definable. Thus, the classic equivalence of star-freeness, aperiodicity, and first-order definability is established for the large and powerful class of OPLs. We argue that the same approach can be exploited to obtain analogous results for visibly pushdown languages too

JIST: just-in-time scheduling translation for parallel processors

The application fields of bytecode virtual machines and VLIW processors overlap in the area of embedded and mobile systems, where the two technologies offer different benefits, namely high code portability, low power consumption and reduced hardware cost. Dynamic compilation makes it possible to bridge the gap between the two technologies, but special attention must be paid to software instruction scheduling, a must for the VLIW architectures. We have implemented JIST, a Virtual Machine and JIT compiler for Java Bytecode targeted to a VLIW processor. We show the impact of various optimizations on the performance of code compiled with JIST through the experimental study on a set of benchmark programs. We report significant speedups, and increments in the number of instructions issued per cycle up to 50% with respect to the non-scheduling version of the JITcompiler. Further optimizations are discussed

Integer compositions and syntactic trees of repeat-until programs

In this work we study some properties of integer compositions in connection with the recognition of rational trace languages. In particular, we introduce some operations defined on integer compositions and present procedures for their computation that work in linear or in quadratic time. These procedures turn out to be useful in the analysis of syntactic trees of certain regular expressions, called repeat-until expressions, which intuitively represent programs of instructions nested in repeat-until loops. Our main aim is to show how, in some cases, such an analysis allows us to design algorithms for the recognition of (rational) trace languages defined by repeat-until expressions, which work in quadratic time independently of the concurrency relation

Intra-Messinian truncation surface in the Levant Basin explained by subaqueous dissolution

The Messinian salinity crisis (MSC) is an extreme event in Earth history during which a salt giant (>1 × 106 km3) accumulated on the Mediterranean seafloor within ~640 k.y. Erosional unconformities extending from the continental margins into the deep basins are key features for reconstructing the MSC; however, the nature of the erosional processes and their subaerial versus subaqueous origin are highly controversial. This study focuses on the top erosion surface (TES) in the deep Levant Basin, which is notably flat, truncating a basinward-tilted Messinian evaporitic succession. Based on high-resolution seismic surveys and wireline logs, we show that (1) the TES is actually an intra-Messinian truncation surface (IMTS) located ~100 m below the Messinian-Zanclean boundary; (2) the topmost, post-truncation Messinian unit is very different from the underlying salt deposits and consists mostly of shale, sand, and anhydrite; and (3) the flat IMTS is a dissolution surface related to significant dilution and stratification of the water column during the transition from stage 2 to stage 3 of the MSC. Dissolution occurred upslope where salt rocks at the seabed were exposed to the upper diluted brine, while downslope, submerged in the deeper halite-saturated layer, the salt rocks were preserved. The model, which requires a stratified water column, is inconsistent with a complete desiccation of the eastern Mediterranean Sea

Locally Chain-Parsable Languages

If a context-free language enjoys the local parsability property then, no matter how the source string is segmented, each segment can be parsed in- dependently, and an efficient parallel parsing algorithm becomes possible. The new class of locally chain-parsable languages (LCPL), included in deterministic context-free languages, is here defined by means of the chain-driven automa- ton and characterized by decidable properties of grammar derivations. Such au- tomaton decides to reduce or not a factor in a way purely driven by the terminal characters, thus extending the well-known concept of Input-Driven (ID) (visibly) pushdown machines. LCPL extend and improve the practically relevant operator- precedence languages (Floyd), which are known to strictly include the ID lan- guages, and for which a parallel-parser generator exists. Consistently with the classical results for ID, chain-compatible LCPL are closed under reversal and Boolean operations, and language inclusion is decidable

Применение метода контрольных возмущений для определения характерных узлов присоединения комплексной нагрузки при расчетах динамической устойчивости

Рассматривается влияние способа замещения комплексной нагрузки на характер электромеханических переходных процессов в электрических системах (ЭС) от действия больших возмущений. Показано, что установить общие рекомендации относительно способа замещения нагрузки в сложных ЭС затруднительно. Предлагается для опреде­ления характерных узлов нагрузки, оказывающих существенное влияние на характер динамического перехода, применять известный метод контрольных возмущений. Приводятся результаты сравнительных расчетов с использованием предлагаемой методики

Provenance of the transparent gypsum crystals (Lapis specularis) and gypsum mortars in the windows from the churches of Rome: S. Sabina sull'Aventino and S. Giorgio al Velabro

The latest example of the traditional Roman use of lapis specularis crystals in windows panels instead of glass are represented by the Paleochristian- Early Medieval churches of Rome. The churches of S. Sabina sull\u2019Aventino and S. Giorgio al Velabro were characterized by some of the most ancient examples of windows frameworks built using gypsum mortar. The light was penetrating the panels throughout openings covered by two or more embricated gypsum cleveage fragments less than 15 cm across and up to 7 mm-thick, a technique unknown in older Roman examples, which is clearly related to the availability of small and low-quality crystals. The petrography of the gypsum mortars and the strontium and sulfur isotope analyses of the crystals indicate two diff erent supply sources. A large group of window frameworks were produced using selenite rocks and lapis specularis crystals probably quarried from Tuscany, Sicily or Cyprus, whereas a window in S. Sabina was built using alabastrine gypsum and lapis specularis crystals quarried in Emilia-Romagna, Sicily, Cyprus or Southern Turkey

Star-Freeness, First-Order Definability and Aperiodicity of Structured Context-Free Languages

A classic result in formal language theory is the equivalence among aperiodic finite automata, star-free regular expressions, and first-order logic on words. Extending these results to structured subclasses of context-free languages, such as tree languages, did not work as smoothly: there are star-free tree languages that are counting. We argue that investigating the same properties within the family of operator precedence languages (OPLs) by going back to string languages rather than tree languages may lead to equivalences that perfectly match those on regular languages. We define operator precedence expressions; we show that they define exactly the class of OPLs and that, when restricted to the star-free subclass, coincide with first-order definable OPLs and are aperiodic. Since operator precedence languages strictly include other classes of structured languages such as visibly pushdown languages, the same results given in this paper hold as trivial corollary for that family too