94 research outputs found
Formal Verification of an SSA-based Middle-end for CompCert
CompCert is a formally verified compiler that generates compact and efficient PowerPC, ARM and x86 code for a large and realistic subset of the C language. However, CompCert foregoes using Static Single Assignment (SSA), an intermediate representation that allows for writing simpler and faster optimizers, and that is used by many compilers. In fact, it has remained an open problem to verify formally an SSA-based compiler middle-end. We report on a formally verified, SSA-based, middle-end for CompCert. Our middle-end performs conversion from CompCert intermediate form to SSA form, optimization of SSA programs, including Global Value Numbering, and transforming out of SSA to intermediate form. In addition to provide the first formally verified SSA-based middle-end, we address two problems raised by Leroy in 2009: giving a simple and intuitive formal semantics to SSA, and leveraging the global properties of SSA to reason locally about program optimizations
Mise en oeuvre d'une approche multi-échelles fondée sur le champ de phase pour caractériser la microstructure des matériaux irradiés : application à l'alliage AgCu
It is of dramatic matter for industry to be able to predict the evolution of material microstructure under working conditions. This requires a clear understanding of the underlying mechanisms, which act on numerous space and time scales. Because it intrinsically performs a scale jump, we chose to use a phase field approach, which is widely used amidst the condensed matter community to study the aging of materials. The first challenge of this work was to extend this formalism beyond its thermodynamic scope and embrace the case of far from equilibrium systems when subjected to irradiation. For that purpose, we adopted the model of ion mixing, developed by Gras Marti to account for ballistic exchanges within a displacements cascade. Based on a numerical scheme and ananalytical method, we were able to describe the generic microstructure signature for materials under irradiation.We then applied this formalism to the particular case of the immiscible binary alloy AgCu.With the joined use of the phase field approach and atomistic methods, we managed to predict how the temperature and the irradiation flux tailor the main microstructure features such as the size, the concentration and the distribution of copper precipitates. This eventually allowed us to simulate a diffraction pattern in grazing incidence, which is directly comparable to experimental ones.Anticiper lâĂ©volution de la microstructure dâun matĂ©riau en condition dâusage est dâune importance cruciale pour lâindustrie. Cette maĂźtrise du vieillissement nĂ©cessite une comprĂ©hension claire des mĂ©canismes sous-jacents, qui agissent sur une large gamme dâĂ©chelles spatiales et temporelles. Dans cette optique, ce travail de thĂšse a choisi dâappliquer la mĂ©thode de champ de phase qui, en raison du saut dâĂ©chelle quâelle rĂ©alise naturellement, est un outil intensivement employĂ© dans le domaine des matĂ©riaux, pour prĂ©dire lâĂ©volution en temps long de la microstructure. Lâenjeu de lâĂ©tude a Ă©tĂ© dâĂ©tendre cette mĂ©thode Ă un systĂšme portĂ© loin de lâĂ©quilibre thermodynamique, en particulier en prĂ©sence dâirradiation. Nous avons ainsi adoptĂ© le formalisme du mĂ©lange ionique, introduit par Gras-Marti pour dĂ©crire le mĂ©lange balistique au sein dâune cascade de dĂ©placements. Par lâutilisation conjointe dâun schĂ©ma numĂ©rique et dâune approche analytique, il nous a Ă©tĂ© possible dâĂ©tablir le diagramme de phase gĂ©nĂ©rique dâun matĂ©riau irradiĂ©. Nous avons ensuite Ă©tudiĂ© le vieillissement de lâalliage binaire test AgCu sous irradiation, par lâutilisation conjointe de la mĂ©thode du champ de phase et dâapproches atomistiques, dans une dĂ©marche multi-Ă©chelles. En fixant les paramĂštres de contrĂŽle que sont le flux dâirradiation et la tempĂ©rature, il nous a ainsi Ă©tĂ© possible de prĂ©dire la taille,la concentration ainsi que la distribution spatiale des nodules de cuivre produits sous irradiation dans cet alliage. La connaissance de ces informations a permis de simuler un diagramme de diffraction en incidence rasante, directement comparable aux diagrammes expĂ©rimentaux
Induced side-branching in smooth and faceted dendrites: theory and Phase-Field simulations
The present work is devoted to the phenomenon of induced side branching
stemming from the disruption of free dendrite growth. Therein, we postulate
that the secondary branching instability can be triggered by the departure of
the morphology of the dendrite from its steady state shape. Thence, the
instability results from the thermodynamic trade-off between non monotonic
variations of interface temperature, surface energy, kinetic anisotropy and
interface velocity within the Gibbs Thomson equation. For purposes of
illustration, the toy model of capillary anisotropy modulation is prospected
both analytically and numerically by means of phase field simulations. It is
evidenced that side branching can befall both smooth and faceted dendrites, at
a normal angle from the front tip which is specific to the nature of the
capillary anisotropy shift applied
Solid state diffusion
The workshop is composed of two main parts: the first part devoted to atomistic Monte Carlo
simulations and the second part devoted to the Phase Field modelling. In each part a lecture will be
accompanied by exercise activities
Theoretical modeling of crystalline symmetry order with dendritic morphology
The stable growth of a crystal with dendritic morphology with n-fold symmetry is modeled. Using the linear stability analysis and solvability theory, a selection criterion for thermally and solutally controlled growth of the dendrite is derived. A complete set of nonlinear equations consisting of the selection criterion and an undercooling balance (which determines the implicit dependencies of the dendrite tip velocity and tip diameter on the total undercooling) is formulated. The growth kinetics of crystals having different lattice symmetry is analyzed. The model predictions are compared with phase field modelling data on ice dendrites grown from pure undercooled water
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