15 June 2014

Just-in-time compilation

One of the known issues of the fem-fenics package was related to the errors during the just-in-time compilation, due to missing include directories. Among my preliminary contributions there is a changeset addressing the problem in the initial build of the package, when installing it into Octave. Now I went on and solved it also during the usage of fem-fenics.

At the moment of the first build, autoconf is in charge of finding out the relevant compiler and linker flags through pkg-config. They are, then, substituted in the Makefile, which compiles the package making use of them. This piece of information is needed also when an UFL file is imported and transformed into the oct-files used to transform the weak formulation at hand into an algebraic system, but until now the user had to supply it by the means of an environment variable.

Currently, I added a new utility function that provides those flags. In the configuration process they are substituted also in get_vars.m, which is called by generate_makefile.m when a differential problem is imported. The latter replaces two placeholders and writes the ad hoc Makefile with all the necessary compile and link options. This way users will not need to provide compilation flags anymore, instead the package will manage this aspect on its own.

As noted in a previous post, however, this just-in-time build is relatively time consuming, taking around half a minute each time. Nonetheless, a common usage pattern could entail the resolution of the same weak formulation on varying meshes or with different boundary conditions, forcing terms or physical parameters. Every mentioned situation does not need the recompilation of the problem's oct-files, since they carry information only about the function space and the formal expressions of the bilinear form and the linear operator. It is useful, then, to take on the build process only when needed.

To add this feature, I created three function to perform appropriate actions. After every successful just-in-time compilation, save_hash.m takes care of hashing the imported UFL file and writing the result to <ufl-filename>.md5sum. On the other hand, at the beginning of every import_ufl_*.m function, a check like this is performed:

         if (check_hash (var_prob) ||
             ! check_oct_files (var_prob, "Problem"))

You can see in it the remaining two functions implemented lately. The first one, check_hash.m, receives as argument the name of the variational problem, reconstructs the UFL file name, looks for a saved hash sum and compares it with the current file's. It returns true if the proper .md5sum file is not found or if the new and old hashes are not the same. Clearly, the oct-files should be rebuilt if one of them is missing: check_oct_files.m looks for the relevant files, with its second option stating which import is underway (thus, which files are expected as output), and returns true if they are all available.

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