Hi
I am using fenics tutorials.
When any script is launched for the first time it starts with a message
"Calling FFC just-in-time (JIT) compiler, this may take some time".
From then on this message no longer appears.
What is it compiled exactly ? And where is it stored ?
Thanks 
One of the original ideas in FEniCS was to exploit automatic code generation. This means that the finite element formulations you write in the symbolic UFL language, the function spaces you create, the expressions you create, and so on, are turned into C/C++ code by FFC, and compiled by dijitso behind the scenes for you just-in-time (JIT).
All of these objects are cached into a directory provided by dijitso so that the compilation need not be done more than necessary. On my system the default cache directory is $HOME/.cache/dijitso where you can see all of the generated code and compiled so libraries.
See the FEniCS book for more information.