Introduce COFFEE optimization level parameter

[Jan Blechta]

Need an interface to set now hard-wired COFFEE optimization: https://bitbucket.org/fenics-project/ffc/src/6b64700a5458fc3af3369c66e070d1d71eecfc57/ffc/tsfc/tsfcoptimization.py?at=master&fileviewer=file-view-default#tsfcoptimization.py-18.

Also check what #pragma coffee is for and eventually get rid of it properly: https://bitbucket.org/fenics-project/ffc/src/6b64700a5458fc3af3369c66e070d1d71eecfc57/ffc/tsfc/tsfcgenerator.py?at=master&fileviewer=file-view-default#tsfcgenerator.py-33.

[Miklós Homolya]

It's another inspiration from C compilers in COFFEE. #pragmas are used to tell the compiler extra information that, e.g., helps with some optimisations. #pragma coffee is to tell things to COFFEE. There are two COFFEE pragmas in use:

• #pragma coffee expression marks the root expression, so COFFEE knows what to optimise.
• #pragma coffee linear loop marks the argument loops, so COFFEE knows which are the argument indices.

For some reason, @FabioLuporini decided to show the former, but hide the latter when printing the AST. It is nice to see them if one needs to debug the COFFEE code generation in C++, otherwise neither means anything to the C compiler.

[Jan Blechta]

This might not be worth of an effort. Lot of COFFEE is being moved to TSFC and optimization interface might change. @miklos1?

[Miklós Homolya]

Yes, the configuration interface will change, as the operation reduction optimisations of COFFEE are being ported by TJ (Tianjiao Sun) to operate on GEM instead.