In the CoSy compiler development system, we like to implement
optimizations at the language and target independent IR
level. However, the benefits of a transformation often depend
on characteristics of the target.

CoSy uses Code Generator feedback to parametrize high level
transformations.  This is possible by viewing the generated
code as an annotation of the IR. It allows calling the code
generator from a high level transformation and then using the
resulting code to decide on the transformation. After that,
the generated code (annotation) can simply be removed.

The importance of CG-feedback is that it keeps all target
specifics in the code generator and remains isolated from
the implementation of the high level transformation.

An example transformation that is currently part of CoSy is
the fully retargetable predicated execution optimization that
replaces if-then-else constructs by conditional execution. The
very same algorithm takes into account such aspects as
generic or specific condition registers, (non-)interlocked
scheduling, non-homogeneous instruction sets that allow only
partial predication, local register pressure and even target
aspects that are not foreseen today. These are all hidden
in the code generator where they must be handled anyway. The
predicated execution algorithm has a very simple interface:
it calls the code generator and simply queries the resulting
code for aspects such as cycle-count.

We will present the CoSy CG-feedback framework and also sketch
how it is used in predicated execution and other optimizations.