Module Flambda2_simplify.Simplify_expr

Simplification proceeds in two passes: the downwards pass and the upwards pass.

The downwards pass proceeds following the scoping of the terms. It collects usage information and exploits the Flambda 2 type system to identify simplifications to be performed. It also does inlining and unboxing calculations.

The upwards pass rebuilds the simplified expressions based on information collected in the downwards pass. This includes placing let-symbol bindings for statically-allocated values.

In general the whole of an expression will be traversed downwards before the upwards pass starts. There is one exception: the right-hand sides of Let-expressions binding Set_of_closures expressions are simplified fully (down and up) during the downwards pass of the surrounding expression. This is so that the Flambda type system can have the necessary simplified function bodies available, enabling inlining of those bodies later on. (In effect we have made the decision to prioritise availability of simplified function bodies over usage information of functions. Having the latter would permit a more extensive data flow analysis whose results could be used before function bodies were rebuilt.)

Data flow equations are solved in the transition from the downwards to the upwards pass.

Also see comments in Simplify_common.