Module Lift_constants

The aim of this pass is to assign symbols to values known to be constant (in other words, whose values we know at compile time), with appropriate sharing of constants, and replace the occurrences of the constants with their corresponding symbols.

This pass uses the results of two other passes, Inconstant_idents and Alias_analysis. The relationship between these two deserves some attention.

Inconstant_idents is a "backwards" analysis that propagates implications about inconstantness of variables and set of closures IDs.

Alias_analysis is a "forwards" analysis that is analogous to the propagation of Simple_value_approx.t values during Inline_and_simplify. It gives us information about relationships between values but not actually about their constantness.

Combining these two into a single pass has been attempted previously, but was not thought to be successful; this experiment could be repeated in the future. (If "constant" is considered as "top" and "inconstant" is considered as "bottom", then Alias_analysis corresponds to a least fixed point and Inconstant_idents corresponds to a greatest fixed point.)

At a high level, this pass operates as follows. Symbols are assigned to variables known to be constant and their defining expressions examined. Based on the results of Alias_analysis, we simplify the destructive elements within the defining expressions (specifically, projection of fields from blocks), to eventually yield Flambda.constant_defining_values that are entirely constructive. These will be bound to symbols in the resulting program.

Another approach to this pass could be to only use the results of Inconstant_idents and then repeatedly lift constants and run Inline_and_simplify until a fixpoint. It was thought more robust to instead use Alias_analysis, where the fixpointing involves a less complicated function.

We still run Inline_and_simplify once after this pass since the lifting of constants may enable more functions to become closed; the simplification pass provides an easy way of cleaning up (e.g. making sure free_vars maps in sets of closures are correct).