Jit: move runtime lookup expansion to lower #35551
Description
This is a follow-up to #341.
The jit currently expands runtime lookups in the importer (impRuntimeLookupToTree). The optimal expansion requires introducing control flow either via GT_QMARK or via IndirectCallTransformer.
The tail call via helpers mechanism introduced in #341 also needs to insert runtime lookups. The expansion of tail calls via helpers happens in morph and it's impossible to change control flow there the way it's done in the importer. Because of that the runtime lookups introduced in morph use the slower helper call expansion:
runtime/src/coreclr/src/jit/morph.cpp
Lines 8032 to 8040 in e1ffadd
The suggested solution is to move the expansion of runtime lookups to lower so that both the importer and morph can just create and insert a new node representing a runtime lookup and have lower expand it in an optimal way. This will involve teaching optimization phases (e.g., value numbering) about the new runtime lookup node.
category:cq
theme:helpers
skill-level:intermediate
cost:medium