The constant pool: Haskell heap objects that native code needs, in an array whose address is passed on every entry. See unison-runtimesrcUnisonRuntimeJIT/design.md, "Appendix: the constant pool", which also says why there is one pool per process rather than one per module.

Indices are assigned when a module is compiled (poolIndices) and are constants in the generated code. The array grows by copying; the old arrays are kept, and new entries are written to them too where they fit, so an address read at entry stays valid for as long as that native run lasts. A run can still meet code that was installed after it began and uses an index past the end of the array the run holds: a function that uses an index of poolStableSize or more checks the array's size at entry, and exits to be entered again with the current array.