Note: %PRIMITIVE can only be used in compiled code. It is a
trapdoor into the compiler, not a general syntax for accessing
“sub-primitives”. It’s main use is in implementation-dependent
compiler transforms. It saves us the effort of defining a “phony
function” (that is not really defined), and also allows direct
communication with the code generator through codegen-info arguments.
Some primitives may be exported from the VM so that %PRIMITIVE
can be used to make it explicit that an escape routine or interpreter
stub is assuming an operation is implemented by the compiler.