[GR-64584] Base-relative code pointers in vtables for non-layered images.

[graalvmbot]

Relocation entries for pointers in the virtual dispatch tables (vtables) of DynamicHub objects require additional space in position-independent executables and shared libraries, cause delays during startup when processed at runtime by the dynamic linker, and decrease sharing between instances of the same image because memory pages must be copied (by the OS) when adjusting pointers. This is magnified with layered images where the base layer(s) in particular are expected to contain large number of methods which may be used in virtual dispatch.

The task at hand is to, as a first step, eliminate relocation entries in vtables for non-layered images, by introducing the notion of a code base and changing vtable entries to be offsets relative to that code base. Therefore, for dispatching a virtual call, it becomes necessary to add the code base to the vtable entry each time.

For prudence and because there will be some initial limitations (PLT/GOT dispatch, LLVM backend, layers), this is an option that is turned off by default.

This will also not eliminate relocation entries outside of vtables, such as in reflection accessors or JNI dispatch records. However, these can typically be assumed to be few compared to those in vtables.