The Shared Classes Cache (SCC) is a feature of OpenJ9 that enables sharing data between JVMs. For more information see [1].
The JIT Compiler uses the SCC to store both AOT code as well as data either
necessary for AOT
(e.g. Class Chains),
or for performace (e.g. IProfiler data). The JIT interfaces with the SCC via
the TR_J9SharedCache class.
When data that is stored into the SCC needs to refer to other data, it is referred to by using offsets. Traditionally, the offset would always be relative to the start of the SCC. However, if the SCC is resized, only offsets of data in the ROM Class section of the SCC will be invariant. Therefore, offsets of all data, which must be in the metadata section, are calculated relative to the start of the metadata section. Another point of clarification is that offsets of data within the ROMClass section is calculated using the start of the ROMClass section as no ROM Structure pointer should point before the start of the section. The offsets that are generated and stored as part of relocation information are first left shifted 1 bit; the low bit is used to determine whether the offset is relative to the start of the ROM Class section or the start of the metadata section.
The APIs in TR_J9SharedCache provide an abstraction layer so that
other parts of the compiler do not need to worry about the details.
However, it is important that one does not use the offsets directly, but
only via the TR_J9SharedCache class.