Rename alloc bit to valid-object bit (VO-bit) #680
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I did not rename comments and functions in malloc space and mark-compact space because when they mention "alloc bit", they do mean a one-bit-per-allocation metadata. They should have used their own local metadata for that purpose.
It is how it is intended to be used. It is set at addresses pointed by `ObjectReference`, not the beginning of an object.
vo_bit is designed for the very purpose of is_mmtk_object, so we don't need another feature for that.
bzero should be unsafe because of potential races.
There is a bug with fetch_and and fetch_or.
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This PR tries to do too many things in one go. I think it is taking way too long. I'll do it in smaller steps. See: #791 |
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This PR does the following:
alloc_bit.rstovo_bit.rsand move it undersrc/util/metadata.ObjectReferenceas parameter instead ofAddress.util::is_mmtk_objectmodule intoutil::metadata::vo_bit.memory_manager::is_mmtk_objecttoutil::metadata::vo_bit::is_valid_mmtk_object, and it takesObjectReferenceparameter.We currently have a so-called "allocation bit", or "alloc bit" metadata. This name is misleading, because the name suggests it is used to identify allocated units in spaces, which is not true. It actually identifies valid objects (not allocation units) at any address, regardless of space. It is primarily designed to help conservative GC to filter potential object references on the stack or in objects. This PR renames it to valid-object bit, or VO-bit for short.
Some existing spaces use VO-bit to identify allocated units. Currently they are
MallocSpaceandMarkCompactSpace.MallocSpaceuses VO-bit to record all the memory returned bymalloc.MarkCompactSpaceuses VO-bit to iterate through all allocation units in the space.These two spaces should switch to local metadata, and such metadata should only exist if the corresponding spaces
MallocSpaceorMarkCompactSpaceexist. It should be independent from the VO-bit.This PR also does some minor refactoring related to things related to VO-bit. Particularly, the Cargo feature "is_mmtk_object" is removed. The
is_mmtk_objectfunction was designed to support conservative GC, but that is actually what VO-bit is designed for. So that function (now renamedutil::metadata::vo_bit::is_valid_mmtk_object) is enabled whenever the "vo_bit" feature is enabled.Things that are NOT done in this PR:
fecth_andandfetch_or. See Fix fetch_and_atomic and fetch_or_atomic #684util::metadata::invalidate_objectfunction. It should simply clear the VO-bit of an object, but it (1) interferes with the way MallocSpace currently works, and (2) requires the tracing code to skip objects without VO-bits. This change may affect performance.