Skip val register in ObjectBarrier impl #294
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In `MMTkBarrierSetAssembler`, `object_reference_write_post` is called after calling `BarrierSetAssembler::store_at` which modifies the `val` register and compresses its value in place. Consequently, `object_reference_write_post` will see a compressed pointer in the `val` register. We decompress the pointer in the slow path before calling into MMTk core.
| // Note: If `compensate_val_reg == true && UseCompressedOops === true`, the `val` register will be | ||
| // holding a compressed pointer to the target object. If the write barrier needs to know the | ||
| // target, we will need to decompress it before passing it to the barrier slow path. However, | ||
| // since we know the semantics of `ObjectReference`, i.e. it logs the object without looking at |
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Did you mean "the semantics of object remembering barrier"?
We need to a check to know this is object remembering barrier somewhere in the code. We can get it from https://github.com/mmtk/mmtk-core/blob/a75309373cd301acad473bbb8ea04670facd6e2a/src/plan/plan_constraints.rs#L35, and store the value somewhere for the C++ code to access.
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Did you mean "the semantics of object remembering barrier"?
Oops. That's a typo. I did mean ObjectBarrier.
We need to a check to know this is object remembering barrier somewhere in the code.
The changed code is in mmtkObjectBarrier.cpp which is dedicated to the object remembering barrier. The class MMTkObjectBarrierSetAssembler is only used if get_selected_barrier() in mmtkBarrierSet.cpp returns new MMTkObjectBarrier(); Therefore we don't need to check here, as long as the get_selected_barrier() chooses the right barrier.
MMTkBarrierSetAssembler::store_atcallsobject_reference_write_postafter callingBarrierSetAssembler::store_at. However,BarrierSetAssembler::store_atmodifies thevalregister to compress its value in place. Consequently,object_reference_write_postwill see a compressed pointer in thevalregister.This PR makes two changes.
Firstly, in
MMTkObjectBarrierSetAssembler::object_reference_write_post, we simply set bothc_rarg1andc_rarg2to 0 before calling the write barrier slow path. We exploit the semantics of theObjectBarrierthat it simply logs the object without looking at the slot or the target. This will fix the assertion error because 0 will be interpreted as aNoneof typeOption<ObjectReference>.Secondly, we add a
bool compensate_val_regparameter toMMTkBarrierSetAssembler::object_reference_write_postso that if we call it afterBarrierSetAssembler::store_at, we can giveobject_reference_write_posta chance to decompress the compressed oop in theval. This is intended for implementing other barriers introduced in the future that may use thevalregister, and keep the developers informed that thevalregister is mutated inBarrierSetAssembler::store_at.Fixes: #291
Related PR: #293