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8212753: Improve oopDesc::forward_to_atomic

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Avoid multiple unnecessary reloads of the mark oop in oopDesc::forward_to_atomic

Reviewed-by: kbarrett, mdoerr
This commit is contained in:
Thomas Schatzl 2018-10-24 16:22:34 +02:00
parent 5dac22b8eb
commit 3a48e68b1a
4 changed files with 19 additions and 24 deletions
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33
src/hotspot/share/oops/oop.inline.hpp
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@ -370,26 +370,21 @@ bool oopDesc::cas_forward_to(oop p, markOop compare, atomic_memory_order order)
return cas_set_mark_raw(m, compare, order) == compare;
}
oop oopDesc::forward_to_atomic(oop p, atomic_memory_order order) {
markOop oldMark = mark_raw();
markOop forwardPtrMark = markOopDesc::encode_pointer_as_mark(p);
markOop curMark;
assert(forwardPtrMark->decode_pointer() == p, "encoding must be reversable");
assert(sizeof(markOop) == sizeof(intptr_t), "CAS below requires this.");
while (!oldMark->is_marked()) {
curMark = cas_set_mark_raw(forwardPtrMark, oldMark, order);
assert(is_forwarded(), "object should have been forwarded");
if (curMark == oldMark) {
return NULL;
}
// If the CAS was unsuccessful then curMark->is_marked()
// should return true as another thread has CAS'd in another
// forwarding pointer.
oldMark = curMark;
oop oopDesc::forward_to_atomic(oop p, markOop compare, atomic_memory_order order) {
// CMS forwards some non-heap value into the mark oop to reserve oops during
// promotion, so the next two asserts do not hold.
assert(UseConcMarkSweepGC || check_obj_alignment(p),
"forwarding to something not aligned");
assert(UseConcMarkSweepGC || Universe::heap()->is_in_reserved(p),
"forwarding to something not in heap");
markOop m = markOopDesc::encode_pointer_as_mark(p);
assert(m->decode_pointer() == p, "encoding must be reversable");
markOop old_mark = cas_set_mark_raw(m, compare, order);
if (old_mark == compare) {
return NULL;
} else {
return (oop)old_mark->decode_pointer();
}
return forwardee();
}
// Note that the forwardee is not the same thing as the displaced_mark.