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Trim identity-hashcode to 25bit #3

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Trim identity-hashcode to 25bit #3

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c3790d0
Trim identity-hashcode to 25bit
May 10, 2021
dfe81d7
Adjust testcase to reflect new header layout
May 10, 2021
c658bb8
Fix docs about new header layout
May 10, 2021
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10 changes: 4 additions & 6 deletions src/hotspot/share/oops/markWord.hpp
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Expand Up @@ -41,8 +41,8 @@
//
// 64 bits:
// --------
// unused:25 hash:31 -->| unused_gap:1 age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 unused_gap:1 age:4 biased_lock:1 lock:2 (biased object)
// unused:32 hash:25 -->| age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
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This does not compute. Where did the unused_gap go in the second case? Is epoch now larger? Or does JavaThread* now takes 55 bits?

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Epoch and JavaThread in the biased case are already overlaid with hashcode. Hashcode and biased state doesn't happen at the same time. 32+25+4+1+2 == 64.

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Oh right, unused_gap is now used, epoch is moved 1 bit to the right and JavaThread is now 55 bits (actually, I guess it is still 54 bits and the uppermost bit is unused). I will correct this.

//
// - hash contains the identity hash value: largest value is
// 31 bits, see os::random(). Also, 64-bit vm's require
Expand Down Expand Up @@ -130,17 +130,15 @@ class markWord {
static const int lock_bits = 2;
static const int biased_lock_bits = 1;
static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits;
static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
static const int unused_gap_bits = LP64_ONLY(1) NOT_LP64(0);
static const int hash_bits = max_hash_bits > 25 ? 25 : max_hash_bits;
static const int epoch_bits = 2;

// The biased locking code currently requires that the age bits be
// contiguous to the lock bits.
static const int lock_shift = 0;
static const int biased_lock_shift = lock_bits;
static const int age_shift = lock_bits + biased_lock_bits;
static const int unused_gap_shift = age_shift + age_bits;
static const int hash_shift = unused_gap_shift + unused_gap_bits;
static const int hash_shift = age_shift + age_bits;
static const int epoch_shift = hash_shift;

static const uintptr_t lock_mask = right_n_bits(lock_bits);
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12 changes: 9 additions & 3 deletions src/hotspot/share/services/heapObjectStatistics.cpp
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Expand Up @@ -137,15 +137,21 @@ void HeapObjectStatistics::begin_sample() {

void HeapObjectStatistics::visit_object(oop obj) {
increase_counter(_num_objects);
if (!obj->mark().has_no_hash()) {
markWord mark = obj->mark();
if (!mark.has_no_hash()) {
increase_counter(_num_ihashed);
if (obj->mark().age() > 0) {
if (mark.age() > 0) {
increase_counter(_num_ihashed_moved);
}
}
if (obj->mark().is_locked()) {
if (mark.is_locked()) {
increase_counter(_num_locked);
}
#ifdef ASSERT
if (!mark.has_displaced_mark_helper()) {
assert((mark.value() & 0xffffffff00000000) == 0, "upper 32 mark bits must be free");
}
#endif
increase_counter(_lds, obj->size());
}

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