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8372564: Convert StringDedup to use Atomic<T> #28524

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8372564: Convert StringDedup to use Atomic<T> #28524

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13 changes: 6 additions & 7 deletions src/hotspot/share/gc/shared/stringdedup/stringDedupProcessor.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -36,7 +36,6 @@
#include "memory/iterator.hpp"
#include "nmt/memTag.hpp"
#include "oops/access.inline.hpp"
#include "runtime/atomicAccess.hpp"
#include "runtime/cpuTimeCounters.hpp"
#include "runtime/interfaceSupport.inline.hpp"
#include "runtime/mutexLocker.hpp"
Expand All @@ -46,17 +45,17 @@

OopStorage* StringDedup::Processor::_storages[2] = {};

StringDedup::StorageUse* volatile StringDedup::Processor::_storage_for_requests = nullptr;
DeferredStatic<Atomic<StringDedup::StorageUse*>> StringDedup::Processor::_storage_for_requests{};
StringDedup::StorageUse* StringDedup::Processor::_storage_for_processing = nullptr;

void StringDedup::Processor::initialize_storage() {
assert(_storages[0] == nullptr, "storage already created");
assert(_storages[1] == nullptr, "storage already created");
assert(_storage_for_requests == nullptr, "storage already created");
assert(_storage_for_processing == nullptr, "storage already created");
_storages[0] = OopStorageSet::create_weak("StringDedup Requests0 Weak", mtStringDedup);
_storages[1] = OopStorageSet::create_weak("StringDedup Requests1 Weak", mtStringDedup);
_storage_for_requests = new StorageUse(_storages[0]);
auto sfr = new StorageUse(_storages[0]); // Work around lack of std::forward.
_storage_for_requests.initialize(sfr);
_storage_for_processing = new StorageUse(_storages[1]);
}

Expand All @@ -75,15 +74,15 @@ void StringDedup::Processor::wait_for_requests() const {
{
ThreadBlockInVM tbivm(_thread);
MonitorLocker ml(StringDedup_lock, Mutex::_no_safepoint_check_flag);
OopStorage* storage = AtomicAccess::load(&_storage_for_requests)->storage();
OopStorage* storage = _storage_for_requests->load_relaxed()->storage();
while ((storage->allocation_count() == 0) &&
!Table::is_dead_entry_removal_needed()) {
ml.wait();
}
}
// Swap the request and processing storage objects.
log_trace(stringdedup)("swapping request storages");
_storage_for_processing = AtomicAccess::xchg(&_storage_for_requests, _storage_for_processing);
_storage_for_processing = _storage_for_requests->exchange(_storage_for_processing);
GlobalCounter::write_synchronize();
// Wait for the now current processing storage object to no longer be used
// by an in-progress GC. Again here, the num-dead notification from the
Expand All @@ -99,7 +98,7 @@ void StringDedup::Processor::wait_for_requests() const {
}

StringDedup::StorageUse* StringDedup::Processor::storage_for_requests() {
return StorageUse::obtain(&_storage_for_requests);
return StorageUse::obtain(_storage_for_requests.get());
}

void StringDedup::Processor::yield() const {
Expand Down
6 changes: 4 additions & 2 deletions src/hotspot/share/gc/shared/stringdedup/stringDedupProcessor.hpp
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* Copyright (c) 2021, 2023, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2021, 2025, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
Expand Down Expand Up @@ -27,7 +27,9 @@

#include "gc/shared/stringdedup/stringDedup.hpp"
#include "memory/allocation.hpp"
#include "runtime/atomic.hpp"
#include "services/cpuTimeUsage.hpp"
#include "utilities/deferredStatic.hpp"
#include "utilities/macros.hpp"

class JavaThread;
Expand All @@ -51,7 +53,7 @@ class StringDedup::Processor : public CHeapObj<mtGC> {
NONCOPYABLE(Processor);

static OopStorage* _storages[2];
static StorageUse* volatile _storage_for_requests;
static DeferredStatic<Atomic<StorageUse*>> _storage_for_requests;
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@xmas92 xmas92 Dec 2, 2025

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What is the reason for the DeferredStatic?

Is it just for the init once semantics and asserts?

Is this a style we should adapt more broadly for our init once static variables?

static StorageUse* _storage_for_processing;

JavaThread* _thread;
Expand Down
13 changes: 6 additions & 7 deletions src/hotspot/share/gc/shared/stringdedup/stringDedupStorageUse.cpp
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Expand Up @@ -23,7 +23,6 @@
*/

#include "gc/shared/stringdedup/stringDedupStorageUse.hpp"
#include "runtime/atomicAccess.hpp"
#include "runtime/javaThread.hpp"
#include "utilities/debug.hpp"
#include "utilities/globalCounter.inline.hpp"
Expand All @@ -34,18 +33,18 @@ StringDedup::StorageUse::StorageUse(OopStorage* storage) :
{}

bool StringDedup::StorageUse::is_used_acquire() const {
return AtomicAccess::load_acquire(&_use_count) > 0;
return _use_count.load_acquire() > 0;
}

StringDedup::StorageUse*
StringDedup::StorageUse::obtain(StorageUse* volatile* ptr) {
StringDedup::StorageUse::obtain(Atomic<StorageUse*>* ptr) {
GlobalCounter::CriticalSection cs(Thread::current());
StorageUse* storage = AtomicAccess::load(ptr);
AtomicAccess::inc(&storage->_use_count);
StorageUse* storage = ptr->load_relaxed();
storage->_use_count.add_then_fetch(1u);
return storage;
}

void StringDedup::StorageUse::relinquish() {
size_t result = AtomicAccess::sub(&_use_count, size_t(1));
assert(result != SIZE_MAX, "use count underflow");
size_t result = _use_count.fetch_then_sub(1u);
assert(result != 0, "use count underflow");
}
7 changes: 4 additions & 3 deletions src/hotspot/share/gc/shared/stringdedup/stringDedupStorageUse.hpp
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2021, 2025, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
Expand Down Expand Up @@ -27,6 +27,7 @@

#include "gc/shared/stringdedup/stringDedup.hpp"
#include "memory/allocation.hpp"
#include "runtime/atomic.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/macros.hpp"

Expand All @@ -35,7 +36,7 @@ class OopStorage;
// Manage access to one of the OopStorage objects used for requests.
class StringDedup::StorageUse : public CHeapObj<mtStringDedup> {
OopStorage* const _storage;
volatile size_t _use_count;
Atomic<size_t> _use_count;

NONCOPYABLE(StorageUse);

Expand All @@ -48,7 +49,7 @@ class StringDedup::StorageUse : public CHeapObj<mtStringDedup> {
bool is_used_acquire() const;

// Get the current requests object, and increment its in-use count.
static StorageUse* obtain(StorageUse* volatile* ptr);
static StorageUse* obtain(Atomic<StorageUse*>* ptr);

// Discard a prior "obtain" request, decrementing the in-use count, and
// permitting the deduplication thread to start processing if needed.
Expand Down