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24
25#ifndef SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP
26#define SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP
27
28//
29// String Deduplication
30//
31// String deduplication aims to reduce the heap live-set by deduplicating identical
32// instances of String so that they share the same backing character array.
33//
34// The deduplication process is divided in two main parts, 1) finding the objects to
35// deduplicate, and 2) deduplicating those objects. The first part is done as part of
36// a normal GC cycle when objects are marked or evacuated. At this time a check is
37// applied on each object to check if it is a candidate for deduplication. If so, the
38// object is placed on the deduplication queue for later processing. The second part,
39// processing the objects on the deduplication queue, is a concurrent phase which
40// starts right after the stop-the-wold marking/evacuation phase. This phase is
41// executed by the deduplication thread, which pulls deduplication candidates of the
42// deduplication queue and tries to deduplicate them.
43//
44// A deduplication hashtable is used to keep track of all unique character arrays
45// used by String objects. When deduplicating, a lookup is made in this table to see
46// if there is already an identical character array somewhere on the heap. If so, the
47// String object is adjusted to point to that character array, releasing the reference
48// to the original array allowing it to eventually be garbage collected. If the lookup
49// fails the character array is instead inserted into the hashtable so that this array
50// can be shared at some point in the future.
51//
52// Candidate selection criteria is GC specific.
53//
54// Interned strings are a bit special. They are explicitly deduplicated just before
55// being inserted into the StringTable (to avoid counteracting C2 optimizations done
56// on string literals), then they also become deduplication candidates if they reach
57// the deduplication age threshold or are evacuated to an old heap region. The second
58// attempt to deduplicate such strings will be in vain, but we have no fast way of
59// filtering them out. This has not shown to be a problem, as the number of interned
60// strings is usually dwarfed by the number of normal (non-interned) strings.
61//
62// For additional information on string deduplication, please see JEP 192,
63// http://openjdk.java.net/jeps/192
64//
65
66#include "gc/shared/stringdedup/stringDedupQueue.hpp"
67#include "gc/shared/stringdedup/stringDedupStat.hpp"
68#include "gc/shared/stringdedup/stringDedupTable.hpp"
69#include "memory/allocation.hpp"
70#include "runtime/thread.hpp"
71
72//
73// Main interface for interacting with string deduplication.
74//
75class StringDedup : public AllStatic {
76private:
77 // Single state for checking if string deduplication is enabled.
78 static bool _enabled;
79
80public:
81 // Returns true if string deduplication is enabled.
82 static bool is_enabled() {
83 return _enabled;
84 }
85
86 // Stop the deduplication thread.
87 static void stop();
88
89 // Immediately deduplicates the given String object, bypassing the
90 // the deduplication queue.
91 static void deduplicate(oop java_string);
92
93 static void parallel_unlink(StringDedupUnlinkOrOopsDoClosure* unlink, uint worker_id);
94
95 static void threads_do(ThreadClosure* tc);
96 static void print_worker_threads_on(outputStream* st);
97 static void verify();
98
99 // GC support
100 static void gc_prologue(bool resize_and_rehash_table);
101 static void gc_epilogue();
102
103protected:
104 // Initialize string deduplication.
105 // Q: String Dedup Queue implementation
106 // S: String Dedup Stat implementation
107 template <typename Q, typename S>
108 static void initialize_impl();
109};
110
111//
112// This closure encapsulates the closures needed when scanning
113// the deduplication queue and table during the unlink_or_oops_do() operation.
114//
115class StringDedupUnlinkOrOopsDoClosure : public StackObj {
116 AlwaysTrueClosure _always_true;
117 DoNothingClosure _do_nothing;
118 BoolObjectClosure* _is_alive;
119 OopClosure* _keep_alive;
120
121public:
122 StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
123 OopClosure* keep_alive);
124
125 bool is_alive(oop o) { return _is_alive->do_object_b(o); }
126
127 void keep_alive(oop* p) { _keep_alive->do_oop(p); }
128};
129
130#endif // SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP
131