1/*
2 * Copyright (c) 1997, 2019, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation.
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13 * accompanied this code).
14 *
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23 */
24
25#include "precompiled.hpp"
26#include "runtime/basicLock.hpp"
27#include "runtime/synchronizer.hpp"
28
29void BasicLock::print_on(outputStream* st) const {
30 st->print("monitor");
31 markOop moop = displaced_header();
32 if (moop != NULL)
33 moop->print_on(st);
34}
35
36void BasicLock::move_to(oop obj, BasicLock* dest) {
37 // Check to see if we need to inflate the lock. This is only needed
38 // if an object is locked using "this" lightweight monitor. In that
39 // case, the displaced_header() is unlocked, because the
40 // displaced_header() contains the header for the originally unlocked
41 // object. However the object could have already been inflated. But it
42 // does not matter, the inflation will just a no-op. For other cases,
43 // the displaced header will be either 0x0 or 0x3, which are location
44 // independent, therefore the BasicLock is free to move.
45 //
46 // During OSR we may need to relocate a BasicLock (which contains a
47 // displaced word) from a location in an interpreter frame to a
48 // new location in a compiled frame. "this" refers to the source
49 // basiclock in the interpreter frame. "dest" refers to the destination
50 // basiclock in the new compiled frame. We *always* inflate in move_to().
51 // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
52 // cause performance problems in code that makes heavy use of a small # of
53 // uncontended locks. (We'd inflate during OSR, and then sync performance
54 // would subsequently plummet because the thread would be forced thru the slow-path).
55 // This problem has been made largely moot on IA32 by inlining the inflated fast-path
56 // operations in Fast_Lock and Fast_Unlock in i486.ad.
57 //
58 // Note that there is a way to safely swing the object's markword from
59 // one stack location to another. This avoids inflation. Obviously,
60 // we need to ensure that both locations refer to the current thread's stack.
61 // There are some subtle concurrency issues, however, and since the benefit is
62 // is small (given the support for inflated fast-path locking in the fast_lock, etc)
63 // we'll leave that optimization for another time.
64
65 if (displaced_header()->is_neutral()) {
66 ObjectSynchronizer::inflate_helper(obj);
67 // WARNING: We can not put check here, because the inflation
68 // will not update the displaced header. Once BasicLock is inflated,
69 // no one should ever look at its content.
70 } else {
71 // Typically the displaced header will be 0 (recursive stack lock) or
72 // unused_mark. Naively we'd like to assert that the displaced mark
73 // value is either 0, neutral, or 3. But with the advent of the
74 // store-before-CAS avoidance in fast_lock/compiler_lock_object
75 // we can find any flavor mark in the displaced mark.
76 }
77 dest->set_displaced_header(displaced_header());
78}
79