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1878
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1 /*
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2 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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20 * or visit www.oracle.com if you need additional information or have any
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21 * questions.
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22 *
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23 */
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24
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1972
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25 #include "precompiled.hpp"
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26 #include "runtime/basicLock.hpp"
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27 #include "runtime/synchronizer.hpp"
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1878
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28
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29 void BasicLock::print_on(outputStream* st) const {
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30 st->print("monitor");
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31 }
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32
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33 void BasicLock::move_to(oop obj, BasicLock* dest) {
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34 // Check to see if we need to inflate the lock. This is only needed
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35 // if an object is locked using "this" lightweight monitor. In that
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36 // case, the displaced_header() is unlocked, because the
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37 // displaced_header() contains the header for the originally unlocked
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38 // object. However the object could have already been inflated. But it
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39 // does not matter, the inflation will just a no-op. For other cases,
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40 // the displaced header will be either 0x0 or 0x3, which are location
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41 // independent, therefore the BasicLock is free to move.
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42 //
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43 // During OSR we may need to relocate a BasicLock (which contains a
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44 // displaced word) from a location in an interpreter frame to a
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45 // new location in a compiled frame. "this" refers to the source
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46 // basiclock in the interpreter frame. "dest" refers to the destination
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47 // basiclock in the new compiled frame. We *always* inflate in move_to().
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48 // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
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49 // cause performance problems in code that makes heavy use of a small # of
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50 // uncontended locks. (We'd inflate during OSR, and then sync performance
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51 // would subsequently plummet because the thread would be forced thru the slow-path).
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52 // This problem has been made largely moot on IA32 by inlining the inflated fast-path
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53 // operations in Fast_Lock and Fast_Unlock in i486.ad.
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54 //
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55 // Note that there is a way to safely swing the object's markword from
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56 // one stack location to another. This avoids inflation. Obviously,
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57 // we need to ensure that both locations refer to the current thread's stack.
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58 // There are some subtle concurrency issues, however, and since the benefit is
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59 // is small (given the support for inflated fast-path locking in the fast_lock, etc)
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60 // we'll leave that optimization for another time.
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61
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62 if (displaced_header()->is_neutral()) {
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63 ObjectSynchronizer::inflate_helper(obj);
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64 // WARNING: We can not put check here, because the inflation
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65 // will not update the displaced header. Once BasicLock is inflated,
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66 // no one should ever look at its content.
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67 } else {
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68 // Typically the displaced header will be 0 (recursive stack lock) or
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69 // unused_mark. Naively we'd like to assert that the displaced mark
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70 // value is either 0, neutral, or 3. But with the advent of the
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71 // store-before-CAS avoidance in fast_lock/compiler_lock_object
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72 // we can find any flavor mark in the displaced mark.
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73 }
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74 // [RGV] The next line appears to do nothing!
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75 intptr_t dh = (intptr_t) displaced_header();
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76 dest->set_displaced_header(displaced_header());
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77 }
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