1 /*
   2  * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "runtime/basicLock.hpp"
  27 #include "runtime/synchronizer.hpp"
  28 
  29 void 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 
  36 void 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 // [RGV] The next line appears to do nothing!
  78   intptr_t dh = (intptr_t) displaced_header();
  79   dest->set_displaced_header(displaced_header());
  80 }