HeapCompact doesn't need to support multiple threads

Now that the per-thread heaps have been shipped, HeapCompact no longer needs to
support multiple threads. This CL removes that code.

BUG=671856

Review-Url: https://codereview.chromium.org/2633463006
Cr-Commit-Position: refs/heads/master@{#443542}

third_party/WebKit/Source/platform/heap/HeapCompact.cpp

@@ -253,7 +253,6 @@ class HeapCompact::MovableObjectFixups final {
 HeapCompact::HeapCompact()
     : m_doCompact(false),
       m_gcCountSinceLastCompaction(0),
-      m_threadCount(0),
       m_freeListSize(0),
       m_compactableArenas(0u),
       m_freedPages(0),
@@ -346,7 +345,6 @@ void HeapCompact::initialize(ThreadState* state) {
   m_doCompact = true;
   m_freedPages = 0;
   m_freedSize = 0;
-  m_threadCount = state->heap().threads().size();
   m_fixups.reset();
   m_gcCountSinceLastCompaction = 0;
   s_forceCompactionGC = false;
@@ -418,7 +416,6 @@ void HeapCompact::startThreadCompaction() {
   if (!m_doCompact)
     return;
 
-  MutexLocker locker(m_mutex);
   if (!m_startCompactionTimeMS)
     m_startCompactionTimeMS = WTF::currentTimeMS();
 }
@@ -427,9 +424,6 @@ void HeapCompact::finishThreadCompaction() {
   if (!m_doCompact)
     return;
 
-  MutexLocker locker(m_mutex);
-  // Final one clears out.
-  if (!--m_threadCount) {
 #if DEBUG_HEAP_COMPACTION
   if (m_fixups)
     m_fixups->dumpDebugStats();
@@ -437,8 +431,7 @@ void HeapCompact::finishThreadCompaction() {
   m_fixups.reset();
   m_doCompact = false;
 
-    double timeForHeapCompaction =
-        WTF::currentTimeMS() - m_startCompactionTimeMS;
+  double timeForHeapCompaction = WTF::currentTimeMS() - m_startCompactionTimeMS;
   DEFINE_STATIC_LOCAL(CustomCountHistogram, timeForHeapCompactionHistogram,
                       ("BlinkGC.TimeForHeapCompaction", 1, 10 * 1000, 50));
   timeForHeapCompactionHistogram.count(timeForHeapCompaction);
@@ -457,18 +450,6 @@ void HeapCompact::finishThreadCompaction() {
   LOG_HEAP_COMPACTION("Compaction stats: freed pages=%zu size=%zu\n",
                       m_freedPages, m_freedSize);
 #endif
-
-    // Compaction has been completed by all participating threads, unblock
-    // them all.
-    m_finished.broadcast();
-  } else {
-    // Letting a thread return here and let it exit its safe point opens up
-    // the possibility of it accessing heaps of other threads that are
-    // still being compacted. It is not in a valid state until objects have
-    // all been moved together, hence all GC-participating threads must
-    // complete compaction together. Grab the condition variable and wait.
-    m_finished.wait(m_mutex);
-  }
 }
 
 void HeapCompact::addCompactingPage(BasePage* page) {

third_party/WebKit/Source/platform/heap/HeapCompact.h

@@ -139,18 +139,6 @@ class PLATFORM_EXPORT HeapCompact final {
   bool m_doCompact;
   size_t m_gcCountSinceLastCompaction;
 
-  // Lock protecting finishedThreadCompaction() signalling.
-  Mutex m_mutex;
-
-  // All threads performing a GC must synchronize on completion
-  // of all heap compactions. Not doing so risks one thread resuming
-  // the mutator, which could perform cross-thread access to a heap
-  // that's still in the process of being compacted.
-  ThreadCondition m_finished;
-
-  // Number of heap threads participating in the compaction.
-  int m_threadCount;
-
   // Last reported freelist size, across all compactable arenas.
   size_t m_freeListSize;