Fix concurrent sweeping in predictable mode and bring --concurrent-sweeping flag back.

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 452 matching lines @@
       new SweeperTask(heap(), heap()->old_pointer_space()),
       v8::Platform::kShortRunningTask);
 }
 
 
 void MarkCompactCollector::EnsureSweepingCompleted() {
   DCHECK(sweeping_in_progress_ == true);
 
   // If sweeping is not completed or not running at all, we try to complete it
   // here.
-  if (FLAG_predictable || !IsSweepingCompleted()) {
+  if (!FLAG_concurrent_sweeping || !IsSweepingCompleted()) {
     SweepInParallel(heap()->paged_space(OLD_DATA_SPACE), 0);
     SweepInParallel(heap()->paged_space(OLD_POINTER_SPACE), 0);
   }
   // Wait twice for both jobs.
-  if (!FLAG_predictable) {
+  if (FLAG_concurrent_sweeping) {
     pending_sweeper_jobs_semaphore_.Wait();
     pending_sweeper_jobs_semaphore_.Wait();
   }
   ParallelSweepSpacesComplete();
   sweeping_in_progress_ = false;
   RefillFreeList(heap()->paged_space(OLD_DATA_SPACE));
   RefillFreeList(heap()->paged_space(OLD_POINTER_SPACE));
   heap()->paged_space(OLD_DATA_SPACE)->ResetUnsweptFreeBytes();
   heap()->paged_space(OLD_POINTER_SPACE)->ResetUnsweptFreeBytes();
 
@@ skipping 3649 matching lines @@
   // non-live objects.
   {
     GCTracer::Scope sweep_scope(heap()->tracer(),
                                 GCTracer::Scope::MC_SWEEP_OLDSPACE);
     {
       SequentialSweepingScope scope(this);
       SweepSpace(heap()->old_pointer_space(), CONCURRENT_SWEEPING);
       SweepSpace(heap()->old_data_space(), CONCURRENT_SWEEPING);
     }
     sweeping_in_progress_ = true;
-    if (!FLAG_predictable) {
+    if (FLAG_concurrent_sweeping) {
       StartSweeperThreads();
     }
   }
   RemoveDeadInvalidatedCode();
 
   {
     GCTracer::Scope sweep_scope(heap()->tracer(),
                                 GCTracer::Scope::MC_SWEEP_CODE);
     SweepSpace(heap()->code_space(), SEQUENTIAL_SWEEPING);
   }
@@ skipping 248 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }
 }  // namespace v8::internal