There are no evacuation candidates and slot buffers when marking is started.

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 705 matching lines @@
       Min(kMaxMaxEvacuationCandidates, max_evacuation_candidates);
 
   int count = 0;
   int fragmentation = 0;
   Candidate* least = NULL;
 
   PageIterator it(space);
   while (it.has_next()) {
     Page* p = it.next();
     if (p->NeverEvacuate()) continue;
-    p->ClearEvacuationCandidate();
+
+    // Invariant: Evacuation candidates are just created when marking is
+    // started. At the end of a GC all evacuation candidates are cleared and
+    // their slot buffers are released.
+    CHECK(!p->IsEvacuationCandidate());
+    CHECK(p->slots_buffer() == NULL);
 
     if (FLAG_stress_compaction) {
       unsigned int counter = space->heap()->ms_count();
       uintptr_t page_number = reinterpret_cast<uintptr_t>(p) >> kPageSizeBits;
       if ((counter & 1) == (page_number & 1)) fragmentation = 1;
     } else if (mode == REDUCE_MEMORY_FOOTPRINT) {
       // Don't try to release too many pages.
       if (estimated_release >= over_reserved) {
         continue;
       }
@@ skipping 3681 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