[heap] Increment MarkCompact epoch right after marking.

Bug: v8:9093
Change-Id: I69aa3edddf5ea52e1cca185fa03f51122cdf02c5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1617937
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61668}

src/heap/mark-compact.cc

@@ -853,8 +853,6 @@ void MarkCompactCollector::VerifyMarking() {
 void MarkCompactCollector::Finish() {
   TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_FINISH);
 
-  epoch_++;
-
 #ifdef DEBUG
   heap()->VerifyCountersBeforeConcurrentSweeping();
 #endif
@@ -1924,6 +1922,8 @@ void MarkCompactCollector::MarkLiveObjects() {
   if (was_marked_incrementally_) {
     heap()->incremental_marking()->Deactivate();
   }
+
+  epoch_++;
 }
 
 void MarkCompactCollector::ClearNonLiveReferences() {

src/heap/mark-compact.h

@@ -909,9 +909,11 @@ class MarkCompactCollector final : public MarkCompactCollectorBase {
   MarkingState marking_state_;
   NonAtomicMarkingState non_atomic_marking_state_;
 
-  // Counts the number of mark-compact collections. This is used for marking
-  // descriptor arrays. See NumberOfMarkedDescriptors. Only lower two bits are
-  // used, so it is okay if this counter overflows and wraps around.
+  // Counts the number of major mark-compact collections. The counter is
+  // incremented right after marking. This is used for:
+  // - marking descriptor arrays. See NumberOfMarkedDescriptors. Only the lower
+  //   two bits are used, so it is okay if this counter overflows and wraps
+  //   around.
   unsigned epoch_ = 0;
 
   friend class FullEvacuator;