Introduced gc flag for fuzzing over incremental marking limit.

Bug: v8:6972
Change-Id: I9e341f980ca392920e0789121707e25916981265
Reviewed-on: https://chromium-review.googlesource.com/730383Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michał Majewski <majeski@google.com>
Cr-Commit-Position: refs/heads/master@{#48886}

src/flag-definitions.h

@@ -674,6 +674,8 @@ DEFINE_BOOL(stress_compaction, false,
             "--force_marking_deque_overflows)")
 DEFINE_BOOL(stress_incremental_marking, false,
             "force incremental marking for small heaps and run it more often")
+DEFINE_INT(stress_incremental_marking_percentage, 0,
+           "force incremental marking after X percent of the standard limit")
 DEFINE_BOOL(manual_evacuation_candidates_selection, false,
             "Test mode only flag. It allows an unit test to select evacuation "
             "candidates pages (requires --stress_compaction).")

src/heap/heap.cc

@@ -5299,7 +5299,31 @@ Heap::IncrementalMarkingLimit Heap::IncrementalMarkingLimitReached() {
     // start marking immediately.
     return IncrementalMarkingLimit::kHardLimit;
   }
+
+  if (FLAG_stress_incremental_marking_percentage > 0) {
+    double gained_since_last_gc =
+        PromotedSinceLastGC() +
+        (external_memory_ - external_memory_at_last_mark_compact_);
+    double size_before_gc = PromotedTotalSize() - gained_since_last_gc;
+    double bytes_to_limit = old_generation_allocation_limit_ - size_before_gc;
+    if (bytes_to_limit > 0) {
+      double current_percent = (gained_since_last_gc / bytes_to_limit) * 100.0;
+
+      if (FLAG_trace_incremental_marking) {
+        isolate()->PrintWithTimestamp(
+            "[IncrementalMarking] %.2lf%% of the memory limit reached\n",
+            current_percent);
+      }
+
+      if (static_cast<int>(current_percent) >=
+          FLAG_stress_incremental_marking_percentage) {
+        return IncrementalMarkingLimit::kHardLimit;
+      }
+    }
+  }
+
   size_t old_generation_space_available = OldGenerationSpaceAvailable();
+
   if (old_generation_space_available > new_space_->Capacity()) {
     return IncrementalMarkingLimit::kNoLimit;
   }