[heap] Add --trace-evacuation-candidates flag

This flag will be useful to analyze (and then improve) fragmentation
and compaction.

Bug: v8:9329
Change-Id: Ie3c5b618fa5717277cf2b9617e2d8f9dc2138056
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1664333Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@google.com>
Cr-Commit-Position: refs/heads/master@{#62285}

src/flags/flag-definitions.h

@@ -776,6 +776,8 @@ DEFINE_BOOL(trace_gc_freelists_verbose, false,
             "prints details of freelists of each page before and after "
             "each major garbage collection")
 DEFINE_IMPLICATION(trace_gc_freelists_verbose, trace_gc_freelists)
+DEFINE_BOOL(trace_evacuation_candidates, false,
+            "Show statistics about the pages evacuation by the compaction")
 
 DEFINE_INT(trace_allocation_stack_interval, -1,
            "print stack trace after <n> free-list allocations")

src/heap/mark-compact.cc

@@ -456,6 +456,14 @@ void MarkCompactCollector::TearDown() {
 
 void MarkCompactCollector::AddEvacuationCandidate(Page* p) {
   DCHECK(!p->NeverEvacuate());
+
+  if (FLAG_trace_evacuation_candidates) {
+    PrintIsolate(
+        isolate(),
+        "Evacuation candidate: Free bytes: %6zu. Free Lists length: %4d.\n",
+        p->area_size() - p->allocated_bytes(), p->FreeListsLength());
+  }
+
   p->MarkEvacuationCandidate();
   evacuation_candidates_.push_back(p);
 }