Report the reason for a garbage collection when verbose GC is enabled.

runtime/vm/scavenger.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/scavenger.h"
 
 #include <map>
 #include <utility>
 
 #include "vm/dart.h"
@@ skipping 512 matching lines @@
 void Scavenger::VisitObjects(ObjectVisitor* visitor) const {
   uword cur = FirstObjectStart();
   while (cur < top_) {
     RawObject* raw_obj = RawObject::FromAddr(cur);
     visitor->VisitObject(raw_obj);
     cur += raw_obj->Size();
   }
 }
 
 
-void Scavenger::Scavenge() {
+void Scavenger::Scavenge(const char* gc_reason) {
   // TODO(cshapiro): Add a decision procedure for determining when the
   // the API callbacks should be invoked.
-  Scavenge(false);
+  Scavenge(false, gc_reason);
 }
 
 
-void Scavenger::Scavenge(bool invoke_api_callbacks) {
+void Scavenger::Scavenge(bool invoke_api_callbacks, const char* gc_reason) {
   // Scavenging is not reentrant. Make sure that is the case.
   ASSERT(!scavenging_);
   scavenging_ = true;
   had_promotion_failure_ = false;
   Isolate* isolate = Isolate::Current();
   NoHandleScope no_handles(isolate);
 
   if (FLAG_verify_before_gc) {
     OS::PrintErr("Verifying before Scavenge...");
     heap_->Verify();
     OS::PrintErr(" done.\n");
   }
 
   if (FLAG_verbose_gc) {
-    OS::PrintErr("Start scavenge\n");
+    OS::PrintErr("Start scavenge for %s collection\n", gc_reason);
   }
   Timer timer(FLAG_verbose_gc, "Scavenge");
   timer.Start();
   // Setup the visitor and run a scavenge.
   ScavengerVisitor visitor(isolate, this);
   Prologue(isolate, invoke_api_callbacks);
   IterateRoots(isolate, &visitor, !invoke_api_callbacks);
   ProcessToSpace(&visitor);
   IterateWeakReferences(isolate, &visitor);
   ScavengerWeakVisitor weak_visitor(this);
@@ skipping 17 matching lines @@
   scavenging_ = false;
 }
 
 
 void Scavenger::WriteProtect(bool read_only) {
   space_->Protect(
       read_only ? VirtualMemory::kReadOnly : VirtualMemory::kReadWrite);
 }
 
 }  // namespace dart