Initial isolate reload support

runtime/vm/become.cc

+// Copyright (c) 2016, 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/become.h"
+
+#include "platform/assert.h"
+#include "platform/utils.h"
+
+#include "vm/dart_api_state.h"
+#include "vm/freelist.h"
+#include "vm/isolate_reload.h"
+#include "vm/object.h"
+#include "vm/raw_object.h"
+#include "vm/safepoint.h"
+#include "vm/timeline.h"
+#include "vm/visitor.h"
+
+namespace dart {
+
+DECLARE_FLAG(bool, trace_reload);
+
+// Free list elements are used as a marker for forwarding objects. This is
+// safe because we cannot reach free list elements from live objects. Ideally
+// forwarding objects would have their own class id. See TODO below.

[siva, 2016/05/27 23:48:00]

We should change this to have a becomes forwarding object instead of overloading
Free list elements like this.

+static bool IsForwardingObject(RawObject* object) {
+  return object->IsHeapObject() && object->IsFreeListElement();
+}
+
+
+static RawObject* GetForwardedObject(RawObject* object) {
+  ASSERT(IsForwardingObject(object));
+  uword addr = reinterpret_cast<uword>(object) - kHeapObjectTag;
+  FreeListElement* forwarder = reinterpret_cast<FreeListElement*>(addr);
+  RawObject* new_target = reinterpret_cast<RawObject*>(forwarder->next());
+  return new_target;
+}
+
+
+static void ForwardObjectTo(RawObject* before_obj, RawObject* after_obj) {
+  const intptr_t size_before = before_obj->Size();
+
+  // TODO(rmacnak): We should use different cids for forwarding corpses and
+  // free list elements.
+  uword corpse_addr = reinterpret_cast<uword>(before_obj) - kHeapObjectTag;
+  FreeListElement* forwarder = FreeListElement::AsElement(corpse_addr,
+                                                          size_before);
+  forwarder->set_next(reinterpret_cast<FreeListElement*>(after_obj));
+  if (!IsForwardingObject(before_obj)) {
+    FATAL("become: ForwardObjectTo failure.");
+  }
+  // Still need to be able to iterate over the forwarding corpse.
+  const intptr_t size_after = before_obj->Size();
+  if (size_before != size_after) {
+    FATAL("become: Before and after sizes do not match.");
+  }
+}
+
+
+class ForwardPointersVisitor : public ObjectPointerVisitor {
+ public:
+  explicit ForwardPointersVisitor(Isolate* isolate)
+      : ObjectPointerVisitor(isolate), visiting_object_(NULL), count_(0) { }
+
+  virtual void VisitPointers(RawObject** first, RawObject** last) {
+    for (RawObject** p = first; p <= last; p++) {
+      RawObject* old_target = *p;
+      if (IsForwardingObject(old_target)) {
+        RawObject* new_target = GetForwardedObject(old_target);
+        if (visiting_object_ == NULL) {
+          *p = new_target;
+        } else {
+          visiting_object_->StorePointer(p, new_target);
+        }
+        count_++;
+      }
+    }
+  }
+
+  void VisitingObject(RawObject* obj) { visiting_object_ = obj; }
+
+  intptr_t count() const { return count_; }
+
+ private:
+  RawObject* visiting_object_;
+  intptr_t count_;
+
+  DISALLOW_COPY_AND_ASSIGN(ForwardPointersVisitor);
+};
+
+
+class ForwardHeapPointersVisitor : public ObjectVisitor {
+ public:
+  explicit ForwardHeapPointersVisitor(ForwardPointersVisitor* pointer_visitor)
+      : pointer_visitor_(pointer_visitor) { }
+
+  virtual void VisitObject(RawObject* obj) {
+    pointer_visitor_->VisitingObject(obj);
+    obj->VisitPointers(pointer_visitor_);
+  }
+
+ private:
+  ForwardPointersVisitor* pointer_visitor_;
+
+  DISALLOW_COPY_AND_ASSIGN(ForwardHeapPointersVisitor);
+};
+
+
+class ForwardHeapPointersHandleVisitor : public HandleVisitor {
+ public:
+  ForwardHeapPointersHandleVisitor()
+      : HandleVisitor(Thread::Current()), count_(0) { }
+
+  virtual void VisitHandle(uword addr) {
+    FinalizablePersistentHandle* handle =
+        reinterpret_cast<FinalizablePersistentHandle*>(addr);
+    if (IsForwardingObject(handle->raw())) {
+      *handle->raw_addr() = GetForwardedObject(handle->raw());
+      count_++;
+    }
+  }
+
+  intptr_t count() const { return count_; }
+
+ private:
+  int count_;
+
+  DISALLOW_COPY_AND_ASSIGN(ForwardHeapPointersHandleVisitor);
+};
+
+
+#if defined(DEBUG)
+class NoFreeListTargetsVisitor : public ObjectPointerVisitor {
+ public:
+  explicit NoFreeListTargetsVisitor(Isolate* isolate)
+      : ObjectPointerVisitor(isolate) { }
+
+  virtual void VisitPointers(RawObject** first, RawObject** last) {
+    for (RawObject** p = first; p <= last; p++) {
+      RawObject* target = *p;
+      if (target->IsHeapObject()) {
+        ASSERT(!target->IsFreeListElement());
+      }
+    }
+  }
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(NoFreeListTargetsVisitor);
+};
+#endif
+
+
+void Become::ElementsForwardIdentity(const Array& before, const Array& after) {
+  Thread* thread = Thread::Current();
+  Isolate* isolate = thread->isolate();
+  Heap* heap = isolate->heap();
+
+  {
+    // TODO(rmacnak): Investigate why this is necessary.
+    heap->CollectGarbage(Heap::kNew);
+  }
+
+  TIMELINE_FUNCTION_GC_DURATION(thread, "Become::ElementsForwardIdentity");
+  HeapIterationScope his;
+
+#if defined(DEBUG)
+  {
+    // There should be no pointers to free list elements / forwarding corpses.
+    NoFreeListTargetsVisitor visitor(isolate);
+    isolate->VisitObjectPointers(&visitor, true);
+    heap->VisitObjectPointers(&visitor);
+  }
+#endif
+
+  // Setup forwarding pointers.
+  ASSERT(before.Length() == after.Length());
+  for (intptr_t i = 0; i < before.Length(); i++) {
+    RawObject* before_obj = before.At(i);
+    RawObject* after_obj = after.At(i);
+
+    if (before_obj == after_obj) {
+      FATAL("become: Cannot self-forward");
+    }
+    if (!before_obj->IsHeapObject()) {
+      FATAL("become: Cannot forward immediates");
+    }
+    if (!after_obj->IsHeapObject()) {
+      FATAL("become: Cannot become an immediates");
+    }
+    if (before_obj->IsVMHeapObject()) {
+      FATAL("become: Cannot forward VM heap objects");
+    }
+    if (after_obj->IsFreeListElement()) {
+      // The Smalltalk become does allow this, and for very special cases
+      // it is important (shape changes to Class or Mixin), but as these
+      // cases do not arise in Dart, better to prohibit it.
+      FATAL("become: No indirect chains of forwarding");
+    }
+
+    ForwardObjectTo(before_obj, after_obj);
+  }
+
+  {
+    // Follow forwarding pointers.
+
+    // C++ pointers
+    ForwardPointersVisitor pointer_visitor(isolate);
+    isolate->VisitObjectPointers(&pointer_visitor, true);
+
+    // Weak persistent handles.
+    ForwardHeapPointersHandleVisitor handle_visitor;
+    isolate->VisitWeakPersistentHandles(&handle_visitor);
+
+    //   Heap pointers (may require updating the remembered set)
+    ForwardHeapPointersVisitor object_visitor(&pointer_visitor);
+    heap->VisitObjects(&object_visitor);
+    pointer_visitor.VisitingObject(NULL);
+
+    TIR_Print("Performed %" Pd " heap and %" Pd " handle replacements\n",
+              pointer_visitor.count(),
+              handle_visitor.count());
+  }
+
+#if defined(DEBUG)
+  for (intptr_t i = 0; i < before.Length(); i++) {
+    ASSERT(before.At(i) == after.At(i));
+  }
+
+  {
+    // There should be no pointers to forwarding corpses.
+    NoFreeListTargetsVisitor visitor(isolate);
+    isolate->VisitObjectPointers(&visitor, true);
+    heap->VisitObjectPointers(&visitor);
+  }
+#endif
+}
+
+}  // namespace dart