Introduce infrastructure for tracing ScriptWrappables.

third_party/WebKit/Source/bindings/core/v8/ScriptWrappableHeapTracer.cpp

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ScriptWrappableHeapTracer.h"
+
+#include "bindings/core/v8/ActiveScriptWrappable.h"
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "bindings/core/v8/ScriptWrappableVisitor.h"
+#include "bindings/core/v8/WrapperTypeInfo.h"
+#include "platform/heap/HeapPage.h"
+
+namespace blink {
+
+ScriptWrappableHeapTracer::~ScriptWrappableHeapTracer()
+{
+}
+
+void ScriptWrappableHeapTracer::TraceRoots(v8::Isolate* isolate)
+{
+    m_tracingInProgress = true;
+    ScriptWrappableVisitor visitor(isolate, this);
+    ActiveScriptWrappable::traceActiveWrappable(&visitor);
+}
+
+void ScriptWrappableHeapTracer::AddHeaderToUnmark(HeapObjectHeader* header)
+{
+    m_headersToUnmark.push_back(header);
+}
+
+void ScriptWrappableHeapTracer::ClearTracingMarks(v8::Isolate* isolate)
+{
+    for (auto header : m_headersToUnmark)
+        header->unmarkDOM();
+
+    m_headersToUnmark.clear();
+    m_tracingInProgress = false;
+}
+
+void ScriptWrappableHeapTracer::TraceWrappableFrom(v8::Isolate* isolate, const std::vector<std::pair<void*, void*>>& hiddenFieldsOfPotentialWrappers)

[haraken, 2016/04/13 12:26:22]

Would you help me understand what this method does?

[Marcel Hlopko, 2016/04/14 16:39:08]

When marking, V8 detects all (possible) wrappers and collects their first two
internal fields. That is enough for blink side to reconstruct script wrappable,
and simple for V8. Had it been a vector of Objects, V8 would had to update
references when it moves objects.

+{
+    ASSERT(m_tracingInProgress);
+    for (auto pair : hiddenFieldsOfPotentialWrappers) {
+        TraceWrappableFrom(isolate, pair);
+    }
+}
+
+
+void ScriptWrappableHeapTracer::TraceWrappableFrom(v8::Isolate* isolate, std::pair<void*, void*> hiddenFields)

[haraken, 2016/04/13 12:26:22]

What is the hidden fields?

[Marcel Hlopko, 2016/04/14 16:39:08]

Renamed to internal fields to be consistent with the v8 api - fields set by the
embedder, not accessible from javascript, appearring as tagged integers to the
V8 gc. It's all we need to reconstruct script wrappables.

+{
+    if (!hiddenFields.first)
+        return;
+    if (reinterpret_cast<intptr_t>(hiddenFields.first) % 2 == 1)

[haraken, 2016/04/13 12:26:22]

What is this checking?

[Marcel Hlopko, 2016/04/14 16:39:08]

That the pointer we got is a valid oilpan pointer, and not tagged pointer to the
V8 object. Is ASSERT_NOT_REACHED correctly used here?

+        ASSERT_NOT_REACHED();
+    if (reinterpret_cast<WrapperTypeInfo*>(hiddenFields.first)->ginEmbedder
+        != gin::GinEmbedder::kEmbedderBlink)
+        return;
+
+    ScriptWrappable* scriptWrappable = reinterpret_cast<ScriptWrappable*>(
+        hiddenFields.second);
+    uint16_t classId = scriptWrappable->wrapperTypeInfo()->wrapperClassId;
+    if (classId != WrapperTypeInfo::NodeClassId
+        && classId != WrapperTypeInfo::ObjectClassId) {
+        ASSERT_NOT_REACHED();
+        return;
+    }
+
+    ScriptWrappableVisitor visitor(isolate, this);
+    visitor.trace(scriptWrappable);
+}
+
+void ScriptWrappableHeapTracer::TraceWrappableFrom(v8::Isolate* isolate, v8::Persistent<v8::Object>* handle)
+{
+    ScriptWrappable* scriptWrappable = toScriptWrappable(*handle);
+    uint16_t classId = scriptWrappable->wrapperTypeInfo()->wrapperClassId;
+    if (classId != WrapperTypeInfo::NodeClassId
+        && classId != WrapperTypeInfo::ObjectClassId) {
+        ASSERT_NOT_REACHED();
+        return;
+    }
+
+    ScriptWrappableVisitor visitor(isolate, this);
+    visitor.trace(scriptWrappable);
+}
+
+}
+