Oilpan: GC_TRACING: Improve object path dump

Source/platform/heap/Heap.cpp

Index: Source/platform/heap/Heap.cpp
diff --git a/Source/platform/heap/Heap.cpp b/Source/platform/heap/Heap.cpp
index 5d6ff063e6de245e9a040cce3ef058e9371e1213..72db0042679745f777c1af6f30db7bd97e194e8e 100644
--- a/Source/platform/heap/Heap.cpp
+++ b/Source/platform/heap/Heap.cpp
@@ -1472,10 +1472,12 @@ public:
 
     static void dumpPathToObjectFromObjectGraph(const ObjectGraph& graph, uintptr_t target)
     {
-        fprintf(stderr, "Path to %lx of %s\n", target, classOf(reinterpret_cast<const void*>(target)).ascii().data());
         ObjectGraph::const_iterator it = graph.find(target);
+        if (it == graph.end())
+            return;
+        fprintf(stderr, "Path to %lx of %s\n", target, classOf(reinterpret_cast<const void*>(target)).ascii().data());
         while (it != graph.end()) {
-            fprintf(stderr, "<- %lx of %s\n", it->value.first, it->value.second.ascii().data());
+            fprintf(stderr, "<- %lx of %s\n", it->value.first, it->value.second.utf8().data());
             it = graph.find(it->value.first);
         }
         fprintf(stderr, "\n");
@@ -1607,6 +1609,37 @@ void Heap::dumpPathToObjectOnNextGC(void* p)
 {
     static_cast<MarkingVisitor*>(s_markingVisitor)->dumpPathToObjectOnNextGC(p);
 }
+
+String Heap::createBacktraceString()
+{
+    // Skip the followings:
+    // - WTFGetBacktrace
+    // - Heap::createBacktraceString
+    // - Persistent::recordBacktrace
+    // - Persistent::Persistent
+    const int framesToSkip = 4;

[haraken, 2014/06/16 01:15:40]

It would be better if we could determine this number dynamically instead of
hard-coding; i.e., find "Persistent::Persistent" and skip the frames below the
"Persistent::Persistent".

[tkent, 2014/06/16 02:34:47]

Done.

+    const int framesToShow = 4;
+    typedef void* FramePointer;
+    FramePointer* stackFrame = static_cast<FramePointer*>(alloca((framesToShow + framesToSkip) * sizeof(FramePointer)));
+    int stackFrameSize = framesToShow + framesToSkip;
+    WTFGetBacktrace(stackFrame, &stackFrameSize);
+
+    StringBuilder builder;
+    builder.append("Persistent");
+    bool gotFirstName = false;
+    for (int i = framesToSkip; i < stackFrameSize; ++i) {
+        FrameToNameScope frameToName(stackFrame[i]);
+        if (!frameToName.nullableName())
+            continue;
+        if (!gotFirstName) {
+            gotFirstName = true;
+            builder.append(" ... Backtrace:");
+        }
+        builder.append("\n\t");
+        builder.append(frameToName.nullableName());
+    }
+    return builder.toString().replace("WebCore::", "");
+}
 #endif
 
 void Heap::pushTraceCallback(void* object, TraceCallback callback)