diff -r cd52a7f89548 js/src/vm/TraceLogging.cpp
--- a/js/src/vm/TraceLogging.cpp	Thu Jul 17 14:10:32 2014 -0400
+++ b/js/src/vm/TraceLogging.cpp	Sat Nov 08 12:30:05 2014 +0100
@@ -106,7 +106,7 @@
 
     char treeFilename[sizeof TRACE_LOG_DIR "tl-tree.100.tl"];
     sprintf(treeFilename, TRACE_LOG_DIR "tl-tree.%d.tl", loggerId);
-    treeFile = fopen(treeFilename, "wb");
+    treeFile = fopen(treeFilename, "wb+");
     if (!treeFile) {
         fclose(dictFile);
         dictFile = nullptr;
@@ -280,7 +280,7 @@
         if (bytesWritten < tree.size())
             return false;
 
-        treeOffset += tree.currentId();
+        treeOffset += tree.nextId();
         tree.clear();
     }
 
@@ -466,7 +466,7 @@
 {
     // Entry is still in memory
     if (treeId >= treeOffset) {
-        *entry = tree[treeId];
+        *entry = tree[treeId - treeOffset];
         return true;
     }
 
@@ -570,8 +570,8 @@
         stackEntry.setActive(false);
         return;
     }
-
-    if (!tree.ensureSpaceBeforeAdd()) {
+ 
+    if (tree.sizeBytes() > 100 * 1024 * 1024 || !tree.ensureSpaceBeforeAdd()) {
         uint64_t start = rdtsc() - traceLoggers.startupTime;
         if (!flush()) {
             fprintf(stderr, "TraceLogging: Couldn't write the data to disk.\n");
@@ -633,8 +633,8 @@
 #endif
 
     if (parent.lastChildId() == 0) {
-        MOZ_ASSERT(!entry.hasChildren());
-        MOZ_ASSERT(parent.treeId() == tree.currentId() + treeOffset);
+        MOZ_ASSERT_IF(tree.nextId() > 0, parent.treeId() == tree.currentId() + treeOffset); 
+        MOZ_ASSERT_IF(tree.nextId() == 0, parent.treeId() == treeOffset - 1);
 
         if (!updateHasChildren(parent.treeId()))
             return false;
diff -r cd52a7f89548 js/src/vm/TraceLogging.h
--- a/js/src/vm/TraceLogging.h	Thu Jul 17 14:10:32 2014 -0400
+++ b/js/src/vm/TraceLogging.h	Sat Nov 08 12:30:05 2014 +0100
@@ -182,6 +182,10 @@
         return next_;
     }
 
+    uint32_t sizeBytes() {
+        return next_ * sizeof(T);
+    }
+
     uint32_t nextId() {
         return next_;
     }