Simple 'why is this object alive' helpers

Based on what Rudi says he was doing (sorry rudi)

src/gc/collector.cpp

@@ -35,6 +35,8 @@
 //#undef VERBOSITY
 //#define VERBOSITY(x) 2
 
+#define DEBUG_MARK_PHASE 0
+
 #ifndef NDEBUG
 #define DEBUG 1
 #else
@@ -44,6 +46,13 @@
 namespace pyston {
 namespace gc {
 
+#if DEBUG_MARK_PHASE
+FILE* trace_fp;
+#define TRACE_LOG(...) fprintf(trace_fp, __VA_ARGS__)
+#else
+#define TRACE_LOG(...)
+#endif
+
 class TraceStack {
 private:
     const int CHUNK_SIZE = 256;
@@ -91,6 +100,7 @@ public:
     }
 
     void push(void* p) {
+        TRACE_LOG("Pushing %p\n", p);
         GCAllocation* al = GCAllocation::fromUserData(p);
         if (isMarked(al))
             return;
@@ -273,21 +283,31 @@ void markPhase() {
     VALGRIND_DISABLE_ERROR_REPORTING;
 #endif
 
+#if DEBUG_MARK_PHASE
+    trace_fp = fopen("gc_trace.txt", "w");
+#endif
+    TRACE_LOG("Starting collection\n");
+
+    TRACE_LOG("Looking at roots\n");
     TraceStack stack(roots);
     GCVisitor visitor(&stack);
 
+    TRACE_LOG("Looking at the stack\n");
     threading::visitAllStacks(&visitor);
 
+    TRACE_LOG("Looking at root handles\n");
     for (auto h : *getRootHandles()) {
         visitor.visit(h->value);
     }
 
+    TRACE_LOG("Looking at potential root ranges\n");
     for (auto& e : potential_root_ranges) {
         visitor.visitPotentialRange((void* const*)e.first, (void* const*)e.second);
     }
 
     // if (VERBOSITY()) printf("Found %d roots\n", stack.size());
     while (void* p = stack.pop()) {
+        TRACE_LOG("Looking at heap object %p\n", p);
         assert(((intptr_t)p) % 8 == 0);
         GCAllocation* al = GCAllocation::fromUserData(p);
 
@@ -336,6 +356,11 @@ void markPhase() {
         }
     }
 
+#if DEBUG_MARK_PHASE
+    fclose(trace_fp);
+    trace_fp = NULL;
+#endif
+
 #ifndef NVALGRIND
     VALGRIND_ENABLE_ERROR_REPORTING;
 #endif

tools/process_gc_trace.py

+import os
+import sys
+
+if __name__ == "__main__":
+    look_at = sys.argv[1]
+
+    why = {}
+    cur = None
+    for l in open("gc_trace.txt"):
+        if l.startswith("Pushing "):
+            ptr = l.split()[1]
+            why.setdefault(ptr, []).append(cur)
+        else:
+            cur = l
+
+    def investigate(ptr):
+        l = why.get(ptr)
+        if not l:
+            print "Not sure why %s is alive!" % ptr
+        else:
+            print ptr, "is alive from '%s'" % l[0].strip(),
+            if len(l) > 1:
+                print "and %d more entries" % (len(l) - 1)
+            else:
+                print
+            if l[0].startswith("Looking at heap object"):
+                prev_ptr = l[0].split()[-1]
+                investigate(prev_ptr)
+
+    investigate(look_at)