Notion of redundant visits to slowly move towards scanning everything

src/gc/collector.cpp

@@ -318,10 +318,6 @@ GCRootHandle::~GCRootHandle() {
     getRootHandles()->erase(this);
 }
 
-bool GCVisitor::isValid(void* p) {
-    return global_heap.getAllocationFromInteriorPointer(p) != NULL;
-}
-
 void GCVisitor::visit(void* p) {
     if ((uintptr_t)p < SMALL_ARENA_START || (uintptr_t)p >= HUGE_ARENA_START + ARENA_SIZE) {
         ASSERT(!p || isNonheapRoot(p), "%p", p);

src/gc/collector.h

@@ -15,6 +15,8 @@
 #ifndef PYSTON_GC_COLLECTOR_H
 #define PYSTON_GC_COLLECTOR_H
 
+#include "gc/gc.h"
+
 namespace pyston {
 
 class Box;
@@ -58,6 +60,16 @@ void invalidateOrderedFinalizerList();
 // assert rather than delaying of the next GC.
 void startGCUnexpectedRegion();
 void endGCUnexpectedRegion();
+
+class GCVisitorNoRedundancy : public GCVisitor {
+    virtual ~GCVisitorNoRedundancy() {}
+
+    virtual void visitRedundant(void* p) { visit(p); }
+    virtual void visitRangeRedundant(void* const* start, void* const* end) { visitRange(start, end); }
+    virtual void visitPotentialRedundant(void* p) { visitPotential(p); }
+    virtual void visitPotentialRangeRedundant(void* const* start, void* const* end) { visitPotentialRange(start, end); }
+    virtual bool shouldVisitRedundants() { return true; }
+};
 }
 }
 

src/gc/gc.h

@@ -16,10 +16,16 @@
 #define PYSTON_GC_GC_H
 
 #include <deque>
+#include <memory>
+#include <stddef.h>
 
 // Files outside of the gc/ folder should only import gc.h or gc_alloc.h
 // which are the "public" memory management interface.
 
+// Some code is only useful towards an effort to implement a
+// moving gc, gate behind this flag for now.
+#define MOVING_GC 0
+
 #define GC_KEEP_ALIVE(t) asm volatile("" : : "X"(t))
 
 #define TRACE_GC_MARKING 0
@@ -42,11 +48,11 @@ namespace gc {
 class TraceStack;
 class GCVisitor {
 private:
-    bool isValid(void* p);
+    TraceStack* stack;
 
 public:
-    TraceStack* stack;
     GCVisitor(TraceStack* stack) : stack(stack) {}
+    virtual ~GCVisitor() {}
 
     // These all work on *user* pointers, ie pointers to the user_data section of GCAllocations
     void visitIf(void* p) {
@@ -57,6 +63,20 @@ class GCVisitor {
     void visitRange(void* const* start, void* const* end);
     void visitPotential(void* p);
     void visitPotentialRange(void* const* start, void* const* end);
+
+    // Some object have fields with pointers to Pyston heap objects that we are confident are
+    // already being scanned elsewhere.
+    //
+    // In a mark-and-sweep collector, scanning those fields would be redundant because the mark
+    // phase only needs to visit each object once, so there would be a performance hit.
+    //
+    // In a moving collector, every reference needs to be visited since the pointer value could
+    // change. We don't have a moving collector yet, but it's good practice to call visit every
+    // pointer value and no-op to avoid the performance hit of the mark-and-sweep case.
+    virtual void visitRedundant(void* p) {}
+    virtual void visitRedundantRange(void** start, void** end) {}
+    virtual void visitPotentialRedundant(void* p) {}
+    virtual void visitPotentialRangeRedundant(void* const* start, void* const* end) {}
 };
 
 enum class GCKind : uint8_t {

src/runtime/hiddenclass.cpp

@@ -32,13 +32,23 @@ void HiddenClass::gc_visit(GCVisitor* visitor) {
     visitor->visitRange((void* const*)&children.vector()[0], (void* const*)&children.vector()[children.size()]);
     visitor->visit(attrwrapper_child);
 
-    // We don't need to visit the keys of the 'children' map, since the children should have those as entries
-    // in the attr_offssets map.
-    // Also, if we have any children, we can skip scanning our attr_offsets map, since it will be a subset
-    // of our child's map.
-    if (children.empty())
+    if (children.empty()) {
         for (auto p : attr_offsets)
             visitor->visit(p.first);
+    } else {
+#if MOVING_GC
+        // If we have any children, the attr_offsets map will be a subset of the child's map.
+        for (const auto& p : attr_offsets)
+            visitor->visitRedundant(p.first);
+#endif
+    }
+
+#if MOVING_GC
+    // The children should have the entries of the keys of the 'children' map in the attr_offsets map.
+    for (const auto& p : children) {
+        visitor->visitRedundant(p.first);
+    }
+#endif
 }
 
 void HiddenClass::appendAttribute(BoxedString* attr) {