xo-interpreter: + toplevel env in VSM

xo-alloc/include/xo/alloc/GC.hpp

@@ -7,6 +7,7 @@
 
 #include "ArenaAlloc.hpp"
 #include "GcStatistics.hpp"
+#include "Object.hpp"
 #include "xo/callback/UpCallbackSet.hpp"
 #include "xo/indentlog/print/array.hpp"
 #include <vector>
@@ -154,6 +155,9 @@ namespace xo {
              **/
             static up<GC> make(const Config & config);
 
+            /** runtime downcast **/
+            static GC * from(IAlloc * mm);
+
             const Config & config() const { return config_; }
             std::uint8_t nursery_polarity() const { return nursery_polarity_; }
             std::uint8_t tenured_polarity() const { return tenured_polarity_; }
@@ -230,6 +234,15 @@ namespace xo {
              *  from @c *addr
              **/
             void add_gc_root(Object ** addr);
+            /** reverse the effect of previous call to @ref add_gc_root **/
+            void remove_gc_root(Object ** addr);
+
+            /** convenience wrapper **/
+            template <typename T>
+            void add_gc_root_dwim(gp<T> * p) { this->add_gc_root(reinterpret_cast<Object**>(p->ptr_address())); }
+            template <typename T>
+            void remove_gc_root_dwim(gp<T> * p) { this->remove_gc_root(reinterpret_cast<Object**>(p->ptr_address())); }
+
             /** may optionally use this to observe GC copy phase.
              *  Will be invoked once _per surviving object_, so not cheap.
              *  Intended for GC visualization.

xo-alloc/src/alloc/GC.cpp

@@ -132,11 +132,15 @@ namespace xo {
         up<GC>
         GC::make(const Config & config)
         {
-            //GC * gc = new GC(config);
-
             return std::make_unique<GC>(config);
         }
 
+        GC *
+        GC::from(IAlloc * mm)
+        {
+            return dynamic_cast<GC *>(mm);
+        }
+
         const std::string &
         GC::name() const
         {
@@ -390,6 +394,19 @@ namespace xo {
             gc_root_v_.push_back(addr);
         }
 
+        void
+        GC::remove_gc_root(Object ** addr)
+        {
+            /* Multithreaded GC not supported */
+
+            assert(!this->gc_in_progress());
+
+            auto new_end_ix = std::remove(gc_root_v_.begin(), gc_root_v_.end(), addr);
+
+            /* erase now-unused slots */
+            gc_root_v_.erase(new_end_ix, gc_root_v_.end());
+        }
+
         auto
         GC::add_gc_copy_callback(up<GcCopyCallback> fn) -> CallbackId
         {