rt: Introduce a self-describing box representation and functions to create and free them

src/rt/rust_box.h

@@ -0,0 +1,18 @@
+/* Rust box representation. */
+
+#ifndef RUST_BOX_H
+#define RUST_BOX_H
+
+#include "rust_internal.h"
+#include <stdint.h>
+
+struct rust_box {
+    RUST_REFCOUNTED(rust_box)
+    type_desc *tydesc;
+    rust_box *gc_next;
+    rust_box *gc_prev;
+    uint8_t data[0];
+};
+
+#endif
+

src/rt/rust_task.h

@@ -10,6 +10,8 @@
 #include "context.h"
 #include "rust_obstack.h"
 
+struct rust_box;
+
 struct stk_seg {
     unsigned int valgrind_id;
     uintptr_t limit;
@@ -57,7 +59,7 @@ rust_task : public kernel_owned<rust_task>, rust_cond
     context ctx;
     stk_seg *stk;
     uintptr_t runtime_sp;      // Runtime sp while task running.
-    void *gc_alloc_chain;      // Linked list of GC allocations.
+    rust_box *gc_alloc_chain;      // Linked list of GC allocations.
     rust_scheduler *sched;
     rust_crate_cache *cache;
 

src/rt/rust_upcall.cpp

@@ -1,3 +1,4 @@
+#include "rust_box.h"
 #include "rust_gc.h"
 #include "rust_internal.h"
 #include "rust_unwind.h"
@@ -76,6 +77,25 @@ upcall_malloc(rust_task *task, size_t nbytes, type_desc *td) {
     return (uintptr_t) p;
 }
 
+extern "C" CDECL rust_box *
+upcall_malloc_box(rust_task *task, size_t nbytes, type_desc *td) {
+    LOG_UPCALL_ENTRY(task);
+
+    gc::maybe_gc(task);
+
+    rust_box *box = reinterpret_cast<rust_box *>
+        (task->malloc(nbytes + sizeof(rust_box), "tdesc", td));
+    box->ref_count = 1;
+    box->tydesc = td;
+
+    box->gc_prev = NULL;
+    if ((box->gc_next = task->gc_alloc_chain) != NULL)
+        box->gc_next->gc_prev = box;
+    task->gc_alloc_chain = box;
+
+    return box;
+}
+
 /**
  * Called whenever an object's ref count drops to zero.
  */
@@ -90,6 +110,24 @@ upcall_free(rust_task *task, void* ptr, uintptr_t is_gc) {
     task->free(ptr, (bool) is_gc);
 }
 
+extern "C" CDECL void
+upcall_free_box(rust_task *task, rust_box *box) {
+    LOG_UPCALL_ENTRY(task);
+
+    assert(!box->ref_count && "Box reference count is nonzero on free!");
+
+    if (box->gc_prev)
+        box->gc_prev->gc_next = box->gc_next;
+    else
+        task->gc_alloc_chain = box->gc_next;
+    if (box->gc_next)
+        box->gc_next->gc_prev = box->gc_prev;
+
+    box->tydesc->drop_glue(NULL, task, (void *)box->tydesc,
+                           box->tydesc->first_param, box->data);
+    task->free(box, false);
+}
+
 extern "C" CDECL uintptr_t
 upcall_shared_malloc(rust_task *task, size_t nbytes, type_desc *td) {
     LOG_UPCALL_ENTRY(task);