Trace pinned objects as roots (mmtk#91)

Following the suggestion in mmtk#89 (comment), trace recently added pinned objects as roots. When we pass those objects as 'nodes' to MMTk, MMTk does not know the slots so MMTk cannot update the reference, thus MMTk will not move those objects. This is essentially the same as pinning those objects before a GC, and unpinning after a GC.

Author: qinsoon

src/ast.c

@@ -797,6 +797,9 @@ static value_t julia_to_list2_noalloc(fl_context_t *fl_ctx, jl_value_t *a, jl_va
 
 static value_t julia_to_scm_(jl_ast_context_t *ctx, jl_value_t *v, int check_valid)
 {
+    // The following code will take internal pointers to v's fields. We need to make sure
+    // that v will not be moved by GC.
+    arraylist_push(&ctx->ast_roots, v);
     value_t retval;
     fl_context_t *fl_ctx = &ctx->fl;
     if (julia_to_scm_noalloc1(fl_ctx, v, &retval))