Implement global identifiers.

stepper.py

@@ -317,7 +317,13 @@ def eval_oper(operand, ssa_env, global_env):
     elif isinstance(operand, ll.Const):
         return operand.val
     elif isinstance(operand, ll.Gid):
-        TODO('eval_oper Gid')
+        gid = operand.val
+        try:
+            return global_env[gid]
+        except KeyError:
+            err('Unable to find @{} in environment:\n{}'
+                .format(global_env))
+            return 0
     elif isinstance(operand, ll.Id):
         id = operand.val
         try:
@@ -325,6 +331,7 @@ def eval_oper(operand, ssa_env, global_env):
         except KeyError:
             err('Unable to find %{} in environment:\n{}'
                 .format(id, ssa_env))
+            return 0
 
 
 def eval_binop(bop, left, right):
@@ -464,6 +471,7 @@ def base_ty2size(base_ty):
             .format(ll.ty2s(base_ty)))
         return 1
 
+
 def alloc_globals(gdecls, heap):
     def alloc_global(ginit):
         next_idx = len(heap)