Code gen for SubArrayRef: handle empty subscript

x-ref: inducer/pymbolic#224

Author: inducer

loopy/symbolic.py

@@ -1072,6 +1072,8 @@ def _get_lower_bound(iname: str) -> int:
             for swept_iname in sar.swept_inames}
 
     result = EvaluatorWithDeficientContext(swept_inames_to_zeros)(sar.subscript)
+    # assert isinstance(result, p.Subscript)
+    # pending https://github.com/inducer/pymbolic/pull/224
     assert isinstance(result, (p.Variable, p.Subscript))
 
     return result

loopy/target/c/codegen/expression.py

@@ -232,8 +232,11 @@ def map_tagged_variable(self, expr, type_context):
     @override
     def map_sub_array_ref(self, expr, type_context):
         from loopy.symbolic import get_start_subscript_from_sar
-        return var("&")(self.rec(get_start_subscript_from_sar(expr, self.kernel),
-            type_context))
+        if not expr.subscript.index_tuple:
+            return var("&")(self.rec(expr.subscript.aggregate, type_context))
+        else:
+            return var("&")(self.rec(get_start_subscript_from_sar(expr, self.kernel),
+                type_context))
 
     @override
     def map_subscript(self, expr, type_context):