Fix add_subexpressions_for_field_reads

src/pystencils/simp/simplifications.py

@@ -190,7 +190,7 @@ def add_subexpressions_for_field_reads(ac, subexpressions=True, main_assignments
             field_reads.update(assignment.rhs.atoms(Field.Access))
 
     if not field_reads:
-        return
+        return ac
 
     substitutions = dict()
     for fa in field_reads:

tests/test_simplifications.py

@@ -146,6 +146,16 @@ def test_add_subexpressions_for_field_reads():
     assert isinstance(ac3.subexpressions[0].lhs, TypedSymbol)
     assert ac3.subexpressions[0].lhs.dtype == BasicType("float32")
 
+    # added check for early out of add_subexpressions_for_field_reads is no fields appear on the rhs (See #92)
+    main = [Assignment(s[0, 0](0), 3.0),
+            Assignment(s[0, 0](1), 4.0)]
+
+    ac4 = AssignmentCollection(main, subexpressions)
+    assert len(ac4.subexpressions) == 0
+    ac5 = add_subexpressions_for_field_reads(ac4)
+    assert ac5 is not None
+    assert ac4 is ac5
+
 
 @pytest.mark.parametrize('target', (ps.Target.CPU, ps.Target.GPU))
 @pytest.mark.parametrize('dtype', ('float32', 'float64'))