Fix for LB collide only step

- sweep updates pdf field in place, but does not load all the values
  before overwriting them
-> added a new transformation on AssignmentCollection that loads all
   read values first

simp/__init__.py

@@ -2,9 +2,10 @@ from .assignment_collection import AssignmentCollection
 from .simplificationstrategy import SimplificationStrategy
 from .simplifications import sympy_cse, sympy_cse_on_assignment_list, \
     apply_to_all_assignments, apply_on_all_subexpressions, subexpression_substitution_in_existing_subexpressions, \
-    subexpression_substitution_in_main_assignments, add_subexpressions_for_divisions
+    subexpression_substitution_in_main_assignments, add_subexpressions_for_divisions, add_subexpressions_for_field_reads
 
 __all__ = ['AssignmentCollection', 'SimplificationStrategy',
            'sympy_cse', 'sympy_cse_on_assignment_list', 'apply_to_all_assignments',
            'apply_on_all_subexpressions', 'subexpression_substitution_in_existing_subexpressions',
-           'subexpression_substitution_in_main_assignments', 'add_subexpressions_for_divisions']
+           'subexpression_substitution_in_main_assignments', 'add_subexpressions_for_divisions',
+           'add_subexpressions_for_field_reads']

simp/simplifications.py

 import sympy as sp
 from typing import Callable, List
+
+from pystencils import Field
 from pystencils.assignment import Assignment
 from pystencils.simp.assignment_collection import AssignmentCollection
 from pystencils.sympyextensions import subs_additive
@@ -83,6 +85,24 @@ def add_subexpressions_for_divisions(ac: AC) -> AC:
     return ac.new_with_substitutions(substitutions, True)
 
 
+def add_subexpressions_for_field_reads(ac: AC, subexpressions=True, main_assignments=True) -> AC:
+    r"""Substitutes field accesses on rhs of assignments with subexpressions
+
+    Can change semantics of the update rule (which is the goal of this transformation)
+    This is useful if a field should be update in place - all values are loaded before into subexpression variables,
+    then the new values are computed and written to the same field in-place.
+    """
+    field_reads = set()
+    if subexpressions:
+        for assignment in ac.subexpressions:
+            field_reads.update(assignment.rhs.atoms(Field.Access))
+    if main_assignments:
+        for assignment in ac.main_assignments:
+            field_reads.update(assignment.rhs.atoms(Field.Access))
+    substitutions = {fa: sp.Dummy() for fa in field_reads}
+    return ac.new_with_substitutions(substitutions, add_substitutions_as_subexpressions=True, substitute_on_lhs=False)
+
+
 def apply_to_all_assignments(operation: Callable[[sp.Expr], sp.Expr]) -> Callable[[AC], AC]:
     """Applies sympy expand operation to all equations in collection."""
     def f(assignment_collection: AC) -> AC: