compiler: catch corner case read after write

devito/ir/clusters/algorithms.py

@@ -231,8 +231,15 @@ def _break_for_parallelism(self, scope, dim, timestamp):
             if any(dep.is_carried(i) for i in candidates):
                 test0 = dep.is_flow and dep.is_lex_negative
                 test1 = dep.is_anti and dep.is_lex_positive
+                if test0:
+                    # If the same access pair is not a flow under logical distance,
+                    # the dep is a buffer/modulo-aliasing artifact and fission is OK
+                    ldist = dep.source.distance(dep.sink, logical=True)
+                    real_flow = (ldist > 0) or \
+                        (ldist == 0 and dep.sink.lex_ge(dep.source))
+                    if not real_flow:
+                        test0 = real_flow
                 if test0 or test1:
-                    # Would break a data dependence
                     return False
 
             test = test or (bool(dep.cause & candidates) and not dep.is_lex_equal)

devito/ir/support/basic.py

@@ -1105,6 +1105,7 @@ def d_flow_gen(self):
                         continue
 
                     distance = dependence.distance
+
                     try:
                         is_flow = distance > 0 or (r.lex_ge(w) and distance == 0)
                     except TypeError:

tests/test_dse.py

@@ -9,10 +9,10 @@
     get_params, skipif
 )
 from devito import (  # noqa
-    NODE, Abs, ConditionalDimension, Constant, DefaultDimension, Derivative, Dimension,
-    Eq, Function, Ge, Grid, Inc, Lt, Operator, SparseTimeFunction, SubDimension,
-    TimeFunction, configuration, cos, dimensions, div, exp, first_derivative, floor, grad,
-    norm, sin, solve, sqrt, switchconfig, transpose
+    NODE, Abs, Buffer, ConditionalDimension, Constant, DefaultDimension, Derivative,
+    Dimension, Eq, Function, Ge, Grid, Inc, Lt, Operator, SparseTimeFunction,
+    SubDimension, TimeFunction, configuration, cos, dimensions, div, exp,
+    first_derivative, floor, grad, norm, sin, solve, sqrt, switchconfig, transpose
 )
 from devito.exceptions import InvalidArgument, InvalidOperator
 from devito.ir import (
@@ -58,6 +58,26 @@ def test_scheduling_after_rewrite():
     assert all(trees[1].root.dim is tree.root.dim for tree in trees[1:])
 
 
+def test_scheduling_no_deriv():
+    grid = Grid((11, 11, 11))
+    x, y, z = grid.dimensions
+
+    image_vs = Function(name='image_vs', grid=grid, space_order=1, staggered=NODE)
+    p_back_xy = TimeFunction(name='p_back_xy', grid=grid, staggered=(x, y),
+                             space_order=4, time_order=1, save=Buffer(1))
+
+    eqns = [Eq(image_vs, p_back_xy + image_vs),
+            Eq(p_back_xy.backward, p_back_xy)]
+
+    op = Operator(eqns)
+
+    assert_structure(
+        op,
+        ['t,x0_blk0,y0_blk0,x,y,z', 't,x1_blk0,y1_blk0,x,y,z'],
+        'tx0_blk0y0_blk0xyzx1_blk0y1_blk0xyz'
+    )
+
+
 @pytest.mark.parametrize('expr,expected', [
     ('2*fa[x] + fb[x]', '2*fa[x] + fb[x]'),
     ('fa[x]**2', 'fa[x]*fa[x]'),