compiler: catch corner case read after write

Author: mloubout

devito/ir/clusters/algorithms.py

@@ -165,7 +165,7 @@ def callback(self, clusters, prefix, backlog=None, known_break=None):
         # Schedule Clusters over different IterationSpaces if this increases
         # parallelism
         for i in range(1, len(clusters)):
-            if self._break_for_parallelism(scope, dim, i):
+            if self._break_for_parallelism(scope, dim, i, prefix[:-1]):
                 return self.callback(clusters[:i], prefix, clusters[i:] + backlog,
                                      candidates | known_break)
 
@@ -204,7 +204,7 @@ def callback(self, clusters, prefix, backlog=None, known_break=None):
 
         return processed + self.callback(backlog, prefix)
 
-    def _break_for_parallelism(self, scope, dim, timestamp):
+    def _break_for_parallelism(self, scope, dim, timestamp, prev):
         candidates = dim._defines
 
         # Do not fission for data locality reasons if there's enough potential
@@ -228,6 +228,12 @@ def _break_for_parallelism(self, scope, dim, timestamp):
                 # Would break a dependence on storage
                 return False
 
+            if any(dep.distance_mapper.get(d, 0) != 0 for d in candidates) and \
+               prev and all(dep.distance_mapper.get(d.dim, -1) == 0 for d in prev) and \
+               dep.read.function is not dep.write.function:
+                # Cannot read/write with `dim` since all previous ones are dependent
+                return True
+
             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

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 +58,25 @@ 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]'),