compiler: Fix handling for determining if functions on subdomains touch the halo

devito/ir/support/basic.py

@@ -10,7 +10,7 @@
 from devito.ir.support.utils import AccessMode, extrema
 from devito.ir.support.vector import LabeledVector, Vector
 from devito.symbolics import (
-    compare_ops, q_affine, q_comp_acc, q_constant, retrieve_indexed
+    compare_ops, q_affine, q_comp_acc, q_constant, retrieve_indexed, search
 )
 from devito.tools import (
     CacheInstances, Tag, as_mapper, as_tuple, cached_hash, filter_sorted, flatten,
@@ -20,6 +20,7 @@
     ComponentAccess, CriticalRegion, Dimension, DimensionTuple, Fence, Function, Symbol,
     TBArray, Temp, TempArray
 )
+from devito.types.dimension import Thickness
 
 __all__ = ['ExprGeometry', 'IterationInstance', 'Scope', 'TimedAccess']
 
@@ -515,8 +516,17 @@ def touched_halo(self, findex):
         # If `m + (self[d] - d) < self.function._size_nodomain[d].left`, then `self`
         # will definitely touch the left-halo, at least when `d=0`
         size_nodomain_left = self.function._size_nodomain[findex].left
+
+        # Symbolic thicknesses may appear in indices for Functions defined on SubDomains
+        # These are used to offset the loop index values to correctly index into the
+        # reduced array allocated on the SubDomain. As such, they do not represent a
+        # shift of index on the physical grid and can (and should) be zeroed for the
+        # purpose of determining if the halo is touched.
+        tkns_subs = {tkn: 0 for tkn in search(self[findex], Thickness)}
+        findex_wo_thickness = self[findex].subs(tkns_subs)
+
         try:
-            touch_halo_left = bool(m + (self[findex] - d) < size_nodomain_left)
+            touch_halo_left = bool(m + (findex_wo_thickness - d) < size_nodomain_left)
         except TypeError:
             # Two reasons we might end up here:
             # * `d` is a constant integer
@@ -529,7 +539,7 @@ def touched_halo(self, findex):
         # If `M + (self[d] - d) > self.function._size_nodomain[d].left`, then
         # `self` will definitely touch the right-halo, at least when `d=d_M`
         try:
-            touch_halo_right = bool(M + (self[findex] - d) > size_nodomain_left)
+            touch_halo_right = bool(M + (findex_wo_thickness - d) > size_nodomain_left)
         except TypeError:
             # See comments in the except block above
             touch_halo_right = True

tests/test_mpi.py

@@ -2323,6 +2323,28 @@ def test_merge_subset_comms_w_overlap(self, mode):
         halo_update1 = body[1].body[0]
         assert isinstance(halo_update1, HaloUpdateList)
 
+    @pytest.mark.parallel(mode=1)
+    def test_function_on_subdomain_no_exchange(self, mode):
+
+        class Boundary(SubDomain):
+            name = 'boundary'
+
+            def define(self, dimensions):
+                x, y, z = dimensions
+                return {x: ('left', 1), y: ('middle', 1, 1), z: ('right', 1)}
+
+        grid = Grid(shape=(6, 6, 6))
+        boundary = Boundary(grid=grid)
+
+        f = TimeFunction(name='f', grid=boundary, space_order=2)
+
+        op = Operator(Eq(f.forward, f))
+        _ = op.cfunction
+
+        # Halo should not be considered touched, despite symbolic offsets in indices due
+        # to Function on SubDomain
+        check_halo_exchanges(op, 0, 0)
+
 
 class TestOperatorAdvanced: