Merge pull request #2369 from devitocodes/expand-time

api: Always expand time derivatives

devito/finite_differences/finite_difference.py

@@ -230,6 +230,11 @@ def generic_derivative(expr, dim, fd_order, deriv_order, matvec=direct, x0=None,
 
 def make_derivative(expr, dim, fd_order, deriv_order, side, matvec, x0, coefficients,
                     expand):
+    # Always expand time derivatives to avoid issue with buffering and streaming.
+    # Time derivative are almost always short stencils and won't benefit from
+    # unexpansion in the rare case the derivative is not evaluated for time stepping.
+    expand = dim.is_Time or expand
+
     # The stencil indices
     indices, x0 = generate_indices(expr, dim, fd_order, side=side, matvec=matvec,
                                    x0=x0)

tests/test_unexpansion.py

@@ -8,7 +8,7 @@
 from devito.arch.compiler import OneapiCompiler
 from devito.ir import Expression, FindNodes, FindSymbols
 from devito.parameters import switchconfig, configuration
-from devito.types import Symbol
+from devito.types import Symbol, Dimension
 
 
 class TestLoopScheduling(object):
@@ -330,6 +330,29 @@ def test_redundant_derivatives(self):
 
         op.cfunction
 
+    def test_buffering_timestencil(self):
+        grid = Grid((11, 11))
+        so = 4
+        nt = 11
+
+        u = TimeFunction(name="u", grid=grid, space_order=so, time_order=2, save=nt)
+        v = TimeFunction(name="v", grid=grid, space_order=so, time_order=2)
+
+        g = Function(name="g", grid=grid, space_order=so)
+
+        # Make sure operator builds with buffering
+        op = Operator([Eq(g, g + u.dt*v.dx + u.dx2)],
+                      opt=('buffering', 'streaming', {'expand': False}))
+
+        exprs = FindNodes(Expression).visit(op)
+        dims = [d for i in FindSymbols().visit(exprs) for d in i.dimensions
+                if isinstance(d, Dimension)]
+
+        # Should only be two stencil dimension for .dx and .dx2
+        assert len([d for d in dims if d.is_Stencil]) == 2
+        # Should only be one buffer dimension
+        assert len([d for d in dims if d.is_Custom]) == 1
+
 
 class Test2Pass(object):