compiler: Fix detection of global distributed reductions

devito/ir/clusters/algorithms.py

@@ -447,14 +447,14 @@ def reduction_comms(clusters):
     processed = []
     fifo = []
     for c in clusters:
-        # Schedule the global reductions encountered before `c`, if the
-        # IterationSpace of `c` is such that the reduction can be carried out
+        # Schedule the global distributed reductions encountered before `c`,
+        # if `c`'s IterationSpace is such that the reduction can be carried out
         found, fifo = split(fifo, lambda dr: dr.ispace.is_subset(c.ispace))
         if found:
             exprs = [Eq(dr.var, dr) for dr in found]
             processed.append(c.rebuild(exprs=exprs))
 
-        # Detect the global reductions in `c`
+        # Detect the global distributed reductions in `c`
         for e in c.exprs:
             op = e.operation
             if op is None or c.is_sparse:
@@ -465,12 +465,23 @@ def reduction_comms(clusters):
             if grid is None:
                 continue
 
-            # The IterationSpace within which the global reduction is carried out
+            # Is Inc/Max/Min/... actually used for a reduction?
             ispace = c.ispace.project(lambda d: d in var.free_symbols)
             if ispace.itdims == c.ispace.itdims:
-                # Inc/Max/Min/... being used for a non-reduction operation
                 continue
 
+            # The reduced Dimensions
+            rdims = set(c.ispace.itdims) - set(ispace.itdims)
+
+            # The reduced Dimensions inducing a global distributed reduction
+            grdims = {d for d in rdims if d._defines & c.dist_dimensions}
+            if not grdims:
+                continue
+
+            # The IterationSpace within which the global distributed reduction
+            # must be carried out
+            ispace = c.ispace.prefix(lambda d: d in var.free_symbols)
+
             fifo.append(DistReduce(var, op=op, grid=grid, ispace=ispace))
 
         processed.append(c)

devito/ir/clusters/cluster.py

@@ -167,6 +167,19 @@ def used_dimensions(self):
         idims = set.union(*[set(e.implicit_dims) for e in self.exprs])
         return {i for i in self.free_symbols if i.is_Dimension} | idims
 
+    @cached_property
+    def dist_dimensions(self):
+        """
+        The Cluster's distributed Dimensions.
+        """
+        ret = set()
+        for f in self.functions:
+            try:
+                ret.update(f._dist_dimensions)
+            except AttributeError:
+                pass
+        return frozenset(ret)
+
     @cached_property
     def scope(self):
         return Scope(self.exprs)

devito/ir/support/space.py

@@ -954,7 +954,7 @@ def prefix(self, key):
         try:
             i = self.project(key)[-1]
         except IndexError:
-            return None
+            return null_ispace
 
         return self[:self.index(i.dim) + 1]
 

tests/test_mpi.py

@@ -4,17 +4,17 @@
 
 from conftest import _R, assert_blocking, assert_structure
 from devito import (Grid, Constant, Function, TimeFunction, SparseFunction,
-                    SparseTimeFunction, Dimension, ConditionalDimension, SubDimension,
-                    SubDomain, Eq, Ne, Inc, NODE, Operator, norm, inner, configuration,
-                    switchconfig, generic_derivative, PrecomputedSparseFunction,
-                    DefaultDimension)
+                    SparseTimeFunction, Dimension, ConditionalDimension,
+                    SubDimension, SubDomain, Eq, Ne, Inc, NODE, Operator, norm,
+                    inner, configuration, switchconfig, generic_derivative,
+                    PrecomputedSparseFunction, DefaultDimension)
 from devito.arch.compiler import OneapiCompiler
 from devito.data import LEFT, RIGHT
 from devito.ir.iet import (Call, Conditional, Iteration, FindNodes, FindSymbols,
                            retrieve_iteration_tree)
 from devito.mpi import MPI
 from devito.mpi.routines import (HaloUpdateCall, HaloUpdateList, MPICall,
-                                 ComputeCall, AllreduceCall)
+                                 ComputeCall)
 from devito.mpi.distributed import CustomTopology
 from devito.tools import Bunch
 
@@ -929,8 +929,7 @@ def test_avoid_haloupdate_as_nostencil_advanced(self, mode):
 
         # No stencil in the expressions, so no halo update required!
         calls = FindNodes(Call).visit(op)
-        assert len(calls) == 2
-        assert all(isinstance(i, AllreduceCall) for i in calls)
+        assert len(calls) == 0
 
     @pytest.mark.parallel(mode=1)
     def test_avoid_redundant_haloupdate(self, mode):