Global with no comm is a Constant (#701)

---------

Co-authored-by: Connor Ward <[email protected]>
Co-authored-by: David A. Ham <[email protected]>

pyop2/global_kernel.py

@@ -13,7 +13,7 @@
 from pyop2.caching import Cached
 from pyop2.configuration import configuration
 from pyop2.datatypes import IntType, as_ctypes
-from pyop2.types import IterationRegion
+from pyop2.types import IterationRegion, Constant, READ
 from pyop2.utils import cached_property, get_petsc_dir
 
 
@@ -277,13 +277,27 @@ def __init__(self, local_kernel, arguments, *,
             return
 
         if not len(local_kernel.accesses) == len(arguments):
-            raise ValueError("Number of arguments passed to the local "
-                             "and global kernels do not match")
+            raise ValueError(
+                "Number of arguments passed to the local and global kernels"
+                " do not match"
+            )
+
+        if any(
+            isinstance(garg, Constant) and larg.access is not READ
+            for larg, garg in zip(local_kernel.arguments, arguments)
+        ):
+            raise ValueError(
+                "Constants can only ever be read in a parloop, not modified"
+            )
 
         if pass_layer_arg and not extruded:
-            raise ValueError("Cannot request layer argument for non-extruded iteration")
+            raise ValueError(
+                "Cannot request layer argument for non-extruded iteration"
+            )
         if constant_layers and not extruded:
-            raise ValueError("Cannot request constant_layers argument for non-extruded iteration")
+            raise ValueError(
+                "Cannot request constant_layers argument for non-extruded iteration"
+            )
 
         self.local_kernel = local_kernel
         self.arguments = arguments

pyop2/op2.py

@@ -39,10 +39,10 @@
 from pyop2.logger import debug, info, warning, error, critical, set_log_level
 from pyop2.mpi import MPI, COMM_WORLD, collective
 
-from pyop2.types import (
+from pyop2.types import (  # noqa: F401
     Set, ExtrudedSet, MixedSet, Subset, DataSet, MixedDataSet,
     Map, MixedMap, PermutedMap, ComposedMap, Sparsity, Halo,
-    Global, GlobalDataSet,
+    Global, Constant, GlobalDataSet,
     Dat, MixedDat, DatView, Mat
 )
 from pyop2.types import (READ, WRITE, RW, INC, MIN, MAX,

pyop2/types/dat.py

@@ -426,12 +426,15 @@ def _iop_kernel(self, op, globalp, other_is_self, dtype):
 
     def _iop(self, other, op):
         from pyop2.parloop import parloop
-        from pyop2.types.glob import Global
+        from pyop2.types.glob import Global, Constant
 
         globalp = False
         if np.isscalar(other):
             other = Global(1, data=other, comm=self.comm)
             globalp = True
+        elif isinstance(other, Constant):
+            other = Global(other, comm=self.comm)
+            globalp = True
         elif other is not self:
             self._check_shape(other)
         args = [self(Access.INC)]