[Fix] GPU Buffer with iteration slices

GPU buffers did not work in combination with iteration slices before. This is resolved here

pystencils/gpucuda/kernelcreation.py

@@ -13,7 +13,7 @@ from pystencils.node_collection import NodeCollection
 from pystencils.gpucuda.indexing import indexing_creator_from_params
 from pystencils.simp.assignment_collection import AssignmentCollection
 from pystencils.transformations import (
-    get_base_buffer_index, get_common_shape, parse_base_pointer_info,
+    get_base_buffer_index, get_common_field, parse_base_pointer_info,
     resolve_buffer_accesses, resolve_field_accesses, unify_shape_symbols)
 
 
@@ -45,8 +45,8 @@ def create_cuda_kernel(assignments: Union[AssignmentCollection, NodeCollection],
         num_buffer_accesses += sum(1 for access in eq.atoms(Field.Access) if FieldType.is_buffer(access.field))
 
     # common shape and field to from the iteration space
-    common_shape = get_common_shape(fields_without_buffers)
-    common_field = list(sorted(fields_without_buffers, key=lambda e: str(e)))[0]
+    common_field = get_common_field(fields_without_buffers)
+    common_shape = common_field.spatial_shape
 
     if iteration_slice is None:
         # determine iteration slice from ghost layers
@@ -160,7 +160,7 @@ def created_indexed_cuda_kernel(assignments: Union[AssignmentCollection, NodeCol
                                 iteration_slice=[slice(None, None, None)] * len(idx_field.spatial_shape))
 
     function_body = Block(coordinate_symbol_assignments + assignments)
-    function_body = indexing.guard(function_body, get_common_shape(index_fields))
+    function_body = indexing.guard(function_body, get_common_field(index_fields).spatial_shape)
     ast = KernelFunction(function_body, Target.GPU, Backend.CUDA, make_python_function,
                          None, function_name, assignments=assignments)
     ast.global_variables.update(indexing.index_variables)