[Fix] GPU Buffer with iteration slices

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

pystencils/gpucuda/indexing.py

@@ -124,12 +124,18 @@ class BlockIndexing(AbstractIndexing):
         self._symbolic_shape = [e if isinstance(e, sp.Basic) else None for e in field.spatial_shape]
         self._compile_time_block_size = compile_time_block_size
 
+    @property
+    def cuda_indices(self):
+        block_size = self._block_size if self._compile_time_block_size else BLOCK_DIM
+        indices = [block_index * bs + thread_idx
+                   for block_index, bs, thread_idx in zip(BLOCK_IDX, block_size, THREAD_IDX)]
+
+        return indices[:self._dim]
+
     @property
     def coordinates(self):
         offsets = _get_start_from_slice(self._iterationSlice)
-        block_size = self._block_size if self._compile_time_block_size else BLOCK_DIM
-        coordinates = [block_index * bs + thread_idx + off
-                       for block_index, bs, thread_idx, off in zip(BLOCK_IDX, block_size, THREAD_IDX, offsets)]
+        coordinates = [c + off for c, off in zip(self.cuda_indices, offsets)]
 
         return coordinates[:self._dim]
 
@@ -159,8 +165,13 @@ class BlockIndexing(AbstractIndexing):
 
     def guard(self, kernel_content, arr_shape):
         arr_shape = arr_shape[:self._dim]
-        conditions = [c < end
-                      for c, end in zip(self.coordinates, _get_end_from_slice(self._iterationSlice, arr_shape))]
+        end = _get_end_from_slice(self._iterationSlice, arr_shape)
+
+        conditions = [c < e for c, e in zip(self.coordinates, end)]
+        for cuda_index, iter_slice in zip(self.cuda_indices, self._iterationSlice):
+            if iter_slice.step > 1:
+                conditions.append(sp.Eq(sp.Mod(cuda_index, iter_slice.step), 0))
+
         condition = conditions[0]
         for c in conditions[1:]:
             condition = sp.And(condition, c)