Destructuring field binding

Add DestructuringBindingsForFieldClass to use pystencils kernels in a more C++-ish way

DestructuringBindingsForFieldClass defines all field-related variables in its subordinated block. However, it leaves a TypedSymbol of type Field for each field undefined. By that trick we can generate kernels that accept structs as kernelparameters. Either to include a pystencils specific Field struct of the following definition:

template<DTYPE_T, DIMENSION>
struct Field
{
    DTYPE_T* data;
    std::array<int64_t, DIMENSION> shape;
    std::array<int64_t, DIMENSION> stride;
}

or to be able to destructure user defined types like pybind11::array, at::Tensor, tensorflow::Tensor.

The test generates a kernel like that:

FUNC_PREFIX void kernel(Field<double, 2>& x, Field<double, 2>& y, Field<double, 2>& z)
{
   _stride_z_1 = z.stride[1];
   _size_x_0 = x.shape[0];
   _stride_x_1 = x.stride[1];
   _stride_z_0 = z.stride[0];
   _size_x_1 = x.shape[1];
   _stride_y_1 = y.stride[1];
   _data_x = x.data;
   _stride_x_0 = x.stride[0];
   _data_z = z.data;
   _stride_y_0 = y.stride[0];
   _data_y = y.data;
   {
      for (int ctr_0 = 0; ctr_0 < _size_x_0; ctr_0 += 1)
      {
         double * RESTRICT _data_z_00 = _data_z + _stride_z_0*ctr_0;
         double * RESTRICT const _data_y_00 = _data_y + _stride_y_0*ctr_0;
         double * RESTRICT const _data_x_00 = _data_x + _stride_x_0*ctr_0;
         for (int ctr_1 = 0; ctr_1 < _size_x_1; ctr_1 += 1)
         {
            _data_z_00[_stride_z_1*ctr_1] = log(_data_x_00[_stride_x_1*ctr_1]*_data_y_00[_stride_y_1*ctr_1])*_data_y_00[_stride_y_1*ctr_1];
         }
      }
   }
}

pystencils/backends/cbackend.py

-import sympy as sp
 from collections import namedtuple
-from sympy.core import S
 from typing import Set
+
+import jinja2
+import sympy as sp
+from sympy.core import S
 from sympy.printing.ccode import C89CodePrinter
 
-from pystencils.cpu.vectorization import vec_any, vec_all
-from pystencils.fast_approximation import fast_division, fast_sqrt, fast_inv_sqrt
+from pystencils.astnodes import (DestructuringBindingsForFieldClass,
+                                 KernelFunction, Node)
+from pystencils.cpu.vectorization import vec_all, vec_any
+from pystencils.data_types import (PointerType, VectorType, cast_func,
+                                   create_type, get_type_of_expression,
+                                   reinterpret_cast_func, vector_memory_access)
+from pystencils.fast_approximation import (fast_division, fast_inv_sqrt,
+                                           fast_sqrt)
+from pystencils.integer_functions import (bit_shift_left, bit_shift_right,
+                                          bitwise_and, bitwise_or, bitwise_xor,
+                                          int_div, int_power_of_2, modulo_ceil)
+from pystencils.kernelparameters import FieldPointerSymbol
 
 try:
     from sympy.printing.ccode import C99CodePrinter as CCodePrinter
 except ImportError:
     from sympy.printing.ccode import CCodePrinter  # for sympy versions < 1.1
 
-from pystencils.integer_functions import bitwise_xor, bit_shift_right, bit_shift_left, bitwise_and, \
-    bitwise_or, modulo_ceil, int_div, int_power_of_2
-from pystencils.astnodes import Node, KernelFunction
-from pystencils.data_types import create_type, PointerType, get_type_of_expression, VectorType, cast_func, \
-    vector_memory_access, reinterpret_cast_func
 
 __all__ = ['generate_c', 'CustomCodeNode', 'PrintNode', 'get_headers', 'CustomSympyPrinter']
 
@@ -224,6 +231,30 @@ class CBackend:
             result += "else " + false_block
         return result
 
+    def _print_DestructuringBindingsForFieldClass(self, node: Node):
+        # Define all undefined symbols
+        undefined_field_symbols = node.symbols_defined
+        destructuring_bindings = ["%s = %s.%s%s;" %
+                                  (u.name,
+                                   u.field_name if hasattr(u, 'field_name') else u.field_names[0],
+                                   DestructuringBindingsForFieldClass.CLASS_TO_MEMBER_DICT[u.__class__],
+                                   "" if type(u) == FieldPointerSymbol else ("[%i]" % u.coordinate))
+                                  for u in undefined_field_symbols
+                                  ]
+        destructuring_bindings.sort()  # only for code aesthetics
+        template = jinja2.Template(
+            """{
+   {% for binding in bindings -%}
+   {{ binding | indent(3) }}
+   {% endfor -%}
+   {{ block | indent(3) }}
+}
+
+""")
+        code = template.render(bindings=destructuring_bindings,
+                               block=self._print(node.body))
+        return code
+
 
 # ------------------------------------------ Helper function & classes -------------------------------------------------