Add global_declarations to cbackend

This enables astnodes.Nodes to have a member required_global_declarations by which they can specify a global declaration required for their usage.

In the test, I added a AST-Node Bogus which requires a global declaration. The global declaration can define symbols required in the kernel that will then not appear in the kernel parameters

// Declaration would go here

FUNC_PREFIX void kernel(double * RESTRICT const _data_x, double * RESTRICT const _data_y, double * RESTRICT _data_z, int64_t const _size_1, int64_t const _stride_z_0, int64_t const _stride_z_1)
{
   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];
      }
   }
   // Bogus would go here
}

I used this code for my CudaBackend (instead of CBackend) to enable the forward declaration of textures and constant memory.

pystencils/backends/cbackend.py

@@ -38,10 +38,39 @@ def generate_c(ast_node: Node, signature_only: bool = False, dialect='c') -> str
     Returns:
         C-like code for the ast node and its descendants
     """
+    global_declarations = get_global_declarations(ast_node)
+    for d in global_declarations:
+        if hasattr(ast_node, "global_variables"):
+            ast_node.global_variables.update(d.symbols_defined)
+        else:
+            ast_node.global_variables = d.symbols_defined
     printer = CBackend(signature_only=signature_only,
                        vector_instruction_set=ast_node.instruction_set,
                        dialect=dialect)
-    return printer(ast_node)
+    code = printer(ast_node)
+    if not signature_only and isinstance(ast_node, KernelFunction):
+        code = "\n" + code
+        for declaration in global_declarations:
+            code = printer(declaration) + "\n" + code
+
+    return code
+
+
+def get_global_declarations(ast):
+    global_declarations = []
+
+    def visit_node(sub_ast):
+        if hasattr(sub_ast, "required_global_declarations"):
+            nonlocal global_declarations
+            global_declarations += sub_ast.required_global_declarations
+
+        if hasattr(sub_ast, "args"):
+            for node in sub_ast.args:
+                visit_node(node)
+
+    visit_node(ast)
+
+    return set(global_declarations)
 
 
 def get_headers(ast_node: Node) -> Set[str]: