Add experimental half precison support

With this MR experimental half-precision support is added

pystencils/cpu/cpujit.py

@@ -43,26 +43,30 @@ Then 'cl.exe' is used to compile.
   For Windows compilers the qualifier should be ``__restrict``
 
 """
+from appdirs import user_cache_dir, user_config_dir
+from collections import OrderedDict
 import hashlib
+import importlib.util
 import json
 import os
 import platform
 import shutil
 import subprocess
+import sysconfig
+import tempfile
 import textwrap
-from collections import OrderedDict
-from sysconfig import get_paths
-from tempfile import TemporaryDirectory, NamedTemporaryFile
+import time
+import warnings
 
 import numpy as np
-from appdirs import user_cache_dir, user_config_dir
 
 from pystencils import FieldType
 from pystencils.astnodes import LoopOverCoordinate
 from pystencils.backends.cbackend import generate_c, get_headers, CFunction
-from pystencils.typing import CastFunc, VectorType, VectorMemoryAccess
+from pystencils.cpu.msvc_detection import get_environment
 from pystencils.include import get_pystencils_include_path
 from pystencils.kernel_wrapper import KernelWrapper
+from pystencils.typing import BasicType, CastFunc, VectorType, VectorMemoryAccess
 from pystencils.utils import atomic_file_write, recursive_dict_update
 
 
@@ -216,12 +220,11 @@ def read_config():
             shutil.rmtree(config['cache']['object_cache'], ignore_errors=True)
 
         create_folder(config['cache']['object_cache'], False)
-        with NamedTemporaryFile('w', dir=os.path.dirname(cache_status_file), delete=False) as f:
+        with tempfile.NamedTemporaryFile('w', dir=os.path.dirname(cache_status_file), delete=False) as f:
             json.dump(config['compiler'], f, indent=4)
         os.replace(f.name, cache_status_file)
 
     if config['compiler']['os'] == 'windows':
-        from pystencils.cpu.msvc_detection import get_environment
         msvc_env = get_environment(config['compiler']['msvc_version'], config['compiler']['arch'])
         if 'env' not in config['compiler']:
             config['compiler']['env'] = {}
@@ -470,18 +473,15 @@ def create_module_boilerplate_code(module_name, names):
 
 
 def load_kernel_from_file(module_name, function_name, path):
-    from importlib.util import spec_from_file_location, module_from_spec
     try:
-        spec = spec_from_file_location(name=module_name, location=path)
-        mod = module_from_spec(spec)
+        spec = importlib.util.spec_from_file_location(name=module_name, location=path)
+        mod = importlib.util.module_from_spec(spec)
         spec.loader.exec_module(mod)
     except ImportError:
-        import time
-        import warnings
         warnings.warn(f"Could not load {path}, trying on more time in 5 seconds ...")
         time.sleep(5)
-        spec = spec_from_file_location(name=module_name, location=path)
-        mod = module_from_spec(spec)
+        spec = importlib.util.spec_from_file_location(name=module_name, location=path)
+        mod = importlib.util.module_from_spec(spec)
         spec.loader.exec_module(mod)
 
     return getattr(mod, function_name)
@@ -520,9 +520,13 @@ class ExtensionModuleCode:
 
         headers = {'<math.h>', '<stdint.h>'}
         for ast in self._ast_nodes:
+            for field in ast.fields_accessed:
+                if isinstance(field.dtype, BasicType) and field.dtype.is_half():
+                    # Add the half precision header only if half precision numbers occur in the AST
+                    headers.add('"half_precision.h"')
             headers.update(get_headers(ast))
-        header_list = list(headers)
-        header_list.sort()
+
+        header_list = sorted(headers)
         header_list.insert(0, '"Python.h"')
         ps_headers = [os.path.join(os.path.dirname(__file__), '..', 'include', h[1:-1]) for h in header_list
                       if os.path.exists(os.path.join(os.path.dirname(__file__), '..', 'include', h[1:-1]))]
@@ -559,7 +563,7 @@ def compile_module(code, code_hash, base_dir, compile_flags=None):
         compile_flags = []
 
     compiler_config = get_compiler_config()
-    extra_flags = ['-I' + get_paths()['include'], '-I' + get_pystencils_include_path()] + compile_flags
+    extra_flags = ['-I' + sysconfig.get_paths()['include'], '-I' + get_pystencils_include_path()] + compile_flags
 
     if compiler_config['os'].lower() == 'windows':
         lib_suffix = '.pyd'
@@ -593,7 +597,6 @@ def compile_module(code, code_hash, base_dir, compile_flags=None):
 
         # Linking
         if windows:
-            import sysconfig
             config_vars = sysconfig.get_config_vars()
             py_lib = os.path.join(config_vars["installed_base"], "libs",
                                   f"python{config_vars['py_version_nodot']}.lib")
@@ -627,7 +630,7 @@ def compile_and_load(ast, custom_backend=None):
         compile_flags = ast.instruction_set['compile_flags']
 
     if cache_config['object_cache'] is False:
-        with TemporaryDirectory() as base_dir:
+        with tempfile.TemporaryDirectory() as base_dir:
             lib_file = compile_module(code, code_hash_str, base_dir, compile_flags=compile_flags)
             result = load_kernel_from_file(code_hash_str, ast.function_name, lib_file)
     else: