Windows: automatically compile with clang if available

setup.py

@@ -11,11 +11,15 @@
 import io
 import os
 import platform
-from shutil import copy2
+from shutil import copy2, copyfile, rmtree
 import sys
+import tempfile
+import atexit
 
 is_win = platform.system() == "Windows"
 is_mac = platform.system() == "Darwin"
+if is_win:
+    import winreg
 
 def set_extension_compile_args(extension):
     rel_lib_path = extension.name.replace('.', '/')
@@ -36,6 +40,44 @@ def run(self):
         ]
         return install_data.run(self)
 
+def win32_get_llvm_reg():
+    REG_PATH = "SOFTWARE\\LLVM\\LLVM"
+    try:
+      return winreg.OpenKey(winreg.HKEY_LOCAL_MACHINE, REG_PATH, 0, winreg.KEY_READ | winreg.KEY_WOW64_32KEY)
+    except FileNotFoundError:
+      pass
+    return winreg.OpenKey(winreg.HKEY_LOCAL_MACHINE, "SOFTWARE\\LLVM\\LLVM", 0, winreg.KEY_READ) 
+
+def win32_find_clang_path():
+    try:
+        with win32_get_llvm_reg() as rkey:
+            return winreg.QueryValueEx(rkey, None)[0]
+    except FileNotFoundError:
+        return None
+
+def win32_use_clang():
+    # Recent (>= 8 ?) LLVM versions does not ship anymore a cl.exe binary in
+    # the msbuild-bin directory. Thus, we need to
+    # * copy-paste bin/clang-cl.exe into a temporary directory
+    # * rename it to cl.exe
+    # * add that path first in %Path%
+    # * clean this mess on exit
+    # We could use the build directory created by distutils for this, but it
+    # seems non trivial to gather
+    # (https://stackoverflow.com/questions/12896367/reliable-way-to-get-the-build-directory-from-within-setup-py).
+    clang_path = win32_find_clang_path()
+    if clang_path is None:
+        return False
+    tmpdir = tempfile.mkdtemp(prefix="llvm")
+    copyfile(os.path.join(clang_path, "bin", "clang-cl.exe"), os.path.join(tmpdir, "cl.exe"))
+    os.environ['Path'] = "%s;%s" % (tmpdir, os.environ["Path"])
+    atexit.register(lambda dir_: rmtree(dir_), tmpdir)
+
+    return True
+
+if is_win:
+    if not win32_use_clang():
+        print("Warning: couldn't find a Clang/LLVM installation. Some runtime functions needed by the jitter won't be compiled.")
 
 def buil_all():
     packages=[