compiler: Handle None-returning function calls used as values

GitHub: Fixes #1493.

artiq/compiler/transforms/llvm_ir_generator.py

@@ -1462,6 +1462,11 @@ class LLVMIRGenerator:
         else:
             llcall = llresult = self.llbuilder.call(llfun, llargs, name=insn.name)
 
+            if isinstance(llresult.type, ll.VoidType):
+                # We have NoneType-returning functions return void, but None is
+                # {} elsewhere.
+                llresult = ll.Constant(llunit, [])
+
             # Never add TBAA nowrite metadata to a functon with sret!
             # This leads to miscompilations.
             if types.is_c_function(functiontyp) and 'nowrite' in functiontyp.flags:

artiq/test/lit/codegen/none_retval.py

@@ -0,0 +1,11 @@
+# RUN: %python -m artiq.compiler.testbench.llvmgen %s
+
+def make_none():
+    return None
+
+def take_arg(arg):
+    pass
+
+def run():
+    retval = make_none()
+    take_arg(retval)