diff --git a/artiq/compiler/ir.py b/artiq/compiler/ir.py
index a411c036f..941b04d36 100644
--- a/artiq/compiler/ir.py
+++ b/artiq/compiler/ir.py
@@ -36,6 +36,47 @@ class TKeyword(types.TMono):
 def is_keyword(typ):
     return isinstance(typ, TKeyword)
 
+
+# See rpc_proto.rs and comm_kernel.py:_{send,receive}_rpc_value.
+def rpc_tag(typ, error_handler):
+    typ = typ.find()
+    if types.is_tuple(typ):
+        assert len(typ.elts) < 256
+        return b"t" + bytes([len(typ.elts)]) + \
+                b"".join([rpc_tag(elt_type, error_handler)
+                            for elt_type in typ.elts])
+    elif builtins.is_none(typ):
+        return b"n"
+    elif builtins.is_bool(typ):
+        return b"b"
+    elif builtins.is_int(typ, types.TValue(32)):
+        return b"i"
+    elif builtins.is_int(typ, types.TValue(64)):
+        return b"I"
+    elif builtins.is_float(typ):
+        return b"f"
+    elif builtins.is_str(typ):
+        return b"s"
+    elif builtins.is_bytes(typ):
+        return b"B"
+    elif builtins.is_bytearray(typ):
+        return b"A"
+    elif builtins.is_list(typ):
+        return b"l" + rpc_tag(builtins.get_iterable_elt(typ), error_handler)
+    elif builtins.is_array(typ):
+        return b"a" + rpc_tag(builtins.get_iterable_elt(typ), error_handler)
+    elif builtins.is_range(typ):
+        return b"r" + rpc_tag(builtins.get_iterable_elt(typ), error_handler)
+    elif is_keyword(typ):
+        return b"k" + rpc_tag(typ.params["value"], error_handler)
+    elif types.is_function(typ) or types.is_method(typ) or types.is_rpc(typ):
+        raise ValueError("RPC tag for functional value")
+    elif '__objectid__' in typ.attributes:
+        return b"O"
+    else:
+        error_handler(typ)
+
+
 class Value:
     """
     An SSA value that keeps track of its uses.
diff --git a/artiq/compiler/transforms/llvm_ir_generator.py b/artiq/compiler/transforms/llvm_ir_generator.py
index dcc2cc03e..70ef64785 100644
--- a/artiq/compiler/transforms/llvm_ir_generator.py
+++ b/artiq/compiler/transforms/llvm_ir_generator.py
@@ -570,7 +570,7 @@ class LLVMIRGenerator:
                 if name == "__objectid__":
                     rpctag = b""
                 else:
-                    rpctag = b"Os" + self._rpc_tag(typ, error_handler=rpc_tag_error) + b":n"
+                    rpctag = b"Os" + ir.rpc_tag(typ, error_handler=rpc_tag_error) + b":n"
 
                 llrpcattrinit = ll.Constant(llrpcattrty, [
                     ll.Constant(lli32, offset),
@@ -1310,49 +1310,6 @@ class LLVMIRGenerator:
 
         return llfun, list(llargs)
 
-    # See session.c:{send,receive}_rpc_value and comm_generic.py:_{send,receive}_rpc_value.
-    def _rpc_tag(self, typ, error_handler):
-        typ = typ.find()
-        if types.is_tuple(typ):
-            assert len(typ.elts) < 256
-            return b"t" + bytes([len(typ.elts)]) + \
-                   b"".join([self._rpc_tag(elt_type, error_handler)
-                             for elt_type in typ.elts])
-        elif builtins.is_none(typ):
-            return b"n"
-        elif builtins.is_bool(typ):
-            return b"b"
-        elif builtins.is_int(typ, types.TValue(32)):
-            return b"i"
-        elif builtins.is_int(typ, types.TValue(64)):
-            return b"I"
-        elif builtins.is_float(typ):
-            return b"f"
-        elif builtins.is_str(typ):
-            return b"s"
-        elif builtins.is_bytes(typ):
-            return b"B"
-        elif builtins.is_bytearray(typ):
-            return b"A"
-        elif builtins.is_list(typ):
-            return b"l" + self._rpc_tag(builtins.get_iterable_elt(typ),
-                                        error_handler)
-        elif builtins.is_array(typ):
-            return b"a" + self._rpc_tag(builtins.get_iterable_elt(typ),
-                                        error_handler)
-        elif builtins.is_range(typ):
-            return b"r" + self._rpc_tag(builtins.get_iterable_elt(typ),
-                                        error_handler)
-        elif ir.is_keyword(typ):
-            return b"k" + self._rpc_tag(typ.params["value"],
-                                        error_handler)
-        elif types.is_function(typ) or types.is_method(typ) or types.is_rpc(typ):
-            raise ValueError("RPC tag for functional value")
-        elif '__objectid__' in typ.attributes:
-            return b"O"
-        else:
-            error_handler(typ)
-
     def _build_rpc(self, fun_loc, fun_type, args, llnormalblock, llunwindblock):
         llservice = ll.Constant(lli32, fun_type.service)
 
@@ -1370,7 +1327,7 @@ class LLVMIRGenerator:
                     {"type": printer.name(arg.type)},
                     arg.loc)
                 self.engine.process(diag)
-            tag += self._rpc_tag(arg.type, arg_error_handler)
+            tag += ir.rpc_tag(arg.type, arg_error_handler)
         tag += b":"
 
         def ret_error_handler(typ):
@@ -1384,7 +1341,7 @@ class LLVMIRGenerator:
                 {"type": printer.name(fun_type.ret)},
                 fun_loc)
             self.engine.process(diag)
-        tag += self._rpc_tag(fun_type.ret, ret_error_handler)
+        tag += ir.rpc_tag(fun_type.ret, ret_error_handler)
 
         lltag = self.llconst_of_const(ir.Constant(tag, builtins.TStr()))
         lltagptr = self.llbuilder.alloca(lltag.type)
diff --git a/artiq/coredevice/comm_kernel.py b/artiq/coredevice/comm_kernel.py
index d3a73c20e..838bad042 100644
--- a/artiq/coredevice/comm_kernel.py
+++ b/artiq/coredevice/comm_kernel.py
@@ -244,7 +244,7 @@ class CommKernel:
 
     _rpc_sentinel = object()
 
-    # See session.c:{send,receive}_rpc_value and llvm_ir_generator.py:_rpc_tag.
+    # See rpc_proto.rs and compiler/ir.py:rpc_tag.
     def _receive_rpc_value(self, embedding_map):
         tag = chr(self._read_int8())
         if tag == "\x00":