diff --git a/artiq/transforms/remove_dead_code.py b/artiq/transforms/remove_dead_code.py
index b176e6a4d..9a58c851d 100644
--- a/artiq/transforms/remove_dead_code.py
+++ b/artiq/transforms/remove_dead_code.py
@@ -1,6 +1,6 @@
 import ast
 
-from artiq.transforms.tools import is_replaceable
+from artiq.transforms.tools import is_ref_transparent
 
 
 class _SourceLister(ast.NodeVisitor):
@@ -22,7 +22,7 @@ class _DeadCodeRemover(ast.NodeTransformer):
             if (not isinstance(target, ast.Name)
                     or target.id in self.kept_targets):
                 new_targets.append(target)
-        if not new_targets and is_replaceable(node.value):
+        if not new_targets and is_ref_transparent(node.value)[0]:
             return None
         else:
             return node
@@ -30,7 +30,7 @@ class _DeadCodeRemover(ast.NodeTransformer):
     def visit_AugAssign(self, node):
         if (isinstance(node.target, ast.Name)
                 and node.target.id not in self.kept_targets
-                and is_replaceable(node.value)):
+                and is_ref_transparent(node.value)[0]):
             return None
         else:
             return node
diff --git a/artiq/transforms/remove_inter_assigns.py b/artiq/transforms/remove_inter_assigns.py
index 2e77339f8..bcc6e2fcd 100644
--- a/artiq/transforms/remove_inter_assigns.py
+++ b/artiq/transforms/remove_inter_assigns.py
@@ -1,7 +1,8 @@
 import ast
 from copy import copy, deepcopy
+from collections import defaultdict
 
-from artiq.transforms.tools import is_replaceable
+from artiq.transforms.tools import is_ref_transparent
 
 
 class _TargetLister(ast.NodeVisitor):
@@ -17,30 +18,42 @@ class _InterAssignRemover(ast.NodeTransformer):
     def __init__(self):
         self.replacements = dict()
         self.modified_names = set()
+        # name -> set of names that depend on it
+        # i.e. when x is modified, dependencies[x] is the set of names that
+        # cannot be replaced anymore
+        self.dependencies = defaultdict(set)
+
+    def invalidate(self, name):
+        try:
+            del self.replacements[name]
+        except KeyError:
+            pass
+        for d in self.dependencies[name]:
+            self.invalidate(d)
+        del self.dependencies[name]
 
     def visit_Name(self, node):
         if isinstance(node.ctx, ast.Load):
             try:
-                return self.replacements[node.id]
+                return deepcopy(self.replacements[node.id])
             except KeyError:
                 return node
         else:
             self.modified_names.add(node.id)
+            self.invalidate(node.id)
             return node
 
     def visit_Assign(self, node):
-        self.generic_visit(node)
-        if is_replaceable(node.value):
+        node.value = self.visit(node.value)
+        node.targets = [self.visit(target) for target in node.targets]
+        rt, depends_on = is_ref_transparent(node.value)
+        if rt:
             for target in node.targets:
                 if isinstance(target, ast.Name):
-                    self.replacements[target.id] = node.value
-        else:
-            for target in node.targets:
-                if isinstance(target, ast.Name):
-                    try:
-                        del self.replacements[target.id]
-                    except KeyError:
-                        pass
+                    if target.id not in depends_on:
+                        self.replacements[target.id] = node.value
+                        for d in depends_on:
+                            self.dependencies[d].add(target.id)
         return node
 
     def visit_AugAssign(self, node):
@@ -62,10 +75,7 @@ class _InterAssignRemover(ast.NodeTransformer):
 
     def modified_names_pop(self, prev_modified_names):
         for name in self.modified_names:
-            try:
-                del self.replacements[name]
-            except KeyError:
-                pass
+            self.invalidate(name)
         self.modified_names |= prev_modified_names
 
     def visit_Try(self, node):
@@ -99,10 +109,7 @@ class _InterAssignRemover(ast.NodeTransformer):
         for n in node.body:
             tl.visit(n)
         for name in tl.targets:
-            try:
-                del self.replacements[name]
-            except KeyError:
-                pass
+            self.invalidate(name)
         node.body = [self.visit(n) for n in node.body]
 
         self.replacements = copy(prev_replacements)
diff --git a/artiq/transforms/tools.py b/artiq/transforms/tools.py
index 8a7b4d08e..45a4bd2c0 100644
--- a/artiq/transforms/tools.py
+++ b/artiq/transforms/tools.py
@@ -84,18 +84,34 @@ def eval_constant(node):
 _replaceable_funcs = {
     "bool", "int", "float", "round",
     "int64", "round64", "Fraction",
+    "time_to_cycles", "cycles_to_time",
     "Quantity"
 }
 
 
-def is_replaceable(expr):
+def _is_ref_transparent(dependencies, expr):
     if isinstance(expr, (ast.NameConstant, ast.Num, ast.Str)):
         return True
+    elif isinstance(expr, ast.Name):
+        dependencies.add(expr.id)
+        return True
+    elif isinstance(expr, ast.UnaryOp):
+        return _is_ref_transparent(dependencies, expr.operand)
     elif isinstance(expr, ast.BinOp):
-        return is_replaceable(expr.left) and is_replaceable(expr.right)
+        return (_is_ref_transparent(dependencies, expr.left)
+            and _is_ref_transparent(dependencies, expr.right))
     elif isinstance(expr, ast.BoolOp):
-        return all(is_replaceable(v) for v in expr.values)
+        return all(_is_ref_transparent(dependencies, v) for v in expr.values)
     elif isinstance(expr, ast.Call) and isinstance(expr.func, ast.Name):
-        return expr.func.id in _replaceable_funcs
+        return (expr.func.id in _replaceable_funcs and
+            all(_is_ref_transparent(dependencies, arg) for arg in expr.args))
     else:
         return False
+
+
+def is_ref_transparent(expr):
+    dependencies = set()
+    if _is_ref_transparent(dependencies, expr):
+        return True, dependencies
+    else:
+        return False, None