diff --git a/artiq/compiler/transforms/interleaver.py b/artiq/compiler/transforms/interleaver.py
index ea2097ad2..f737d839c 100644
--- a/artiq/compiler/transforms/interleaver.py
+++ b/artiq/compiler/transforms/interleaver.py
@@ -45,55 +45,57 @@ class Interleaver:
 
         postdom_tree = None
         for insn in func.instructions():
-            if isinstance(insn, ir.Parallel):
-                # Lazily compute dominators.
-                if postdom_tree is None:
-                    postdom_tree = domination.PostDominatorTree(func)
+            if not isinstance(insn, ir.Parallel):
+                continue
 
-                interleave_until = postdom_tree.immediate_dominator(insn.basic_block)
-                assert (interleave_until is not None) # no nonlocal flow in `with parallel`
+            # Lazily compute dominators.
+            if postdom_tree is None:
+                postdom_tree = domination.PostDominatorTree(func)
 
-                target_block  = insn.basic_block
-                target_time   = 0
-                source_blocks = insn.basic_block.successors()
-                source_times  = [0 for _ in source_blocks]
+            interleave_until = postdom_tree.immediate_dominator(insn.basic_block)
+            assert (interleave_until is not None) # no nonlocal flow in `with parallel`
 
-                while len(source_blocks) > 0:
-                    def iodelay_of_block(block):
-                        terminator = block.terminator()
-                        if isinstance(terminator, ir.Delay):
-                            # We should be able to fold everything without free variables.
-                            assert iodelay.is_const(terminator.expr)
-                            return terminator.expr.value
-                        else:
-                            return 0
+            target_block  = insn.basic_block
+            target_time   = 0
+            source_blocks = insn.basic_block.successors()
+            source_times  = [0 for _ in source_blocks]
 
-                    def time_after_block(pair):
-                        index, block = pair
-                        return source_times[index] + iodelay_of_block(block)
-
-                    index, source_block = min(enumerate(source_blocks), key=time_after_block)
-                    source_block_delay  = iodelay_of_block(source_block)
-
-                    target_terminator = target_block.terminator()
-                    if isinstance(target_terminator, (ir.Delay, ir.Branch)):
-                        target_terminator.set_target(source_block)
-                    elif isinstance(target_terminator, ir.Parallel):
-                        target_terminator.replace_with(ir.Branch(source_block))
+            while len(source_blocks) > 0:
+                def iodelay_of_block(block):
+                    terminator = block.terminator()
+                    if isinstance(terminator, ir.Delay):
+                        # We should be able to fold everything without free variables.
+                        assert iodelay.is_const(terminator.expr)
+                        return terminator.expr.value
                     else:
-                        assert False
+                        return 0
 
-                    new_source_block = postdom_tree.immediate_dominator(source_block)
-                    assert (new_source_block is not None)
-                    assert delay_free_subgraph(source_block, new_source_block)
+                def time_after_block(pair):
+                    index, block = pair
+                    return source_times[index] + iodelay_of_block(block)
 
-                    target_block          = source_block
-                    target_time          += source_block_delay
+                index, source_block = min(enumerate(source_blocks), key=time_after_block)
+                source_block_delay  = iodelay_of_block(source_block)
 
-                    if new_source_block == interleave_until:
-                        # We're finished with this branch.
-                        del source_blocks[index]
-                        del source_times[index]
-                    else:
-                        source_blocks[index] = new_source_block
-                        source_times[index]  = target_time
+                target_terminator = target_block.terminator()
+                if isinstance(target_terminator, (ir.Delay, ir.Branch)):
+                    target_terminator.set_target(source_block)
+                elif isinstance(target_terminator, ir.Parallel):
+                    target_terminator.replace_with(ir.Branch(source_block))
+                else:
+                    assert False
+
+                target_block  = source_block
+                target_time  += source_block_delay
+
+                new_source_block = postdom_tree.immediate_dominator(source_block)
+                assert (new_source_block is not None)
+                assert delay_free_subgraph(source_block, new_source_block)
+
+                if new_source_block == interleave_until:
+                    # We're finished with this branch.
+                    del source_blocks[index]
+                    del source_times[index]
+                else:
+                    source_blocks[index] = new_source_block
+                    source_times[index]  = target_time