diff --git a/artiq/compiler/transforms/interleaver.py b/artiq/compiler/transforms/interleaver.py
index 282c84534..873e55eac 100644
--- a/artiq/compiler/transforms/interleaver.py
+++ b/artiq/compiler/transforms/interleaver.py
@@ -5,6 +5,7 @@ the timestamp would always monotonically nondecrease.
 
 from .. import types, builtins, ir, iodelay
 from ..analyses import domination
+from ..algorithms import inline
 
 def delay_free_subgraph(root, limit):
     visited = set()
@@ -25,9 +26,24 @@ def delay_free_subgraph(root, limit):
 
     return True
 
+def iodelay_of_block(block):
+    terminator = block.terminator()
+    if isinstance(terminator, ir.Delay):
+        # We should be able to fold everything without free variables.
+        folded_expr = terminator.expr.fold()
+        assert iodelay.is_const(folded_expr)
+        return folded_expr.value
+    else:
+        return 0
+
 def is_pure_delay(insn):
     return isinstance(insn, ir.Builtin) and insn.op in ("delay", "delay_mu")
 
+def is_impure_delay_block(block):
+    terminator = block.terminator()
+    return isinstance(terminator, ir.Delay) and \
+            not is_pure_delay(terminator.decomposition())
+
 class Interleaver:
     def __init__(self, engine):
         self.engine = engine
@@ -64,25 +80,24 @@ class Interleaver:
             source_times  = [0 for _ in source_blocks]
 
             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.
-                        folded_expr = terminator.expr.fold()
-                        assert iodelay.is_const(folded_expr)
-                        return folded_expr.value
-                    else:
-                        return 0
-
                 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)
+                # Always prefer impure blocks (with calls) to pure blocks, because
+                # impure blocks may expand with smaller delays appearing, and in
+                # case of a tie, if a pure block is preferred, this would violate
+                # the timeline monotonicity.
+                available_source_blocks = list(filter(is_impure_delay_block, source_blocks))
+                if not any(available_source_blocks):
+                    available_source_blocks = source_blocks
+
+                index, source_block = min(enumerate(available_source_blocks), key=time_after_block)
                 source_block_delay  = iodelay_of_block(source_block)
 
                 new_target_time   = source_times[index] + source_block_delay
                 target_time_delta = new_target_time - target_time
+                assert target_time_delta >= 0
 
                 target_terminator = target_block.terminator()
                 if isinstance(target_terminator, ir.Parallel):
@@ -93,42 +108,32 @@ class Interleaver:
 
                 source_terminator = source_block.terminator()
 
-                if isinstance(source_terminator, ir.Delay):
-                    old_decomp = source_terminator.decomposition()
-                else:
-                    old_decomp = None
-
-                if target_time_delta > 0:
-                    assert isinstance(source_terminator, ir.Delay)
-
-                    if is_pure_delay(old_decomp):
-                        new_decomp_expr = ir.Constant(int(target_time_delta), builtins.TInt64())
-                        new_decomp = ir.Builtin("delay_mu", [new_decomp_expr], builtins.TNone())
-                        new_decomp.loc = old_decomp.loc
-                        source_terminator.basic_block.insert(source_terminator, new_decomp)
-                    else: # It's a call.
-                        need_to_inline = False
-                        for other_source_block in filter(lambda block: block != source_block,
-                                                         source_blocks):
-                            other_source_terminator = other_source_block.terminator()
-                            if not (is_pure_delay(other_source_terminator.decomposition()) and \
-                                    iodelay.is_const(other_source_terminator.expr) and \
-                                    other_source_terminator.expr.fold().value >= source_block_delay):
-                                need_to_inline = True
-                                break
-
-                        if need_to_inline:
-                            assert False
-                        else:
-                            old_decomp, new_decomp = None, old_decomp
-
-                    source_terminator.replace_with(ir.Delay(iodelay.Const(target_time_delta), {},
-                                                            new_decomp, source_terminator.target()))
-                else:
+                if not isinstance(source_terminator, ir.Delay):
                     source_terminator.replace_with(ir.Branch(source_terminator.target()))
+                else:
+                    old_decomp = source_terminator.decomposition()
+                    if is_pure_delay(old_decomp):
+                        if target_time_delta > 0:
+                            new_decomp_expr = ir.Constant(int(target_time_delta), builtins.TInt64())
+                            new_decomp = ir.Builtin("delay_mu", [new_decomp_expr], builtins.TNone())
+                            new_decomp.loc = old_decomp.loc
 
-                if old_decomp is not None:
-                    old_decomp.erase()
+                            source_terminator.basic_block.insert(new_decomp, before=source_terminator)
+                            source_terminator.expr = iodelay.Const(target_time_delta)
+                            source_terminator.set_decomposition(new_decomp)
+                        else:
+                            source_terminator.replace_with(ir.Branch(source_terminator.target()))
+                        old_decomp.erase()
+                    else: # It's a call.
+                        need_to_inline = len(source_blocks) > 1
+                        if need_to_inline:
+                            inline(old_decomp)
+                            postdom_tree = domination.PostDominatorTree(func)
+                            continue
+                        elif target_time_delta > 0:
+                            source_terminator.expr = iodelay.Const(target_time_delta)
+                        else:
+                            source_terminator.replace_with(ir.Branch(source_terminator.target()))
 
                 target_block = source_block
                 target_time  = new_target_time