Add option collect_from_back to CollectMultiQBlocks (#13612)

* adding option collect_from_back

* new option, tests, reno

* typo

* improving test following review

* test fix

qiskit/transpiler/passes/optimization/collect_multiqubit_blocks.py

@@ -33,13 +33,18 @@ class CollectMultiQBlocks(AnalysisPass):
     Some gates may not be present in any block (e.g. if the number
     of operands is greater than ``max_block_size``)
 
+    By default, blocks are collected in the direction from the inputs towards the
+    outputs of the DAG. The option ``collect_from_back`` allows to change this
+    direction, that is to collect blocks from the outputs towards the inputs.
+    Note that the blocks are still reported in a valid topological order.
+
     A Disjoint Set Union data structure (DSU) is used to maintain blocks as
     gates are processed. This data structure points each qubit to a set at all
     times and the sets correspond to current blocks. These change over time
     and the data structure allows these changes to be done quickly.
     """
 
-    def __init__(self, max_block_size=2):
+    def __init__(self, max_block_size=2, collect_from_back=False):
         super().__init__()
         self.parent = {}  # parent array for the union
 
@@ -49,6 +54,7 @@ def __init__(self, max_block_size=2):
         self.gate_groups = {}  # current gate lists for the groups
 
         self.max_block_size = max_block_size  # maximum block size
+        self.collect_from_back = collect_from_back  # backward collection
 
     def find_set(self, index):
         """DSU function for finding root of set of items
@@ -127,6 +133,10 @@ def collect_key(x):
 
         op_nodes = dag.topological_op_nodes(key=collect_key)
 
+        # When collecting from the back, the order of nodes is reversed
+        if self.collect_from_back:
+            op_nodes = reversed(list(op_nodes))
+
         for nd in op_nodes:
             can_process = True
             makes_too_big = False
@@ -222,6 +232,11 @@ def collect_key(x):
             if item == index and len(self.gate_groups[index]) != 0:
                 block_list.append(self.gate_groups[index][:])
 
+        # When collecting from the back, both the order of the blocks
+        # and the order of nodes in each block should be reversed.
+        if self.collect_from_back:
+            block_list = [block[::-1] for block in block_list[::-1]]
+
         self.property_set["block_list"] = block_list
 
         return dag

releasenotes/notes/add-option-collect-from-back-cde10ee5e2e4ea9f.yaml

@@ -0,0 +1,11 @@
+---
+features_transpiler:
+  - |
+    Added a new option, ``collect_from_back``, to 
+    :class:`~qiskit.transpiler.passes.CollectMultiQBlocks`.
+    When set to ``True``, the blocks are collected in the reverse direction, 
+    from the outputs towards the inputs of the circuit. The blocks are still
+    reported following the normal topological order. 
+    This leads to an additional flexibility provided by the pass, and 
+    additional optimization opportunities when combined with a circuit 
+    resynthesis method.

test/python/transpiler/test_collect_multiq_blocks.py

@@ -290,6 +290,46 @@ def test_larger_blocks(self):
 
         pass_manager.run(qc)
 
+    def test_collect_from_back(self):
+        """Test the option to collect blocks from the outputs towards
+        the inputs.
+             ┌───┐
+        q_0: ┤ H ├──■────■────■───────
+             └───┘┌─┴─┐  │    │
+        q_1: ─────┤ X ├──┼────┼───────
+                  └───┘┌─┴─┐  │
+        q_2: ──────────┤ X ├──┼───────
+                       └───┘┌─┴─┐┌───┐
+        q_3: ───────────────┤ X ├┤ H ├
+                            └───┘└───┘
+        """
+        qc = QuantumCircuit(4)
+        qc.h(0)
+        qc.cx(0, 1)
+        qc.cx(0, 2)
+        qc.cx(0, 3)
+        qc.h(3)
+
+        dag = circuit_to_dag(qc)
+        # For the circuit above, the topological order is unique
+        topo_ops = list(dag.topological_op_nodes())
+
+        # When collecting blocks of size-3 using the default direction,
+        # the first block should contain the H-gate and two CX-gates,
+        # and the second block should contain a single CX-gate and an H-gate.
+        pass_ = CollectMultiQBlocks(max_block_size=3, collect_from_back=False)
+        pass_.run(dag)
+        expected_blocks = [[topo_ops[0], topo_ops[1], topo_ops[2]], [topo_ops[3], topo_ops[4]]]
+        self.assertEqual(pass_.property_set["block_list"], expected_blocks)
+
+        # When collecting blocks of size-3 using the opposite direction,
+        # the first block should contain the H-gate and a single CX-gate,
+        # and the second block should contain two CX-gates and an H-gate.
+        pass_ = CollectMultiQBlocks(max_block_size=3, collect_from_back=True)
+        pass_.run(dag)
+        expected_blocks = [[topo_ops[0], topo_ops[1]], [topo_ops[2], topo_ops[3], topo_ops[4]]]
+        self.assertEqual(pass_.property_set["block_list"], expected_blocks)
+
 
 if __name__ == "__main__":
     unittest.main()