Modify and fix CliffordResynthesis pass (#1476)

pytket/binders/passes.cpp

@@ -846,8 +846,8 @@ PYBIND11_MODULE(passes, m) {
 
   m.def(
       "CliffordResynthesis", &gen_clifford_resynthesis_pass,
-      "An optimisation pass that resynhesises all Clifford subcircuits and "
-      "then applies some rewrite rules to simplify them further."
+      "An optimisation pass that resynthesises Clifford subcircuits, trying "
+      "to reduce the 2-qubit gate count as much as possible."
       "\n\n:param transform: optional user-provided resynthesis method to "
       "apply to all Clifford subcircuits (a function taking a Clifford "
       "circuit as an argument and returning an equivalent circuit); if not "

pytket/conanfile.py

@@ -32,7 +32,7 @@ def package(self):
         cmake.install()
 
     def requirements(self):
-        self.requires("tket/1.3.12@tket/stable")
+        self.requires("tket/1.3.13@tket/stable")
         self.requires("tklog/0.3.3@tket/stable")
         self.requires("tkrng/0.3.3@tket/stable")
         self.requires("tkassert/0.3.4@tket/stable")

pytket/docs/changelog.rst

@@ -9,6 +9,7 @@ Unreleased
 * Add `PauliSynthStrat.Greedy` strategy to `TermSequenceBox`.
 * Update version requirements on dependencies, removing all upper bounds (in
   particular allowing compatibility with numpy 2.0).
+* Fix bug in `CliffordResynthesis()` pass.
 
 1.29.2 (June 2024)
 ------------------

pytket/pytket/_tket/passes.pyi

@@ -250,7 +250,7 @@ def CliffordPushThroughMeasures() -> BasePass:
     """
 def CliffordResynthesis(transform: typing.Callable[[pytket._tket.circuit.Circuit], pytket._tket.circuit.Circuit] | None = None, allow_swaps: bool = True) -> BasePass:
     """
-    An optimisation pass that resynhesises all Clifford subcircuits and then applies some rewrite rules to simplify them further.
+    An optimisation pass that resynthesises Clifford subcircuits, trying to reduce the 2-qubit gate count as much as possible.
     
     :param transform: optional user-provided resynthesis method to apply to all Clifford subcircuits (a function taking a Clifford circuit as an argument and returning an equivalent circuit); if not provided, a default resynthesis method is applied
     :param allow_swaps: whether the rewriting may introduce wire swaps (only relevant to the default resynthesis method used when the `transform` argument is not provided)

tket/conanfile.py

@@ -23,7 +23,7 @@
 
 class TketConan(ConanFile):
     name = "tket"
-    version = "1.3.12"
+    version = "1.3.13"
     package_type = "library"
     license = "Apache 2"
     homepage = "https://github.com/CQCL/tket"

tket/src/Transformations/CliffordResynthesis.cpp

@@ -14,7 +14,11 @@
 
 #include "tket/Transformations/CliffordResynthesis.hpp"
 
+#include <algorithm>
+#include <vector>
+
 #include "tket/Circuit/Circuit.hpp"
+#include "tket/Circuit/DAGDefs.hpp"
 #include "tket/Clifford/UnitaryTableau.hpp"
 #include "tket/Converters/Converters.hpp"
 #include "tket/Transformations/BasicOptimisation.hpp"
@@ -27,19 +31,20 @@ namespace tket {
 namespace Transforms {
 
 static Circuit resynthesised_circuit(
-    Circuit &circ,
+    const Circuit &circ,
     std::optional<std::function<Circuit(const Circuit &)>> transform = nullptr,
     bool allow_swaps = true) {
   if (transform.has_value()) {
     return (*transform)(circ);
   } else {
+    Circuit circ1(circ);
     // Ensure all recognized Clifford gates are Clifford types:
-    decompose_multi_qubits_CX().apply(circ);
-    decompose_cliffords_std().apply(circ);
-    remove_redundancies().apply(circ);
+    decompose_multi_qubits_CX().apply(circ1);
+    decompose_cliffords_std().apply(circ1);
+    remove_redundancies().apply(circ1);
 
     // Convert to tableau and back:
-    const UnitaryTableau tab = circuit_to_unitary_tableau(circ);
+    const UnitaryTableau tab = circuit_to_unitary_tableau(circ1);
     Circuit newcirc = unitary_tableau_to_circuit(tab);
 
     // Simplify:
@@ -49,18 +54,46 @@ static Circuit resynthesised_circuit(
   }
 }
 
+// Returns the two-qubit-gate reduction resulting from resynthesising the given
+// (convex, connected) vertex set using the given transform.
+static int n_2q_reduction(
+    const Circuit &circ, const VertexSet &verts,
+    std::optional<std::function<Circuit(const Circuit &)>> transform,
+    bool allow_swaps) {
+  Subcircuit subcircuit = circ.make_subcircuit(verts);
+  const Circuit subc = circ.subcircuit(subcircuit);
+  Circuit newsubc = resynthesised_circuit(subc, transform, allow_swaps);
+  return int(subc.count_n_qubit_gates(2)) - int(newsubc.count_n_qubit_gates(2));
+}
+
 static bool resynthesise_cliffords(
     Circuit &circ,
     std::optional<std::function<Circuit(const Circuit &)>> transform = nullptr,
     bool allow_swaps = true) {
   bool changed = false;
-  for (const VertexSet &verts :
-       circ.get_subcircuits([](Op_ptr op) { return op->is_clifford(); })) {
+  while (true) {
+    std::vector<VertexSet> subcircuits =
+        circ.get_subcircuits([](Op_ptr op) { return op->is_clifford(); });
+    if (subcircuits.empty()) {
+      break;
+    }
+    // Pick the one that reduces the 2q gate count the most.
+    VertexSet verts = *std::max_element(
+        subcircuits.begin(), subcircuits.end(),
+        [&circ, &transform, &allow_swaps](
+            const VertexSet &A, const VertexSet &B) {
+          return n_2q_reduction(circ, A, transform, allow_swaps) <
+                 n_2q_reduction(circ, B, transform, allow_swaps);
+        });
     Subcircuit subcircuit = circ.make_subcircuit(verts);
-    Circuit subc = circ.subcircuit(subcircuit);
+    const Circuit subc = circ.subcircuit(subcircuit);
     Circuit newsubc = resynthesised_circuit(subc, transform, allow_swaps);
-    circ.substitute(newsubc, subcircuit);
-    changed = true;
+    if (newsubc.count_n_qubit_gates(2) < subc.count_n_qubit_gates(2)) {
+      circ.substitute(newsubc, subcircuit);
+      changed = true;
+    } else {
+      break;
+    }
   }
   return changed;
 }

tket/test/src/Passes/test_CliffordResynthesis.cpp

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <algorithm>
 #include <catch2/catch_test_macros.hpp>
+#include <vector>
 
 #include "../testutil.hpp"
 #include "tket/Circuit/Circuit.hpp"
@@ -23,7 +25,7 @@
 namespace tket {
 namespace test_CliffordResynthesis {
 
-void check_clifford_resynthesis(const Circuit& c) {
+void check_clifford_resynthesis(const Circuit &c) {
   Circuit c0 = c;
   CompilationUnit cu(c0);
   gen_clifford_resynthesis_pass()->apply(cu);
@@ -120,6 +122,28 @@ SCENARIO("SynthesiseCliffordResynthesis correctness") {
     c.add_op<unsigned>(OpType::NPhasedX, {0.5, 0.0}, {0, 1});
     check_clifford_resynthesis(c);
   }
+
+  GIVEN("A troublesome circuit (3)") {
+    // https://github.com/CQCL/tket/issues/1468
+    Circuit c0(6);
+    c0.add_op<unsigned>(OpType::XXPhase, 0.5, {1, 4});
+    c0.add_op<unsigned>(OpType::XXPhase, 1.5, {2, 3});
+    c0.add_op<unsigned>(OpType::XXPhase, 2.5, {1, 3});
+    c0.add_op<unsigned>(OpType::YYPhase, 0.5, {4, 5});
+    c0.add_op<unsigned>(OpType::YYPhase, 1.5, {4, 2});
+    c0.add_op<unsigned>(OpType::YYPhase, 2.5, {3, 1});
+    c0.add_op<unsigned>(OpType::ZZPhase, 0.5, {0, 3});
+    c0.add_op<unsigned>(OpType::ZZPhase, 1.5, {4, 1});
+    c0.add_op<unsigned>(OpType::ZZPhase, 2.5, {0, 5});
+    CompilationUnit cu(c0);
+    gen_clifford_resynthesis_pass()->apply(cu);
+    Circuit c1 = cu.get_circ_ref();
+    std::vector<Command> cmds = c1.get_commands();
+    CHECK(std::all_of(cmds.begin(), cmds.end(), [](const Command &cmd) {
+      return cmd.get_op_ptr()->is_clifford();
+    }));
+    CHECK(c1.count_n_qubit_gates(2) <= c0.count_n_qubit_gates(2));
+  }
 }
 
 }  // namespace test_CliffordResynthesis