Avoid blow-up of complexity when squashing symbolic circuits (#969)

pytket/binders/passes.cpp

@@ -504,8 +504,13 @@ PYBIND11_MODULE(passes, m) {
       "already in this set."
       "\n:param tk1_replacement: A function which, given the parameters of "
       "an Rz(a)Rx(b)Rz(c) triple, returns an equivalent circuit in the "
-      "desired basis.",
-      py::arg("singleqs"), py::arg("tk1_replacement"));
+      "desired basis."
+      "\n:param always_squash_symbols: If true, always squash symbolic gates "
+      "regardless of the blow-up in complexity. Default is false, meaning that "
+      "symbolic gates are only squashed if doing so reduces the overall "
+      "symbolic complexity.",
+      py::arg("singleqs"), py::arg("tk1_replacement"),
+      py::arg("always_squash_symbols") = false);
   m.def(
       "DelayMeasures", &DelayMeasures,
       "Commutes Measure operations to the end of the circuit. Throws an "

pytket/conanfile.py

@@ -32,7 +32,7 @@ def package(self):
         cmake.install()
 
     def requirements(self):
-        self.requires("tket/1.2.33@tket/stable")
+        self.requires("tket/1.2.34@tket/stable")
         self.requires("tklog/0.3.3@tket/stable")
         self.requires("tkrng/0.3.3@tket/stable")
         self.requires("tkassert/0.3.3@tket/stable")

pytket/docs/changelog.rst

@@ -8,6 +8,10 @@ Minor new features:
 
 * Implement equality checking for all boxes.
 * Add ``Op.is_clifford`` to python binding.
+* Single-qubit squashing ignores chains of symbolic gates if squashing them
+  would increase the overall complexity of the expressions. This behaviour can
+  be overridden using the ``always_squash_symbols`` parameter to
+  ``SquashCustom``.
 
 1.18.0 (August 2023)
 --------------------

schemas/compiler_pass_v1.json

@@ -183,6 +183,10 @@
           "type": "string",
           "description": "A method for generating optimised single-qubit unitary circuits in a target gate set. This string should be interpreted by Python \"dill\" into a function. Used in \"RebaseCustom\" and \"SquashCustom\"."
         },
+        "always_squash_symbols": {
+          "type": "boolean",
+          "description": "Whether to always squash symbolic gates regardless of the complexity blow-up. Used in \"SquashCustom\"."
+        },
         "euler_p": {
           "type": "string",
           "description": "The choice of P rotation for \"EulerAngleReduction\" for P-Q-P and Q-P-Q triples.",
@@ -351,9 +355,10 @@
           "then": {
             "required": [
               "basis_singleqs",
-              "basis_tk1_replacement"
+              "basis_tk1_replacement",
+              "always_squash_symbols"
             ],
-            "maxProperties": 3
+            "maxProperties": 4
           }
         },
         {

tket/conanfile.py

@@ -23,7 +23,7 @@
 
 class TketConan(ConanFile):
     name = "tket"
-    version = "1.2.33"
+    version = "1.2.34"
     package_type = "library"
     license = "Apache 2"
     homepage = "https://github.com/CQCL/tket"

tket/include/tket/Predicates/PassGenerators.hpp

@@ -51,7 +51,8 @@ PassPtr gen_rebase_pass_via_tk2(
 PassPtr gen_squash_pass(
     const OpTypeSet& singleqs,
     const std::function<Circuit(const Expr&, const Expr&, const Expr&)>&
-        tk1_replacement);
+        tk1_replacement,
+    bool always_squash_symbols = false);
 PassPtr gen_euler_pass(const OpType& q, const OpType& p, bool strict = false);
 PassPtr gen_clifford_simp_pass(bool allow_swaps = true);
 

tket/include/tket/Transformations/BasicOptimisation.hpp

@@ -110,7 +110,8 @@ Transform squash_1qb_to_pqp(
 Transform squash_factory(
     const OpTypeSet& singleqs,
     const std::function<Circuit(const Expr&, const Expr&, const Expr&)>&
-        tk1_replacement);
+        tk1_replacement,
+    bool always_squash_symbols = false);
 
 // commutes single qubit gates through SWAP gates, leaving them on the
 // PhysicalQubit with best fidelity for the given OP Expects: any single qubit

tket/include/tket/Transformations/SingleQubitSquash.hpp

@@ -96,18 +96,20 @@ class SingleQubitSquash {
    * @param circ The circuit to be squashed.
    * @param reversed Whether squashing is made back to front or front to back
    *      (default: false, ie front to back).
+   * @param always_squash_symbols Whether to squash symbolic gates regardless of
+   *      the complexity blow-up (default: false, i.e. symbolic gates are only
+   *      squashed if the overall complexity of the expressions is reduced).
    */
   SingleQubitSquash(
       std::unique_ptr<AbstractSquasher> squasher, Circuit &circ,
-      bool reversed = false)
-      : squasher_(std::move(squasher)), circ_(circ), reversed_(reversed) {}
+      bool reversed = false, bool always_squash_symbols = false);
 
   // rule of 5
   SingleQubitSquash(const SingleQubitSquash &other);
-  SingleQubitSquash &operator=(const SingleQubitSquash &other);
+  SingleQubitSquash &operator=(const SingleQubitSquash &other) = delete;
   ~SingleQubitSquash() = default;
-  SingleQubitSquash(SingleQubitSquash &&other);
-  SingleQubitSquash &operator=(SingleQubitSquash &&other);
+  SingleQubitSquash(SingleQubitSquash &&other) = delete;
+  SingleQubitSquash &operator=(SingleQubitSquash &&other) = delete;
 
   /**
    * @brief Squash entire circuit, one qubit at a time.
@@ -135,6 +137,7 @@ class SingleQubitSquash {
   std::unique_ptr<AbstractSquasher> squasher_;
   Circuit &circ_;
   bool reversed_;
+  bool always_squash_symbols_;
 
   // substitute chain by a sub circuit, handling conditions
   // and backing up + restoring current edge

tket/src/Predicates/PassGenerators.cpp

@@ -107,8 +107,10 @@ PassPtr gen_rebase_pass_via_tk2(
 PassPtr gen_squash_pass(
     const OpTypeSet& singleqs,
     const std::function<Circuit(const Expr&, const Expr&, const Expr&)>&
-        tk1_replacement) {
-  Transform t = Transforms::squash_factory(singleqs, tk1_replacement);
+        tk1_replacement,
+    bool always_squash_symbols) {
+  Transform t = Transforms::squash_factory(
+      singleqs, tk1_replacement, always_squash_symbols);
   PostConditions postcon = {{}, {}, Guarantee::Preserve};
   PredicatePtrMap precons;
   // record pass config
@@ -117,6 +119,7 @@ PassPtr gen_squash_pass(
   j["basis_singleqs"] = singleqs;
   j["basis_tk1_replacement"] =
       "SERIALIZATION OF FUNCTIONS IS NOT YET SUPPORTED";
+  j["always_squash_symbols"] = always_squash_symbols;
   return std::make_shared<StandardPass>(precons, t, postcon, j);
 }
 

tket/src/Transformations/SingleQubitSquash.cpp

@@ -14,37 +14,30 @@
 
 #include "tket/Transformations/SingleQubitSquash.hpp"
 
+#include <cstddef>
+#include <numeric>
+
+#include "Circuit/Command.hpp"
+#include "Gate/GatePtr.hpp"
 #include "tket/Circuit/Circuit.hpp"
 #include "tket/Circuit/DAGDefs.hpp"
 #include "tket/Gate/Gate.hpp"
 
 namespace tket {
 
+SingleQubitSquash::SingleQubitSquash(
+    std::unique_ptr<AbstractSquasher> squasher, Circuit &circ, bool reversed,
+    bool always_squash_symbols)
+    : squasher_(std::move(squasher)),
+      circ_(circ),
+      reversed_(reversed),
+      always_squash_symbols_(always_squash_symbols) {}
+
 SingleQubitSquash::SingleQubitSquash(const SingleQubitSquash &other)
     : squasher_(other.squasher_->clone()),
       circ_(other.circ_),
-      reversed_(other.reversed_) {}
-
-SingleQubitSquash &SingleQubitSquash::operator=(
-    const SingleQubitSquash &other) {
-  squasher_ = other.squasher_->clone();
-  circ_ = other.circ_;
-  reversed_ = other.reversed_;
-  return *this;
-}
-
-SingleQubitSquash::SingleQubitSquash(SingleQubitSquash &&other)
-    : squasher_(std::move(other.squasher_)),
-      circ_(other.circ_),
-      reversed_(other.reversed_) {}
-
-SingleQubitSquash &SingleQubitSquash::operator=(SingleQubitSquash &&other) {
-  squasher_ = std::move(other.squasher_);
-  circ_ = other.circ_;
-  reversed_ = other.reversed_;
-  return *this;
-}
-
+      reversed_(other.reversed_),
+      always_squash_symbols_(other.always_squash_symbols_) {}
 bool SingleQubitSquash::squash() {
   bool success = false;
 
@@ -199,8 +192,25 @@ void SingleQubitSquash::insert_left_over_gate(
 bool SingleQubitSquash::sub_is_better(
     const Circuit &sub, const std::vector<Gate_ptr> chain) const {
   const unsigned n_gates = sub.n_gates();
-  return n_gates < chain.size() ||
-         (n_gates == chain.size() && !is_equal(sub, chain, reversed_));
+  if (n_gates > chain.size()) {
+    return false;
+  }
+  if (!sub.is_symbolic() || always_squash_symbols_) {
+    return n_gates < chain.size() || !is_equal(sub, chain, reversed_);
+  }
+  // For symbolic circuits, we don't want to squash gates if it blows up the
+  // complexity of the expressions. As a crude but adequate measure, we compare
+  // the total size of the string representations.
+  return std::accumulate(
+             sub.begin(), sub.end(), std::size_t{0},
+             [](std::size_t a, const Command &cmd) {
+               return a + cmd.get_op_ptr()->get_name().size();
+             }) <
+         std::accumulate(
+             chain.begin(), chain.end(), std::size_t{0},
+             [](std::size_t a, const Gate_ptr &gpr) {
+               return a + gpr->get_name().size();
+             });
 }
 
 // returns a description of the condition of current vertex

tket/src/Transformations/StandardSquash.cpp

@@ -83,17 +83,22 @@ std::unique_ptr<AbstractSquasher> StandardSquasher::clone() const {
 static bool standard_squash(
     Circuit &circ, const OpTypeSet &singleqs,
     const std::function<Circuit(const Expr &, const Expr &, const Expr &)>
-        &tk1_replacement) {
+        &tk1_replacement,
+    bool always_squash_symbols) {
   auto squasher = std::make_unique<StandardSquasher>(singleqs, tk1_replacement);
-  return SingleQubitSquash(std::move(squasher), circ, false).squash();
+  return SingleQubitSquash(
+             std::move(squasher), circ, false, always_squash_symbols)
+      .squash();
 }
 
 Transform squash_factory(
     const OpTypeSet &singleqs,
     const std::function<Circuit(const Expr &, const Expr &, const Expr &)>
-        &tk1_replacement) {
+        &tk1_replacement,
+    bool always_squash_symbols) {
   return Transform([=](Circuit &circ) {
-    return standard_squash(circ, singleqs, tk1_replacement);
+    return standard_squash(
+        circ, singleqs, tk1_replacement, always_squash_symbols);
   });
 }
 

tket/test/src/Circuit/test_Symbolic.cpp

@@ -17,6 +17,7 @@
 #include <tket/Transformations/BasicOptimisation.hpp>
 #include <tket/Transformations/CliffordOptimisation.hpp>
 #include <tket/Transformations/OptimisationPass.hpp>
+#include <tket/Transformations/PQPSquash.hpp>
 #include <vector>
 
 #include "symengine/eval_double.h"
@@ -50,6 +51,8 @@ SCENARIO("Symbolic squashing, correctness") {
   Expr alpha(asym);
   Sym bsym = SymEngine::symbol("b");
   Expr beta(bsym);
+  Sym csym = SymEngine::symbol("c");
+  Expr gamma(csym);
 
   GIVEN("squash_1qb_to_pqp") {
     Circuit circ(1);
@@ -147,8 +150,8 @@ SCENARIO("Symbolic squashing, correctness") {
     symbol_map_t smap = {{asym, 0}, {bsym, 0}};
     circ.symbol_substitution(smap);
     std::vector<Command> cmds = circ.get_commands();
-    CHECK(cmds.size() == 1);
-    Op_ptr op = cmds[0].get_op_ptr();
+    CHECK(cmds.size() == 4);
+    Op_ptr op = cmds[1].get_op_ptr();
     CHECK(op->get_type() == OpType::TK1);
     std::vector<Expr> params = op->get_params();
     CHECK_NOTHROW(SymEngine::eval_double(params[0]));
@@ -169,8 +172,8 @@ SCENARIO("Symbolic squashing, correctness") {
     symbol_map_t smap = {{asym, 0}};
     circ.symbol_substitution(smap);
     std::vector<Command> cmds = circ.get_commands();
-    CHECK(cmds.size() == 1);
-    Op_ptr op = cmds[0].get_op_ptr();
+    CHECK(cmds.size() == 4);
+    Op_ptr op = cmds[1].get_op_ptr();
     CHECK(op->get_type() == OpType::TK1);
     std::vector<Expr> params = op->get_params();
     CHECK_NOTHROW(SymEngine::eval_double(params[0]));
@@ -179,6 +182,29 @@ SCENARIO("Symbolic squashing, correctness") {
     CHECK(approx_0(params[1]));
     CHECK(approx_0(params[0] + params[2]));
   }
+
+  GIVEN("Squashing where expressions are allowed to expand") {
+    auto squash_circuit = [](Circuit &c, bool always_squash_symbols) {
+      auto squasher =
+          std::make_unique<Transforms::PQPSquasher>(OpType::Ry, OpType::Rz);
+      return SingleQubitSquash(
+                 std::move(squasher), c, false, always_squash_symbols)
+          .squash();
+    };
+    Circuit circ0(1);
+    circ0.add_op<unsigned>(OpType::Rz, 0.5, {0});
+    circ0.add_op<unsigned>(OpType::Ry, 0.5, {0});
+    circ0.add_op<unsigned>(OpType::Rz, {alpha}, {0});
+    circ0.add_op<unsigned>(OpType::Ry, {beta}, {0});
+    circ0.add_op<unsigned>(OpType::Rz, {gamma}, {0});
+    Circuit circ1 = circ0;
+    CHECK_FALSE(squash_circuit(circ0, false));
+    CHECK(squash_circuit(circ1, true));
+    symbol_map_t smap = {{bsym, 0.3}, {csym, 0.4}};
+    circ0.symbol_substitution(smap);
+    circ1.symbol_substitution(smap);
+    check_equiv(circ0, circ1);
+  }
 }
 
 }  // namespace test_Symbolic

tket/test/src/test_CompilerPass.cpp

@@ -769,6 +769,16 @@ SCENARIO("PeepholeOptimise2Q and FullPeepholeOptimise") {
     CompilationUnit cu1(circ1);
     REQUIRE(FullPeepholeOptimise()->apply(cu1));
   }
+  GIVEN("Symbolic circuit, FullPeepholeOptimise TK2") {
+    // https://github.com/CQCL/tket/issues/963
+    Sym a = SymEngine::symbol("a");
+    Circuit circ(3);
+    circ.add_op<unsigned>(OpType::CX, {0, 1});
+    circ.add_op<unsigned>(OpType::Rz, Expr(a), {0});
+    circ.add_op<unsigned>(OpType::CX, {0, 2});
+    CompilationUnit cu(circ);
+    REQUIRE(FullPeepholeOptimise(true, OpType::TK2)->apply(cu));
+  }
   GIVEN("YYPhase") {
     // TKET-1302
     Circuit circ(2);