feat: bus updates (#7522)

Adds a second calldata column (secondary_calldata) to the databus (which now has 3 columns).
This is needed to support kernel circuits which will have two calldata
inputs (one from previous kernel, one from a function circuit).

barretenberg/cpp/src/barretenberg/circuit_checker/ultra_circuit_checker.cpp

@@ -169,15 +169,20 @@ template <typename Builder> bool UltraCircuitChecker::check_databus_read(auto& v
 
         // Determine the type of read based on selector values
         bool is_calldata_read = (values.q_l == 1);
-        bool is_return_data_read = (values.q_r == 1);
-        ASSERT(is_calldata_read || is_return_data_read);
+        bool is_secondary_calldata_read = (values.q_r == 1);
+        bool is_return_data_read = (values.q_o == 1);
+        ASSERT(is_calldata_read || is_secondary_calldata_read || is_return_data_read);
 
         // Check that the claimed value is present in the calldata/return data at the corresponding index
         FF bus_value;
         if (is_calldata_read) {
             auto calldata = builder.get_calldata();
             bus_value = builder.get_variable(calldata[raw_read_idx]);
         }
+        if (is_secondary_calldata_read) {
+            auto secondary_calldata = builder.get_secondary_calldata();
+            bus_value = builder.get_variable(secondary_calldata[raw_read_idx]);
+        }
         if (is_return_data_read) {
             auto return_data = builder.get_return_data();
             bus_value = builder.get_variable(return_data[raw_read_idx]);

barretenberg/cpp/src/barretenberg/relations/databus_lookup_relation.hpp

@@ -47,39 +47,53 @@ template <typename FF_> class DatabusLookupRelationImpl {
   public:
     using FF = FF_;
     static constexpr size_t LENGTH = 5;          // 1 + polynomial degree of this relation
-    static constexpr size_t NUM_BUS_COLUMNS = 2; // calldata, return data
+    static constexpr size_t NUM_BUS_COLUMNS = 3; // calldata, return data
+
+    static constexpr size_t INVERSE_SUBREL_LENGTH = 5; // deg + 1 of inverse correctness subrelation
+    static constexpr size_t LOOKUP_SUBREL_LENGTH = 5;  // deg + 1 of log-deriv lookup subrelation
 
     // Note: Inverse correctness subrelations are actually LENGTH-1; taking advantage would require additional work
     static constexpr std::array<size_t, NUM_BUS_COLUMNS * 2> SUBRELATION_PARTIAL_LENGTHS{
-        LENGTH, // inverse polynomial correctness subrelation
-        LENGTH, // log-derivative lookup argument subrelation
-        LENGTH, // inverse polynomial correctness subrelation
-        LENGTH  // log-derivative lookup argument subrelation
+        INVERSE_SUBREL_LENGTH, // inverse polynomial correctness subrelation (bus_idx 0)
+        LOOKUP_SUBREL_LENGTH,  // log-derivative lookup argument subrelation (bus_idx 0)
+        INVERSE_SUBREL_LENGTH, // inverse polynomial correctness subrelation (bus_idx 1)
+        LOOKUP_SUBREL_LENGTH,  // log-derivative lookup argument subrelation (bus_idx 1)
+        INVERSE_SUBREL_LENGTH, // inverse polynomial correctness subrelation (bus_idx 2)
+        LOOKUP_SUBREL_LENGTH   // log-derivative lookup argument subrelation (bus_idx 2)
     };
 
+    static constexpr size_t INVERSE_SUBREL_WITNESS_DEGREE = 4; // witness degree of inverse correctness subrelation
+    static constexpr size_t LOOKUP_SUBREL_WITNESS_DEGREE = 4;  // witness degree of log-deriv lookup subrelation
+
     /**
      * @brief For ZK-Flavors: Upper bound on the degrees of subrelations considered as polynomials only in witness
-polynomials,
-     * i.e. all selectors and public polynomials are treated as constants. The subrelation witness degree does not
-     * exceed the subrelation partial degree, which is given by LENGTH - 1 in this case.
+     * polynomials, i.e. all selectors and public polynomials are treated as constants. The subrelation witness degree
+     * does not exceed the subrelation partial degree, which is given by LENGTH - 1 in this case.
      */
     static constexpr std::array<size_t, NUM_BUS_COLUMNS * 2> SUBRELATION_WITNESS_DEGREES{
-        LENGTH - 1, // inverse polynomial correctness subrelation
-        LENGTH - 1, // log-derivative lookup argument subrelation
-        LENGTH - 1, // inverse polynomial correctness subrelation
-        LENGTH - 1  // log-derivative lookup argument subrelation
+        INVERSE_SUBREL_WITNESS_DEGREE, // inverse polynomial correctness subrelation (bus_idx 0)
+        LOOKUP_SUBREL_WITNESS_DEGREE,  // log-derivative lookup argument subrelation (bus_idx 0)
+        INVERSE_SUBREL_WITNESS_DEGREE, // inverse polynomial correctness subrelation (bus_idx 1)
+        LOOKUP_SUBREL_WITNESS_DEGREE,  // log-derivative lookup argument subrelation (bus_idx 1)
+        INVERSE_SUBREL_WITNESS_DEGREE, // inverse polynomial correctness subrelation (bus_idx 2)
+        LOOKUP_SUBREL_WITNESS_DEGREE   // log-derivative lookup argument subrelation (bus_idx 2)
     };
 
+    static constexpr bool INVERSE_SUBREL_LIN_INDEPENDENT = true; // to be satisfied independently at each row
+    static constexpr bool LOOKUP_SUBREL_LIN_INDEPENDENT = false; // to be satisfied as a sum across all rows
+
     // The lookup subrelations are "linearly dependent" in the sense that they establish the value of a sum across the
     // entire execution trace rather than a per-row identity.
     static constexpr std::array<bool, NUM_BUS_COLUMNS* 2> SUBRELATION_LINEARLY_INDEPENDENT = {
-        true, false, true, false
+        INVERSE_SUBREL_LIN_INDEPENDENT, LOOKUP_SUBREL_LIN_INDEPENDENT,  INVERSE_SUBREL_LIN_INDEPENDENT,
+        LOOKUP_SUBREL_LIN_INDEPENDENT,  INVERSE_SUBREL_LIN_INDEPENDENT, LOOKUP_SUBREL_LIN_INDEPENDENT
     };
 
     template <typename AllEntities> inline static bool skip([[maybe_unused]] const AllEntities& in)
     {
         // Ensure the input does not contain a read gate or data that is being read
-        return in.q_busread.is_zero() && in.calldata_read_counts.is_zero() && in.return_data_read_counts.is_zero();
+        return in.q_busread.is_zero() && in.calldata_read_counts.is_zero() &&
+               in.secondary_calldata_read_counts.is_zero() && in.return_data_read_counts.is_zero();
     }
 
     // Interface for easy access of databus components by column (bus_idx)
@@ -95,10 +109,20 @@ polynomials,
         static auto& read_tags(const AllEntities& in) { return in.calldata_read_tags; }
     };
 
-    // Specialization for return data (bus_idx = 1)
+    // Specialization for secondary_calldata (bus_idx = 1)
     template <typename AllEntities> struct BusData</*bus_idx=*/1, AllEntities> {
-        static auto& values(const AllEntities& in) { return in.return_data; }
+        static auto& values(const AllEntities& in) { return in.secondary_calldata; }
         static auto& selector(const AllEntities& in) { return in.q_r; }
+        static auto& inverses(AllEntities& in) { return in.secondary_calldata_inverses; }
+        static auto& inverses(const AllEntities& in) { return in.secondary_calldata_inverses; } // const version
+        static auto& read_counts(const AllEntities& in) { return in.secondary_calldata_read_counts; }
+        static auto& read_tags(const AllEntities& in) { return in.secondary_calldata_read_tags; }
+    };
+
+    // Specialization for return data (bus_idx = 2)
+    template <typename AllEntities> struct BusData</*bus_idx=*/2, AllEntities> {
+        static auto& values(const AllEntities& in) { return in.return_data; }
+        static auto& selector(const AllEntities& in) { return in.q_o; }
         static auto& inverses(AllEntities& in) { return in.return_data_inverses; }
         static auto& inverses(const AllEntities& in) { return in.return_data_inverses; } // const version
         static auto& read_counts(const AllEntities& in) { return in.return_data_read_counts; }
@@ -217,8 +241,12 @@ polynomials,
                 is_read = q_busread == 1 && polynomials.q_l[i] == 1;
                 nonzero_read_count = polynomials.calldata_read_counts[i] > 0;
             }
-            if constexpr (bus_idx == 1) { // return data
+            if constexpr (bus_idx == 1) { // secondary_calldata
                 is_read = q_busread == 1 && polynomials.q_r[i] == 1;
+                nonzero_read_count = polynomials.secondary_calldata_read_counts[i] > 0;
+            }
+            if constexpr (bus_idx == 2) { // return data
+                is_read = q_busread == 1 && polynomials.q_o[i] == 1;
                 nonzero_read_count = polynomials.return_data_read_counts[i] > 0;
             }
             // We only compute the inverse if this row contains a read gate or data that has been read

barretenberg/cpp/src/barretenberg/stdlib/honk_recursion/verifier/protogalaxy_recursive_verifier.cpp

@@ -69,10 +69,10 @@ void ProtoGalaxyRecursiveVerifier_<VerifierInstances>::receive_and_finalise_inst
 
     // If Goblin (i.e. using DataBus) receive commitments to log-deriv inverses polynomial
     if constexpr (IsGoblinFlavor<Flavor>) {
-        witness_commitments.calldata_inverses = transcript->template receive_from_prover<Commitment>(
-            domain_separator + "_" + commitment_labels.calldata_inverses);
-        witness_commitments.return_data_inverses = transcript->template receive_from_prover<Commitment>(
-            domain_separator + "_" + commitment_labels.return_data_inverses);
+        for (auto [commitment, label] :
+             zip_view(witness_commitments.get_databus_inverses(), commitment_labels.get_databus_inverses())) {
+            commitment = transcript->template receive_from_prover<Commitment>(domain_separator + "_" + label);
+        }
     }
 
     witness_commitments.z_perm =

barretenberg/cpp/src/barretenberg/stdlib/honk_recursion/verifier/ultra_recursive_verifier.cpp

@@ -107,13 +107,14 @@ std::array<typename Flavor::GroupElement, 2> UltraRecursiveVerifier_<Flavor>::ve
     commitments.lookup_inverses =
         transcript->template receive_from_prover<Commitment>(commitment_labels.lookup_inverses);
 
-    // If Goblin (i.e. using DataBus) receive commitments to log-deriv inverses polynomial
+    // If Goblin (i.e. using DataBus) receive commitments to log-deriv inverses polynomials
     if constexpr (IsGoblinFlavor<Flavor>) {
-        commitments.calldata_inverses =
-            transcript->template receive_from_prover<Commitment>(commitment_labels.calldata_inverses);
-        commitments.return_data_inverses =
-            transcript->template receive_from_prover<Commitment>(commitment_labels.return_data_inverses);
+        for (auto [commitment, label] :
+             zip_view(commitments.get_databus_inverses(), commitment_labels.get_databus_inverses())) {
+            commitment = transcript->template receive_from_prover<Commitment>(label);
+        }
     }
+
     const FF public_input_delta = compute_public_input_delta<Flavor>(
         public_inputs, beta, gamma, circuit_size, static_cast<uint32_t>(key->pub_inputs_offset));
 

barretenberg/cpp/src/barretenberg/stdlib/primitives/databus/databus.hpp

@@ -51,6 +51,7 @@ template <typename Builder> class databus {
   public:
     // The columns of the DataBus
     bus_vector calldata{ BusId::CALLDATA };
+    bus_vector secondary_calldata{ BusId::SECONDARY_CALLDATA };
     bus_vector return_data{ BusId::RETURNDATA };
 };
 } // namespace bb::stdlib

barretenberg/cpp/src/barretenberg/stdlib_circuit_builders/databus.hpp

@@ -54,7 +54,7 @@ struct BusVector {
  * in-circuit as we would with public inputs).
  *
  */
-using DataBus = std::array<BusVector, 2>;
-enum class BusId { CALLDATA, RETURNDATA };
+using DataBus = std::array<BusVector, 3>;
+enum class BusId { CALLDATA, SECONDARY_CALLDATA, RETURNDATA };
 
 } // namespace bb

barretenberg/cpp/src/barretenberg/stdlib_circuit_builders/mega_circuit_builder.cpp

@@ -40,6 +40,12 @@ template <typename FF> void MegaCircuitBuilder_<FF>::add_gates_to_ensure_all_pol
     auto read_idx = this->add_variable(raw_read_idx);
     read_calldata(read_idx);
 
+    // Create an arbitrary secondary_calldata read gate
+    add_public_secondary_calldata(this->add_variable(25)); // ensure there is at least one entry in secondary_calldata
+    raw_read_idx = static_cast<uint32_t>(get_secondary_calldata().size()) - 1; // read data that was just added
+    read_idx = this->add_variable(raw_read_idx);
+    read_secondary_calldata(read_idx);
+
     // Create an arbitrary return data read gate
     add_public_return_data(this->add_variable(17)); // ensure there is at least one entry in return data
     raw_read_idx = static_cast<uint32_t>(get_return_data().size()) - 1; // read data that was just added
@@ -238,17 +244,24 @@ template <typename FF> void MegaCircuitBuilder_<FF>::apply_databus_selectors(con
     case BusId::CALLDATA: {
         block.q_1().emplace_back(1);
         block.q_2().emplace_back(0);
+        block.q_3().emplace_back(0);
         break;
     }
-    case BusId::RETURNDATA: {
+    case BusId::SECONDARY_CALLDATA: {
         block.q_1().emplace_back(0);
         block.q_2().emplace_back(1);
+        block.q_3().emplace_back(0);
+        break;
+    }
+    case BusId::RETURNDATA: {
+        block.q_1().emplace_back(0);
+        block.q_2().emplace_back(0);
+        block.q_3().emplace_back(1);
         break;
     }
     }
     block.q_busread().emplace_back(1);
     block.q_m().emplace_back(0);
-    block.q_3().emplace_back(0);
     block.q_c().emplace_back(0);
     block.q_delta_range().emplace_back(0);
     block.q_arith().emplace_back(0);

barretenberg/cpp/src/barretenberg/stdlib_circuit_builders/mega_circuit_builder.hpp

@@ -171,6 +171,16 @@ template <typename FF> class MegaCircuitBuilder_ : public UltraCircuitBuilder_<M
      */
     void add_public_calldata(const uint32_t& in) { return append_to_bus_vector(BusId::CALLDATA, in); }
 
+    /**
+     * @brief Add a witness variable to secondary_calldata.
+     * @details In practice this is used in aztec by the kernel circuit to recieve output from a function circuit
+     *
+     */
+    void add_public_secondary_calldata(const uint32_t& in)
+    {
+        return append_to_bus_vector(BusId::SECONDARY_CALLDATA, in);
+    }
+
     /**
      * @brief Add a witness variable to the public return_data.
      *
@@ -190,6 +200,17 @@ template <typename FF> class MegaCircuitBuilder_ : public UltraCircuitBuilder_<M
         return read_bus_vector(BusId::CALLDATA, read_idx_witness_idx);
     };
 
+    /**
+     * @brief Read from secondary_calldata and create a corresponding databus read gate
+     *
+     * @param read_idx_witness_idx Witness index for the secondary_calldata read index
+     * @return uint32_t Witness index for the result of the read
+     */
+    uint32_t read_secondary_calldata(const uint32_t& read_idx_witness_idx)
+    {
+        return read_bus_vector(BusId::SECONDARY_CALLDATA, read_idx_witness_idx);
+    };
+
     /**
      * @brief Read from return_data and create a corresponding databus read gate
      *
@@ -207,6 +228,7 @@ template <typename FF> class MegaCircuitBuilder_ : public UltraCircuitBuilder_<M
     }
 
     const BusVector& get_calldata() { return databus[static_cast<size_t>(BusId::CALLDATA)]; }
+    const BusVector& get_secondary_calldata() { return databus[static_cast<size_t>(BusId::SECONDARY_CALLDATA)]; }
     const BusVector& get_return_data() { return databus[static_cast<size_t>(BusId::RETURNDATA)]; }
 
     void create_poseidon2_external_gate(const poseidon2_external_gate_<FF>& in);