Branch Prediction API (#139)

A code introducing branch prediction API.

Author: arupc-vmicro

core/BranchPredIF.hpp

@@ -0,0 +1,45 @@
+// <Branch.hpp> -*- C++ -*-
+
+//!
+//! \file BranchPred.hpp
+//! \brief Definition of Branch Prediction API
+//!
+
+/*
+ * This file defines the Branch Prediction API.
+ * The goal is to define an API that is generic and yet flexible enough to support various
+ * branch prediction microarchitecture.
+ * To the end, we envision a generic branch predictor as a black box with following inputs
+ * and outputs:
+ *   * A generic Prediction output
+ *   * A generic Prediction input
+ *   * A generic Update input
+ *
+ * The generic branch predictor may have two operations:
+ *   * getPrediction: produces Prediction output based on the Prediction input.
+ *   * updatePredictor: updates Predictor with Update input.
+ *
+ * It is intended that an implementation of branch predictor must also specify
+ * implementations of Prediction output, Prediction input and Update input, along with
+ * implementations of getPrediction and updatePredictor operations.
+ * */
+#pragma once
+
+namespace olympia
+{
+namespace BranchPredictor
+{
+
+    template <class PredictionT, class UpdateT, class InputT>
+    class BranchPredictorIF
+    {
+    public:
+        // TODO: create constexpr for bytes per compressed and uncompressed inst
+        static constexpr uint8_t bytes_per_inst = 4;
+        virtual ~BranchPredictorIF() { };
+        virtual PredictionT getPrediction(const InputT &) = 0;
+        virtual void updatePredictor(const UpdateT &) = 0;
+    };
+
+} // namespace BranchPredictor
+} // namespace olympia

core/CMakeLists.txt

@@ -1,6 +1,7 @@
 project (core)
 add_library(core
   Core.cpp
+  SimpleBranchPred.cpp
   Fetch.cpp
   Decode.cpp
   Rename.cpp

core/SimpleBranchPred.cpp

@@ -0,0 +1,79 @@
+#include "SimpleBranchPred.hpp"
+
+/*
+ * The algorithm used for prediction / update is as follows:
+ * Prediction:
+ *    - look up BHT to determine if the branch is predicted taken or not
+ *      using 2-bit saturated counter
+ *       - value 3: strongly taken
+ *       - value 2: weakly taken
+ *       - value 1: weakly not taken
+ *       - value 0: strongly not taken
+ *    - look up BTB to see if an entry exists for the input fetch pc
+ *       - if present in BTB and predicted taken, BTB entry is used to determine
+ *         prediction branch idx and predicted_PC
+ *       - if present in BTB but predicted not taken, BTB entry is used to determine
+ *         prediction branch idx, while predicted_PC is the fall through addr
+ *       - if not present in BTB entry, prediction branch idx is the last instr of
+ *         the FetchPacket, while predicted PC is the fall through addr. Also, create
+ *         a new BTB entry
+ * Update:
+ *    - a valid BTB entry must be present for fetch PC
+ *    - TBD
+ *
+ */
+namespace olympia
+{
+namespace BranchPredictor
+{
+
+    void SimpleBranchPredictor::updatePredictor(const DefaultUpdate & update) {
+
+        sparta_assert(branch_target_buffer_.find(update.fetch_PC) != branch_target_buffer_.end());
+        branch_target_buffer_[update.fetch_PC].branch_idx = update.branch_idx;
+        if (update.actually_taken) {
+            branch_history_table_[update.fetch_PC] =
+                (branch_history_table_[update.fetch_PC] == 3) ? 3 :
+                 branch_history_table_[update.fetch_PC] + 1;
+            branch_target_buffer_[update.fetch_PC].predicted_PC = update.corrected_PC;
+        } else {
+            branch_history_table_[update.fetch_PC] =
+                (branch_history_table_[update.fetch_PC] == 0) ? 0 :
+                 branch_history_table_[update.fetch_PC] - 1;
+        }
+    }
+
+    DefaultPrediction SimpleBranchPredictor::getPrediction(const DefaultInput & input) {
+        bool predictTaken = false;
+        if (branch_history_table_.find(input.fetch_PC) != branch_history_table_.end()) {
+            predictTaken = (branch_history_table_[input.fetch_PC] > 1);
+        } else {
+            // add a new entry to BHT, biased towards not taken
+            branch_history_table_.insert(std::pair<uint64_t, uint8_t>(input.fetch_PC, 1));
+        }
+
+        DefaultPrediction prediction;
+        if (branch_target_buffer_.find(input.fetch_PC) != branch_target_buffer_.end()) {
+            // BTB hit
+            const BTBEntry & btb_entry = branch_target_buffer_[input.fetch_PC];
+            prediction.branch_idx = btb_entry.branch_idx;
+            if (predictTaken) {
+                prediction.predicted_PC = btb_entry.predicted_PC;
+            } else {
+                // fall through address
+                prediction.predicted_PC = input.fetch_PC + prediction.branch_idx + BranchPredictorIF::bytes_per_inst;
+            }
+        } else {
+            // BTB miss
+            prediction.branch_idx = max_fetch_insts_;
+            prediction.predicted_PC = input.fetch_PC + max_fetch_insts_ * bytes_per_inst;
+            // add new entry to BTB
+            branch_target_buffer_.insert(std::pair<uint64_t,BTBEntry>(
+                input.fetch_PC, BTBEntry(prediction.branch_idx, prediction.predicted_PC)));
+        }
+
+        return prediction;
+    }
+
+} // namespace BranchPredictor
+} // namespace olympia

core/SimpleBranchPred.hpp

@@ -0,0 +1,89 @@
+// <SimpleBranchPred.hpp> -*- C++ -*-
+
+//!
+//! \file SimpleBranchPred.hpp
+//! \brief Class definition of a simple brranch prediction using branch prediction interface
+//!
+
+/*
+ * This file defines the class SimpleBranchPredictor, as well as, a default Prediction 
+ * output class, a default Prediction input class, a defeault Prediction input class
+ * as required by Olympia's Branch Prediction inteface
+ * */
+#pragma once
+
+#include <cstdint>
+#include <map>
+#include <limits>
+#include "sparta/utils/SpartaAssert.hpp"
+#include "BranchPredIF.hpp"
+
+namespace olympia
+{
+namespace BranchPredictor
+{
+
+    // following class definitions are example inputs & outputs for a very simple branch
+    // predictor
+    class DefaultPrediction
+    {
+    public:
+        // index of branch instruction in the fetch packet
+        // branch_idx can vary from 0 to (FETCH_WIDTH - 1)
+        // initialized to default max to catch errors
+        uint32_t branch_idx = std::numeric_limits<uint32_t>::max();
+        // predicted target PC
+        uint64_t predicted_PC = std::numeric_limits<uint64_t>::max();
+    };
+
+    class DefaultUpdate
+    {
+    public:
+        uint64_t fetch_PC = std::numeric_limits<uint64_t>::max();
+        uint32_t branch_idx = std::numeric_limits<uint32_t>::max();
+        uint64_t corrected_PC = std::numeric_limits<uint64_t>::max();
+        bool actually_taken = false;
+    };
+
+    class DefaultInput
+    {
+    public:
+        // PC of first instruction of fetch packet
+        uint64_t fetch_PC = std::numeric_limits<uint64_t>::max();
+    };
+
+    class BTBEntry
+    {
+    public:
+        // use of BTBEntry in std:map operator [] requires default constructor
+        BTBEntry() = default;
+        BTBEntry(uint32_t bidx, uint64_t predPC) :
+            branch_idx(bidx),
+            predicted_PC(predPC)
+        {}
+        uint32_t branch_idx {std::numeric_limits<uint32_t>::max()};
+        uint64_t predicted_PC {std::numeric_limits<uint64_t>::max()};
+    };
+
+    // Currently SimpleBranchPredictor works only with uncompressed instructions
+    // TODO: generalize SimpleBranchPredictor for both compressed and uncompressed instructions
+    class SimpleBranchPredictor : public BranchPredictorIF<DefaultPrediction, DefaultUpdate, DefaultInput>
+    {
+    public:
+        SimpleBranchPredictor(uint32_t max_fetch_insts) :
+            max_fetch_insts_(max_fetch_insts)
+        {}
+        DefaultPrediction getPrediction(const DefaultInput &);
+        void updatePredictor(const DefaultUpdate &);
+    private:
+        // maximum number of instructions in a FetchPacket
+        const uint32_t max_fetch_insts_;
+        // BHT and BTB of SimpleBranchPredictor is unlimited in size
+        // a map of branch PC to 2 bit staurating counter tracking branch history
+        std::map <uint64_t, uint8_t> branch_history_table_; // BHT
+        // a map of branch PC to target of the branch
+        std::map <uint64_t, BTBEntry> branch_target_buffer_; // BTB
+    };
+
+} // namespace BranchPredictor
+} // namespace olympia

test/CMakeLists.txt

@@ -32,3 +32,4 @@ add_subdirectory(core/l2cache)
 add_subdirectory(core/rename)
 add_subdirectory(core/lsu)
 add_subdirectory(core/issue_queue)
+add_subdirectory(core/branch_pred)

test/core/branch_pred/BranchPred_test.cpp

@@ -0,0 +1,43 @@
+#include "SimpleBranchPred.hpp"
+#include "sparta/utils/SpartaTester.hpp"
+
+TEST_INIT
+
+void runTest(int argc, char **argv)
+{
+   olympia::BranchPredictor::SimpleBranchPredictor predictor(4); //specify max num insts to fetch
+
+   olympia::BranchPredictor::DefaultInput input;
+   input.fetch_PC = 0x0;
+
+   // BTB miss
+   olympia::BranchPredictor::DefaultPrediction prediction = predictor.getPrediction(input);
+
+   EXPECT_EQUAL(prediction.branch_idx, 4);
+   EXPECT_EQUAL(prediction.predicted_PC, 16);
+
+   // there was a taken branch at the 3rd instruction from fetchPC, with target 0x100
+   olympia::BranchPredictor::DefaultUpdate update;
+   update.fetch_PC = 0x0;
+   update.branch_idx = 2;
+   update.corrected_PC = 0x100;
+   update.actually_taken = true;
+   predictor.updatePredictor(update);
+
+   // try the same input with fetchPC 0x0 again
+   prediction = predictor.getPrediction(input);
+   
+   EXPECT_EQUAL(prediction.branch_idx, 2);
+   EXPECT_EQUAL(prediction.predicted_PC, 0x100);
+
+   // TODO: add more tests
+            
+}
+
+int main(int argc, char **argv)
+{
+    runTest(argc, argv);
+
+    REPORT_ERROR;
+    return (int)ERROR_CODE;
+}

test/core/branch_pred/CMakeLists.txt

@@ -0,0 +1,6 @@
+project(BranchPred_test)
+
+add_executable(BranchPred_test BranchPred_test.cpp)
+target_link_libraries(BranchPred_test core common_test SPARTA::sparta)
+
+sparta_named_test(BranchPred_test_Run  BranchPred_test)