AP_NavEKF2: moved intermediate variables to common memory

this moves intermediate variables from being per-core to being common
between cores. This saves memory on systems with more than one core by
avoiding allocating this memory on every core.

This is an alternative to #11717 which moves memory onto the stack. It
doesn't save as much memory as #11717, but avoids creating large stack
frames

libraries/AP_NavEKF2/AP_NavEKF2.cpp

@@ -660,16 +660,27 @@ bool NavEKF2::InitialiseFilter(void)
             gcs().send_text(MAV_SEVERITY_CRITICAL, "NavEKF2: allocation failed");
             return false;
         }
+
+        // allocate common intermediate variable space
+        core_common = (struct CoreCommon *)hal.util->malloc_type(NavEKF2_core::get_core_common_size(), AP_HAL::Util::MEM_FAST);
+        if (core_common == nullptr) {
+            _enable.set(0);
+            hal.util->free_type(core, sizeof(NavEKF2_core)*num_cores, AP_HAL::Util::MEM_FAST);
+            core = nullptr;
+            gcs().send_text(MAV_SEVERITY_CRITICAL, "NavEKF2: common allocation failed");
+            return false;
+        }
+
+        //Call Constructors on all cores
         for (uint8_t i = 0; i < num_cores; i++) {
-            //Call Constructors
-            new (&core[i]) NavEKF2_core();
+            new (&core[i]) NavEKF2_core(this);
         }
 
         // set the IMU index for the cores
         num_cores = 0;
         for (uint8_t i=0; i<7; i++) {
             if (_imuMask & (1U<<i)) {
-                if(!core[num_cores].setup_core(this, i, num_cores)) {
+                if(!core[num_cores].setup_core(i, num_cores)) {
                     return false;
                 }
                 num_cores++;

libraries/AP_NavEKF2/AP_NavEKF2.h

@@ -492,6 +492,11 @@ private:
     // time of last lane switch
     uint32_t lastLaneSwitch_ms;
 
+    /*
+      common intermediate variables used by all cores
+    */
+    void *core_common;
+
     // update the yaw reset data to capture changes due to a lane switch
     // new_primary - index of the ekf instance that we are about to switch to as the primary
     // old_primary - index of the ekf instance that we are currently using as the primary

libraries/AP_NavEKF2/AP_NavEKF2_core.cpp

@@ -20,7 +20,7 @@ extern const AP_HAL::HAL& hal;
 #define GYRO_BIAS_LIMIT 0.5f
 
 // constructor
-NavEKF2_core::NavEKF2_core(void) :
+NavEKF2_core::NavEKF2_core(NavEKF2 *_frontend) :
     _perf_UpdateFilter(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_UpdateFilter")),
     _perf_CovariancePrediction(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_CovariancePrediction")),
     _perf_FuseVelPosNED(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_FuseVelPosNED")),
@@ -28,7 +28,19 @@ NavEKF2_core::NavEKF2_core(void) :
     _perf_FuseAirspeed(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_FuseAirspeed")),
     _perf_FuseSideslip(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_FuseSideslip")),
     _perf_TerrainOffset(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_TerrainOffset")),
-    _perf_FuseOptFlow(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_FuseOptFlow"))
+    _perf_FuseOptFlow(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_FuseOptFlow")),
+    frontend(_frontend),
+    // setup the intermediate variables shared by all cores (to save memory)
+    common((struct core_common *)_frontend->core_common),
+    Kfusion(common->Kfusion),
+    KH(common->KH),
+    KHP(common->KHP),
+    nextP(common->nextP),
+    processNoise(common->processNoise),
+    SF(common->SF),
+    SG(common->SG),
+    SQ(common->SQ),
+    SPP(common->SPP)
 {
     _perf_test[0] = hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_Test0");
     _perf_test[1] = hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK2_Test1");
@@ -43,9 +55,8 @@ NavEKF2_core::NavEKF2_core(void) :
 }
 
 // setup this core backend
-bool NavEKF2_core::setup_core(NavEKF2 *_frontend, uint8_t _imu_index, uint8_t _core_index)
+bool NavEKF2_core::setup_core(uint8_t _imu_index, uint8_t _core_index)
 {
-    frontend = _frontend;
     imu_index = _imu_index;
     gyro_index_active = _imu_index;
     accel_index_active = _imu_index;
@@ -156,8 +167,7 @@ void NavEKF2_core::InitialiseVariables()
     lastKnownPositionNE.zero();
     prevTnb.zero();
     memset(&P[0][0], 0, sizeof(P));
-    memset(&nextP[0][0], 0, sizeof(nextP));
-    memset(&processNoise[0], 0, sizeof(processNoise));
+    memset(common, 0, sizeof(*common));
     flowDataValid = false;
     rangeDataToFuse  = false;
     Popt = 0.0f;
@@ -515,6 +525,12 @@ void NavEKF2_core::CovarianceInit()
 // Update Filter States - this should be called whenever new IMU data is available
 void NavEKF2_core::UpdateFilter(bool predict)
 {
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+    // fill the common variables with NaN, so we catch any cases in
+    // SITL where they are used without initialisation
+    fill_nanf((float *)common, sizeof(*common)/sizeof(float));
+#endif
+
     // Set the flag to indicate to the filter that the front-end has given permission for a new state prediction cycle to be started
     startPredictEnabled = predict;
 

libraries/AP_NavEKF2/AP_NavEKF2_core.h

@@ -62,10 +62,10 @@ class NavEKF2_core
 {
 public:
     // Constructor
-    NavEKF2_core(void);
+    NavEKF2_core(NavEKF2 *_frontend);
 
     // setup this core backend
-    bool setup_core(NavEKF2 *_frontend, uint8_t _imu_index, uint8_t _core_index);
+    bool setup_core(uint8_t _imu_index, uint8_t _core_index);
     
     // Initialise the states from accelerometer and magnetometer data (if present)
     // This method can only be used when the vehicle is static
@@ -327,6 +327,9 @@ public:
     // return true when external nav data is also being used as a yaw observation
     bool isExtNavUsedForYaw(void);
 
+    // return size of core_common for use in frontend allocation
+    static uint32_t get_core_common_size(void) { return sizeof(core_common); }
+
 private:
     // Reference to the global EKF frontend for parameters
     NavEKF2 *frontend;
@@ -482,6 +485,26 @@ private:
         bool            posReset;   // true when the position measurement has been reset
     };
 
+    /*
+      common intermediate variables used by all cores. These save a
+      lot memory by avoiding allocating these arrays on every core
+      Having these as stack variables would save even more memory, but
+      would give us very high stack usage in some functions, which
+      poses a risk of stack overflow until we have infrastructure in
+      place to calculate maximum stack usage using static analysis.
+     */
+    struct core_common {
+        Vector28 Kfusion;
+        Matrix24 KH;
+        Matrix24 KHP;
+        Matrix24 nextP;
+        Vector24 processNoise;
+        Vector25 SF;
+        Vector5 SG;
+        Vector8 SQ;
+        Vector23 SPP;
+    } *common;
+
     // bias estimates for the IMUs that are enabled but not being used
     // by this core.
     struct {
@@ -799,9 +822,9 @@ private:
     bool badIMUdata;                // boolean true if the bad IMU data is detected
 
     float gpsNoiseScaler;           // Used to scale the  GPS measurement noise and consistency gates to compensate for operation with small satellite counts
-    Vector28 Kfusion;               // Kalman gain vector
-    Matrix24 KH;                    // intermediate result used for covariance updates
-    Matrix24 KHP;                   // intermediate result used for covariance updates
+    Vector28 &Kfusion;              // Kalman gain vector
+    Matrix24 &KH;                   // intermediate result used for covariance updates
+    Matrix24 &KHP;                  // intermediate result used for covariance updates
     Matrix24 P;                     // covariance matrix
     imu_ring_buffer_t<imu_elements> storedIMU;      // IMU data buffer
     obs_ring_buffer_t<gps_elements> storedGPS;      // GPS data buffer
@@ -856,12 +879,12 @@ private:
     bool allMagSensorsFailed;       // true if all magnetometer sensors have timed out on this flight and we are no longer using magnetometer data
     uint32_t lastYawTime_ms;        // time stamp when yaw observation was last fused (msec)
     uint32_t ekfStartTime_ms;       // time the EKF was started (msec)
-    Matrix24 nextP;                 // Predicted covariance matrix before addition of process noise to diagonals
-    Vector24 processNoise;          // process noise added to diagonals of predicted covariance matrix
-    Vector25 SF;                    // intermediate variables used to calculate predicted covariance matrix
-    Vector5 SG;                     // intermediate variables used to calculate predicted covariance matrix
-    Vector8 SQ;                     // intermediate variables used to calculate predicted covariance matrix
-    Vector23 SPP;                   // intermediate variables used to calculate predicted covariance matrix
+    Matrix24 &nextP;                // Predicted covariance matrix before addition of process noise to diagonals
+    Vector24 &processNoise;         // process noise added to diagonals of predicted covariance matrix
+    Vector25 &SF;                   // intermediate variables used to calculate predicted covariance matrix
+    Vector5 &SG;                    // intermediate variables used to calculate predicted covariance matrix
+    Vector8 &SQ;                    // intermediate variables used to calculate predicted covariance matrix
+    Vector23 &SPP;                  // intermediate variables used to calculate predicted covariance matrix
     Vector2f lastKnownPositionNE;   // last known position
     uint32_t lastDecayTime_ms;      // time of last decay of GPS position offset
     float velTestRatio;             // sum of squares of GPS velocity innovation divided by fail threshold