AP_NavEKF3: 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

@@ -662,7 +662,7 @@ bool NavEKF2::InitialiseFilter(void)
         }
 
         // allocate common intermediate variable space
-        core_common = (struct CoreCommon *)hal.util->malloc_type(NavEKF2_core::get_core_common_size(), AP_HAL::Util::MEM_FAST);
+        core_common = (void *)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);

libraries/AP_NavEKF3/AP_NavEKF3.cpp

@@ -683,6 +683,16 @@ bool NavEKF3::InitialiseFilter(void)
             return false;
         }
 
+        // allocate common intermediate variable space
+        core_common = (void *)hal.util->malloc_type(NavEKF3_core::get_core_common_size(), AP_HAL::Util::MEM_FAST);
+        if (core_common == nullptr) {
+            _enable.set(0);
+            hal.util->free_type(core, sizeof(NavEKF3_core)*num_cores, AP_HAL::Util::MEM_FAST);
+            core = nullptr;
+            gcs().send_text(MAV_SEVERITY_CRITICAL, "NavEKF3: common allocation failed");
+            return false;
+        }
+
         // Call constructors on all cores
         for (uint8_t i = 0; i < num_cores; i++) {
             new (&core[i]) NavEKF3_core(this);

libraries/AP_NavEKF3/AP_NavEKF3.h

@@ -496,6 +496,11 @@ private:
     // time of last lane switch
     uint32_t lastLaneSwitch_ms;
 
+    /*
+      common intermediate variables used by all cores
+    */
+    void *core_common;
+
     struct {
         uint32_t last_function_call;  // last time getLastYawYawResetAngle was called
         bool core_changed;            // true when a core change happened and hasn't been consumed, false otherwise

libraries/AP_NavEKF3/AP_NavEKF3_core.cpp

@@ -20,7 +20,13 @@ NavEKF3_core::NavEKF3_core(NavEKF3 *_frontend) :
     _perf_TerrainOffset(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK3_TerrainOffset")),
     _perf_FuseOptFlow(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK3_FuseOptFlow")),
     _perf_FuseBodyOdom(hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK3_FuseBodyOdom")),
-    frontend(_frontend)
+    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)
 {
     _perf_test[0] = hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK3_Test0");
     _perf_test[1] = hal.util->perf_alloc(AP_HAL::Util::PC_ELAPSED, "EK3_Test1");
@@ -211,9 +217,7 @@ void NavEKF3_core::InitialiseVariables()
     lastKnownPositionNE.zero();
     prevTnb.zero();
     memset(&P[0][0], 0, sizeof(P));
-    memset(&KH[0][0], 0, sizeof(KH));
+    memset(common, 0, sizeof(*common));
-    memset(&KHP[0][0], 0, sizeof(KHP));
-    memset(&nextP[0][0], 0, sizeof(nextP));
     flowDataValid = false;
     rangeDataToFuse  = false;
     Popt = 0.0f;
@@ -569,6 +573,12 @@ void NavEKF3_core::CovarianceInit()
 // Update Filter States - this should be called whenever new IMU data is available
 void NavEKF3_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_NavEKF3/AP_NavEKF3_core.h

@@ -367,6 +367,9 @@ public:
     // get timing statistics structure
     void getTimingStatistics(struct ekf_timing &timing);
 
+    // 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
     NavEKF3 *frontend;
@@ -532,6 +535,21 @@ private:
         uint8_t     type;           // type specifiying Euler rotation order used, 1 = 312 (ZXY), 2 = 321 (ZYX)
     };
 
+    /*
+      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;
+    } *common;
+    
     // bias estimates for the IMUs that are enabled but not being used
     // by this core.
     struct {
@@ -854,6 +872,9 @@ 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
     Matrix24 P;                     // covariance matrix
     imu_ring_buffer_t<imu_elements> storedIMU;      // IMU data buffer
     obs_ring_buffer_t<gps_elements> storedGPS;      // GPS data buffer
@@ -907,6 +928,7 @@ private:
     bool allMagSensorsFailed;       // true if all magnetometer sensors have timed out on this flight and we are no longer using magnetometer data
     uint32_t lastSynthYawTime_ms;   // time stamp when synthetic yaw measurement was last fused to maintain covariance health (msec)
     uint32_t ekfStartTime_ms;       // time the EKF was started (msec)
+    Matrix24 &nextP;                // Predicted covariance matrix before addition of process noise to diagonals
     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