[2.0.x] better reverse pass (#8722)

* repair reverse_pass()

And make it readeble.
This was broken a long time ago.
Not competely unfunctional but far from optimal.

* Minor speedup when calling calculate_trapezoid_for_block

2 float / to 1 foat / and 2 float *

* Various style changes

Marlin/src/module/planner.cpp

 
 
 // The kernel called by recalculate() when scanning the plan from last to first entry.
-void Planner::reverse_pass_kernel(block_t* const current, const block_t *next) {
+void Planner::reverse_pass_kernel(block_t* const current, const block_t * const next) {
   if (!current || !next) return;
   // If entry speed is already at the maximum entry speed, no need to recheck. Block is cruising.
   // If not, block in state of acceleration or deceleration. Reset entry speed to maximum and
  * Once in reverse and once forward. This implements the reverse pass.
  */
 void Planner::reverse_pass() {
-
   if (movesplanned() > 3) {
-
-    block_t* block[3] = { NULL, NULL, NULL };
-
-    // Make a local copy of block_buffer_tail, because the interrupt can alter it
-    // Is a critical section REALLY needed for a single byte change?
-    //CRITICAL_SECTION_START;
-    uint8_t tail = block_buffer_tail;
-    //CRITICAL_SECTION_END
-
-    uint8_t b = BLOCK_MOD(block_buffer_head - 3);
-    while (b != tail) {
-      if (block[0] && TEST(block[0]->flag, BLOCK_BIT_START_FROM_FULL_HALT)) break;
-      b = prev_block_index(b);
-      block[2] = block[1];
-      block[1] = block[0];
-      block[0] = &block_buffer[b];
-      reverse_pass_kernel(block[1], block[2]);
-    }
+    const uint8_t endnr = BLOCK_MOD(block_buffer_tail + 2); // tail is running. tail+1 shouldn't be altered because it's connected to the running block.
+                                                            // tail+2 because the index is not yet advanced when checked
+    uint8_t blocknr = prev_block_index(block_buffer_head);
+    block_t* current = &block_buffer[blocknr];
+
+    do {
+      const block_t * const next = current;
+      blocknr = prev_block_index(blocknr);
+      current = &block_buffer[blocknr];
+      if (TEST(current->flag, BLOCK_BIT_START_FROM_FULL_HALT)) // Up to this every block is already optimized.
+        break;
+      reverse_pass_kernel(current, next);
+    } while (blocknr != endnr);
   }
 }
 
 // The kernel called by recalculate() when scanning the plan from first to last entry.
-void Planner::forward_pass_kernel(const block_t* previous, block_t* const current) {
+void Planner::forward_pass_kernel(const block_t * const previous, block_t* const current) {
   if (!previous) return;
 
   // If the previous block is an acceleration block, but it is not long enough to complete the
       // Recalculate if current block entry or exit junction speed has changed.
       if (TEST(current->flag, BLOCK_BIT_RECALCULATE) || TEST(next->flag, BLOCK_BIT_RECALCULATE)) {
         // NOTE: Entry and exit factors always > 0 by all previous logic operations.
-        float nom = current->nominal_speed;
-        calculate_trapezoid_for_block(current, current->entry_speed / nom, next->entry_speed / nom);
+        const float nomr = 1.0 / current->nominal_speed;
+        calculate_trapezoid_for_block(current, current->entry_speed * nomr, next->entry_speed * nomr);
         CBI(current->flag, BLOCK_BIT_RECALCULATE); // Reset current only to ensure next trapezoid is computed
       }
     }
   }
   // Last/newest block in buffer. Exit speed is set with MINIMUM_PLANNER_SPEED. Always recalculated.
   if (next) {
-    float nom = next->nominal_speed;
-    calculate_trapezoid_for_block(next, next->entry_speed / nom, (MINIMUM_PLANNER_SPEED) / nom);
+    const float nomr = 1.0 / next->nominal_speed;
+    calculate_trapezoid_for_block(next, next->entry_speed * nomr, (MINIMUM_PLANNER_SPEED) * nomr);
     CBI(next->flag, BLOCK_BIT_RECALCULATE);
   }
 }
       #endif
     );
   }
-  float inverse_millimeters = 1.0 / block->millimeters;  // Inverse millimeters to remove multiple divides
+  const float inverse_millimeters = 1.0 / block->millimeters;  // Inverse millimeters to remove multiple divides
 
   // Calculate inverse time for this move. No divide by zero due to previous checks.
   // Example: At 120mm/s a 60mm move takes 0.5s. So this will give 2.0.
     //FMM update ring buffer used for delay with filament measurements
     if (extruder == FILAMENT_SENSOR_EXTRUDER_NUM && filwidth_delay_index[1] >= 0) {  //only for extruder with filament sensor and if ring buffer is initialized
 
-      const int MMD_CM = MAX_MEASUREMENT_DELAY + 1, MMD_MM = MMD_CM * 10;
+      constexpr int MMD_CM = MAX_MEASUREMENT_DELAY + 1, MMD_MM = MMD_CM * 10;
 
       // increment counters with next move in e axis
       filwidth_e_count += delta_mm[E_AXIS];
 
   #endif // LIN_ADVANCE
 
-  calculate_trapezoid_for_block(block, block->entry_speed / block->nominal_speed, safe_speed / block->nominal_speed);
+  const float bnsr = 1.0 / block->nominal_speed;
+  calculate_trapezoid_for_block(block, block->entry_speed * bnsr, safe_speed * bnsr);
 
   // Move buffer head
   block_buffer_head = next_buffer_head;
 
   // Update the position (only when a move was queued)
-  static_assert(COUNT(target) > 1, "array as function parameter should be declared as reference and with count");
+  static_assert(COUNT(target) > 1, "Parameter to _buffer_steps must be (&target)[XYZE]!");
   COPY(position, target);
 
   recalculate();

Marlin/src/module/planner.h

 #define BLOCK_MOD(n) ((n)&(BLOCK_BUFFER_SIZE-1))
 
 class Planner {
-
   public:
 
     /**
-     * A ring buffer of moves described in steps
+     * The move buffer, calculated in stepper steps
+     *
+     * block_buffer is a ring buffer...
+     *
+     *             head,tail : indexes for write,read
+     *            head==tail : the buffer is empty
+     *            head!=tail : blocks are in the buffer
+     *   head==(tail-1)%size : the buffer is full
+     *
+     *  Writer of head is Planner::_buffer_line().
+     *  Reader of tail is Stepper::isr(). Always consider tail busy / read-only
      */
     static block_t block_buffer[BLOCK_BUFFER_SIZE];
-    static volatile uint8_t block_buffer_head,  // Index of the next block to be pushed
-                            block_buffer_tail;
+    static volatile uint8_t block_buffer_head,      // Index of the next block to be pushed
+                            block_buffer_tail;      // Index of the busy block, if any
 
     #if ENABLED(DISTINCT_E_FACTORS)
-      static uint8_t last_extruder;             // Respond to extruder change
+      static uint8_t last_extruder;                 // Respond to extruder change
     #endif
 
-    static int16_t flow_percentage[EXTRUDERS]; // Extrusion factor for each extruder
+    static int16_t flow_percentage[EXTRUDERS];      // Extrusion factor for each extruder
 
     static float e_factor[EXTRUDERS],               // The flow percentage and volumetric multiplier combine to scale E movement
                  filament_size[EXTRUDERS],          // diameter of filament (in millimeters), typically around 1.75 or 2.85, 0 disables the volumetric calculations for the extruder
                  volumetric_multiplier[EXTRUDERS];  // Reciprocal of cross-sectional area of filament (in mm^2). Pre-calculated to reduce computation in the planner
                                                     // May be auto-adjusted by a filament width sensor
 
-    static float max_feedrate_mm_s[XYZE_N],     // Max speeds in mm per second
+    static float max_feedrate_mm_s[XYZE_N],         // Max speeds in mm per second
                  axis_steps_per_mm[XYZE_N],
                  steps_to_mm[XYZE_N];
     static uint32_t max_acceleration_steps_per_s2[XYZE_N],
     /**
      * Number of moves currently in the planner
      */
-    static uint8_t movesplanned() { return BLOCK_MOD(block_buffer_head - block_buffer_tail + BLOCK_BUFFER_SIZE); }
+    FORCE_INLINE static uint8_t movesplanned() { return BLOCK_MOD(block_buffer_head - block_buffer_tail + BLOCK_BUFFER_SIZE); }
 
-    static bool is_full() { return (block_buffer_tail == BLOCK_MOD(block_buffer_head + 1)); }
+    FORCE_INLINE static bool is_full() { return block_buffer_tail == next_block_index(block_buffer_head); }
 
     // Update multipliers based on new diameter measurements
     static void calculate_volumetric_multipliers();
     /**
      * Get the index of the next / previous block in the ring buffer
      */
-    static int8_t next_block_index(const int8_t block_index) { return BLOCK_MOD(block_index + 1); }
-    static int8_t prev_block_index(const int8_t block_index) { return BLOCK_MOD(block_index - 1); }
+    static constexpr int8_t next_block_index(const int8_t block_index) { return BLOCK_MOD(block_index + 1); }
+    static constexpr int8_t prev_block_index(const int8_t block_index) { return BLOCK_MOD(block_index - 1); }
 
     /**
      * Calculate the distance (not time) it takes to accelerate
 
     static void calculate_trapezoid_for_block(block_t* const block, const float &entry_factor, const float &exit_factor);
 
-    static void reverse_pass_kernel(block_t* const current, const block_t *next);
-    static void forward_pass_kernel(const block_t *previous, block_t* const current);
+    static void reverse_pass_kernel(block_t* const current, const block_t * const next);
+    static void forward_pass_kernel(const block_t * const previous, block_t* const current);
 
     static void reverse_pass();
     static void forward_pass();