Reorder members of MapBlock for performance

Before and after as obtained via `pahole -C MapBlock bin/minetest`:
/* size: 336, cachelines: 6, members: 23 */
/* sum members: 329, holes: 4, sum holes: 7 */
vs.
/* size: 336, cachelines: 6, members: 23 */
/* sum members: 329, holes: 2, sum holes: 7 */

There is not much to be gained by packing but I made sure
to move the most important data (mainly for the client) into
the first cache line.

src/mapblock.h

@@ -437,6 +437,11 @@ public:
 	// clearObject and return removed objects count
 	u32 clearObjects();
 
+	static const u32 ystride = MAP_BLOCKSIZE;
+	static const u32 zstride = MAP_BLOCKSIZE * MAP_BLOCKSIZE;
+
+	static const u32 nodecount = MAP_BLOCKSIZE * MAP_BLOCKSIZE * MAP_BLOCKSIZE;
+
 private:
 	/*
 		Private methods
@@ -444,59 +449,73 @@ private:
 
 	void deSerialize_pre22(std::istream &is, u8 version, bool disk);
 
-public:
 	/*
-		Public member variables
+	 * PLEASE NOTE: When adding something here be mindful of position and size
+	 * of member variables! This is also the reason for the weird public-private
+	 * interleaving.
+	 * If in doubt consult `pahole` to see the effects.
 	 */
 
+public:
 #ifndef SERVER // Only on client
 	MapBlockMesh *mesh = nullptr;
+
+	// marks the sides which are opaque: 00+Z-Z+Y-Y+X-X
+	u8 solid_sides = 0;
 #endif
 
-	NodeMetadataList m_node_metadata;
-	StaticObjectList m_static_objects;
-
-	static const u32 ystride = MAP_BLOCKSIZE;
-	static const u32 zstride = MAP_BLOCKSIZE * MAP_BLOCKSIZE;
-
-	static const u32 nodecount = MAP_BLOCKSIZE * MAP_BLOCKSIZE * MAP_BLOCKSIZE;
-
-	//// ABM optimizations ////
-	// Cache of content types
-	// This is actually a set but for the small sizes we have a vector should be
-	// more efficient.
-	// Can be empty, in which case nothing was cached yet.
-	std::vector<content_t> contents;
-	// True if we never want to cache content types for this block
-	bool do_not_cache_contents = false;
-	// marks the sides which are opaque: 00+Z-Z+Y-Y+X-X
-	u8 solid_sides {0};
-
 private:
-	/*
-		Private member variables
-	*/
+	// see isOrphan()
+	bool m_orphan = false;
 
 	// Position in blocks on parent
 	v3s16 m_pos;
 
-	/* This is the precalculated m_pos_relative value
+	/* Precalculated m_pos_relative value
 	 * This caches the value, improving performance by removing 3 s16 multiplications
-	* at runtime on each getPosRelative call
-	* For a 5 minutes runtime with valgrind this removes 3 * 19M s16 multiplications
-	* The gain can be estimated in Release Build to 3 * 100M multiply operations for 5 mins
+	 * at runtime on each getPosRelative call.
+	 * For a 5 minutes runtime with valgrind this removes 3 * 19M s16 multiplications.
+	 * The gain can be estimated in Release Build to 3 * 100M multiply operations for 5 mins.
 	 */
 	v3s16 m_pos_relative;
 
 	/*
-	 * Note that this is not an inline array because that has implications on
+		Reference count; currently used for determining if this block is in
+		the list of blocks to be drawn.
+	*/
+	short m_refcount = 0;
+
+	/*
+	 * Note that this is not an inline array because that has implications for
 	 * heap fragmentation (the array is exactly 16K), CPU caches and/or
 	 * optimizability of algorithms working on this array.
 	 */
 	MapNode *const data; // of `nodecount` elements
 
+	// provides the item and node definitions
 	IGameDef *m_gamedef;
 
+	/*
+		When the block is accessed, this is set to 0.
+		Map will unload the block when this reaches a timeout.
+	*/
+	float m_usage_timer = 0;
+
+public:
+	//// ABM optimizations ////
+	// True if we never want to cache content types for this block
+	bool do_not_cache_contents = false;
+	// Cache of content types
+	// This is actually a set but for the small sizes we have a vector should be
+	// more efficient.
+	// Can be empty, in which case nothing was cached yet.
+	std::vector<content_t> contents;
+
+private:
+	// Whether day and night lighting differs
+	bool m_day_night_differs = false;
+	bool m_day_night_differs_expired = true;
+
 	/*
 		- On the server, this is used for telling whether the
 		  block has been modified from the one on disk.
@@ -506,14 +525,12 @@ private:
 	u32 m_modified_reason = MOD_REASON_INITIAL;
 
 	/*
-		When propagating sunlight and the above block doesn't exist,
-		sunlight is assumed if this is false.
-
-		In practice this is set to true if the block is completely
-		undeground with nothing visible above the ground except
-		caves.
+		When block is removed from active blocks, this is set to gametime.
+		Value BLOCK_TIMESTAMP_UNDEFINED=0xffffffff means there is no timestamp.
 	*/
-	bool is_underground = false;
+	u32 m_timestamp = BLOCK_TIMESTAMP_UNDEFINED;
+	// The on-disk (or to-be on-disk) timestamp value
+	u32 m_disk_timestamp = BLOCK_TIMESTAMP_UNDEFINED;
 
 	/*!
 	 * Each bit indicates if light spreading was finished
@@ -525,35 +542,24 @@ private:
 	*/
 	u16 m_lighting_complete = 0xFFFF;
 
-	// Whether day and night lighting differs
-	bool m_day_night_differs = false;
-	bool m_day_night_differs_expired = true;
-
-	// see isOrphan()
-	bool m_orphan = false;
 	// Whether mapgen has generated the content of this block (persisted)
 	bool m_generated = false;
 
 	/*
-		When block is removed from active blocks, this is set to gametime.
-		Value BLOCK_TIMESTAMP_UNDEFINED=0xffffffff means there is no timestamp.
-	*/
-	u32 m_timestamp = BLOCK_TIMESTAMP_UNDEFINED;
-	// The on-disk (or to-be on-disk) timestamp value
-	u32 m_disk_timestamp = BLOCK_TIMESTAMP_UNDEFINED;
+		When propagating sunlight and the above block doesn't exist,
+		sunlight is assumed if this is false.
 
-	/*
-		When the block is accessed, this is set to 0.
-		Map will unload the block when this reaches a timeout.
+		In practice this is set to true if the block is completely
+		undeground with nothing visible above the ground except
+		caves.
 	*/
-	float m_usage_timer = 0;
+	bool is_underground = false;
 
-	/*
-		Reference count; currently used for determining if this block is in
-		the list of blocks to be drawn.
-	*/
-	short m_refcount = 0;
+public:
+	NodeMetadataList m_node_metadata;
+	StaticObjectList m_static_objects;
 
+private:
 	NodeTimerList m_node_timers;
 };