[Maintenance] Remove the `operator =` overloads for Subpixels

At least we've now documented their negative effects.

Part of P0110, funded by [Anonymous] and Blue Bolt.

Research/Borland C++ decompilation.md

@@ -130,6 +130,47 @@ case it's part of an arithmetic expression that was promoted to `int`.
                  // Note the opposite order of variables!
   ```
 
+* For trivially copyable structures, copy assignments are optimized to an
+  equivalent of `memcpy()`:
+
+  | Structure size | (no flags)  |  -G                  |
+  |----------------|-------------|----------------------|
+  |              1 | via `AL`    | via `AL`             |
+  |              2 | via `AX`    | via `AX`             |
+  |              3 | `SCOPY@`    | via `AX` and `AL`    |
+  |              4 | via `DX:AX` | via `DX:AX`          |
+  |        5, 7, 9 | `SCOPY@`    | via `AX` and `AL`    |
+  |           6, 8 | `SCOPY@`    | via `AX`             |
+  |  10, 12, 14, … | `SCOPY@`    | `REP MOVSW`          |
+  |  11, 13, 15, … | `SCOPY@`    | `REP MOVSW`, `MOVSB` |
+
+  (With the `-3` flag, `EAX` is used instead of `DX:AX` in the 4-byte case,
+  but everything else stays the same.)
+
+  Breaking triviality by overloading `operator =` in any of the structure
+  members also breaks this optimization. In some cases, it might be possible
+  to recreate it, by simulating triviality in an overloaded copy assignment
+  operator inside the class in question:
+
+  ```c++
+  struct Nontrivial {
+    nontrivial_char_t e[100];
+    // Functions containing local classes aren't expanded inline, so...
+    struct Trivial {
+      char e[100];
+    };
+
+    void operator =(const Nontrivial &other) {
+      reinterpret_cast<Trivial &>(*this) = (
+          reinterpret_cast<const Trivial &>(other)
+      );
+    }
+  };
+  ```
+
+  However, this only generates identical code to the original optimization if
+  passing the `other` parameter can be inlined, which isn't always the case.
+
 ## `switch` statements
 
 * Sequence of the individual cases is identical in both C and ASM

th01/main/bullet/pellet.cpp

@@ -82,7 +82,7 @@ bool16 pattern_velocity_set(
 	switch(pattern) {
 	case PP_1:
 		ret_y.v = speed;
-		ret_x = 0.0f;
+		ret_x.set(0.0f);
 		done = true;
 		break;
 	case PP_1_AIMED:
@@ -179,7 +179,7 @@ inline subpixel_t base_speed_for_rank(void)
 #define pellet_init(pellet, left, top, pattern) \
 	pellet->decay_frame = 0; \
 	pellet->cur_left.v = TO_SP(left); \
-	pellet->cur_top = top; \
+	pellet->cur_top.v = TO_SP(top); \
 	pellet->cloud_left = left; \
 	pellet->cloud_top = top; \
 	if(spawn_with_cloud) { \
@@ -271,7 +271,7 @@ void CPellets::add_single(
 		p->age = 0;
 		alive_count++;
 		p->spin_center.x.v = TO_SP(spin_center_x);
-		p->spin_center.y = spin_center_y;
+		p->spin_center.y.v = TO_SP(spin_center_y);
 		if(motion_type == PM_SPIN) {
 			vector2(vel_x.v, vel_y.v, speed_for_motion_fixed, angle);
 			p->spin_velocity.x.v = vel_x.v;
@@ -339,27 +339,27 @@ void CPellets::motion_type_apply_for_cur(void)
 			|| (p->cur_top.v <= to_sp(PELLET_BOUNCE_TOP_MIN))
 		) {
 			p->velocity.x.v = -p->velocity.x.v;
-			p->velocity.y = 0.0f;
+			p->velocity.y.set(0.0f);
 			p->motion_type = PM_GRAVITY;
 			// Nope, this doesn't help.
 			if(p->cur_left.v <= to_sp(PELLET_LEFT_MIN)) {
-				p->cur_left = (PELLET_LEFT_MIN + 1.0f);
+				p->cur_left.set(PELLET_LEFT_MIN + 1.0f);
 			}
 			if(p->cur_left.v >= to_sp(PELLET_LEFT_MAX)) {
-				p->cur_left = (PELLET_LEFT_MAX - 1.0f);
+				p->cur_left.set(PELLET_LEFT_MAX - 1.0f);
 			}
 			if(p->cur_top.v <= to_sp(PELLET_TOP_MIN)) {
-				p->cur_top = (PELLET_TOP_MIN + 1.0f);
+				p->cur_top.set(PELLET_TOP_MIN + 1.0f);
 			}
 		}
 		break;
 	case PM_FALL_STRAIGHT_FROM_TOP_THEN_NORMAL:
 		if(p->cur_top.to_screen() <= PELLET_BOUNCE_TOP_MIN) {
-			p->velocity.x = 0.0f;
+			p->velocity.x.set(0.0f);
 			p->velocity.y.v = p->speed.v;
 			p->motion_type = PM_NORMAL;
 			if(p->cur_top.to_screen() <= PLAYFIELD_TOP) {
-				p->cur_top = (PLAYFIELD_TOP + 1.0f);
+				p->cur_top.set(PLAYFIELD_TOP + 1.0f);
 			}
 		}
 		break;
@@ -370,8 +370,7 @@ void CPellets::motion_type_apply_for_cur(void)
 		p->cur_top.v = (to_spin_circle(Sin8(p->angle)) + p->spin_center.y.v);
 		p->spin_center.x.v += p->spin_velocity.x.v;
 		p->spin_center.y.v += p->spin_velocity.y.v;
-		p->velocity.x = 0.0f;
-		p->velocity.y = 0.0f;
+		p->velocity.set(0.0f, 0.0f);
 		p->angle += PELLET_SPIN_DELTA_ANGLE;
 		#undef to_spin_circle
 		break;

th01/math/subpixel.hpp

@@ -30,11 +30,13 @@ public:
 		this->v -= static_cast<T>(to_sp(screen_v));
 	}
 
-	void operator =(float screen_v) {
+	// No overloads of `operator =()`, since the class needs to be trivially
+	// copyable.
+	void set(float screen_v) {
 		v = static_cast<T>(to_sp(screen_v));
 	}
 
-	void operator =(const T &screen_v) {
+	void set(const T &screen_v) {
 		v = TO_SP(screen_v);
 	}
 
@@ -55,13 +57,8 @@ template <class T> struct SPPointBase {
 	T x, y;
 
 	void set(float screen_x, float screen_y) {
-		x = screen_x;
-		y = screen_y;
-	}
-
-	void set(const T &screen_x, const T &screen_y) {
-		x = screen_x;
-		y = screen_y;
+		x.set(screen_x);
+		y.set(screen_y);
 	}
 };
 

th04/math/motion.hpp

@@ -4,9 +4,9 @@ typedef struct {
 	SPPoint velocity;
 
 	void init(float screen_x, float screen_y) {
-		cur.x = screen_x;
-		prev.x = screen_x;
-		cur.y = screen_y;
-		prev.y = screen_y;
+		cur.x.set(screen_x);
+		prev.x.set(screen_x);
+		cur.y.set(screen_y);
+		prev.y.set(screen_y);
 	}
 } motion_t;

th05/main/player/shot.hpp

@@ -66,8 +66,8 @@ struct ShotAddIterator {
 #define MISSILE_R shot->type = ST_MISSILE_RIGHT
 #define MISSILE_S shot->type = ST_MISSILE_STRAIGHT
 
-#define VELOCITY_X(screen_x) shot->pos.velocity.x = screen_x;
-#define VELOCITY_Y(screen_y) shot->pos.velocity.y = screen_y;
+#define VELOCITY_X(screen_x) shot->pos.velocity.x.set(screen_x);
+#define VELOCITY_Y(screen_y) shot->pos.velocity.y.set(screen_y);
 
 #define VELOCITY_XY(screen_x, screen_y) \
 	VELOCITY_X(screen_x); \

th05/main/stage/setup.cpp

@@ -40,7 +40,7 @@ void pascal near stage2_setup(void)
 	midboss_render_func = midboss2_render;
 	midboss.frames_until = 2750;
 	midboss.pos.init(192, -32);
-	midboss.pos.velocity.set(0, 0.5f);
+	midboss.pos.velocity.set(0.0f, 0.5f);
 	midboss.hp = 1400;
 	midboss.sprite = 202;
 
@@ -67,7 +67,7 @@ void pascal near stage3_setup(void)
 	midboss_render_func = midboss3_render;
 	midboss.frames_until = 5750;
 	midboss.pos.init(192, -32);
-	midboss.pos.velocity.set(0, 0.5f);
+	midboss.pos.velocity.set(0.0f, 0.5f);
 	midboss.hp = 1400;
 	midboss.sprite = 208;
 
@@ -97,7 +97,7 @@ void pascal near stage4_setup(void)
 	midboss_render_func = midboss4_render;
 	midboss.frames_until = 3900;
 	midboss.pos.init(192, -32);
-	midboss.pos.velocity.set(0, 0.5f);
+	midboss.pos.velocity.set(0.0f, 0.5f);
 	midboss.hp = 1100;
 	midboss.sprite = 208;
 
@@ -126,7 +126,7 @@ void pascal near stage5_setup(void)
 	midboss_render_func = midboss5_render;
 	midboss.frames_until = 4800;
 	midboss.pos.init(192, -32);
-	midboss.pos.velocity.set(0, 0.5f);
+	midboss.pos.velocity.set(0.0f, 0.5f);
 	midboss.hp = 1550;
 	midboss.sprite = 212;
 

th05/p_mima.cpp

@@ -18,7 +18,7 @@ void pascal near shot_mima_l2(void)
 		if(sai.i <= 2) {
 			if(sai.i == 2) { shot->from_option_l(); }
 			else/*i == 1*/ { shot->from_option_r(); }
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			shot->set_option_sprite_and_damage(5);
 		} else {
 			shot->set_random_angle_forwards();
@@ -37,12 +37,12 @@ void pascal near shot_mima_l3(void)
 		if(sai.i <= 2) {
 			if(sai.i == 2) { shot->from_option_l(); }
 			else/*i == 1*/ { shot->from_option_r(); }
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			shot->set_option_sprite_and_damage(5);
 		} else {
 			if(sai.i == 4) { shot->pos.cur.x -= 8.0f; }
 			else/*i == 3*/ { shot->pos.cur.x += 8.0f; }
-			shot->pos.velocity.y = -12.0f;
+			shot->pos.velocity.y.set(-12.0f);
 			shot->damage = 7;
 		}
 		if(sai.next() <= 0) {
@@ -63,12 +63,12 @@ void pascal near shot_mima_l4(void)
 			case 3:	                  	shot->from_option_l();	break;
 			case 2:	                  	shot->from_option_r();	break;
 			}
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			shot->set_option_sprite_and_damage(4);
 		} else {
 			if(sai.i == 6) { shot->pos.cur.x -= 8.0f; }
 			else/* i== 5*/ { shot->pos.cur.x += 8.0f; }
-			shot->pos.velocity.y = -12.0f;
+			shot->pos.velocity.y.set(-12.0f);
 			shot->damage = 7;
 		}
 		if(sai.next() <= 0) {
@@ -90,7 +90,7 @@ void pascal near shot_mima_l5(void)
 			case 1:	VELOCITY_X(-1.0f);	shot->from_option_r();	break;
 			case 2:	                  	shot->from_option_r();	break;
 			}
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			shot->set_option_sprite_and_damage(4);
 		} else {
 			shot_velocity_set(&shot->pos.velocity, sai.angle);
@@ -115,7 +115,7 @@ void pascal near shot_mima_l6(void)
 			case 2:	VELOCITY_X(-0.5f);	shot->from_option_r();	break;
 			case 3:	VELOCITY_X( 0.5f);	shot->from_option_l();	break;
 			}
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			shot->set_option_sprite_and_damage(4);
 		} else {
 			shot_velocity_set(&shot->pos.velocity, sai.angle);
@@ -143,7 +143,7 @@ void pascal near shot_mima_l7(void)
 			case 3:	VELOCITY_X( 0.5f);	shot->from_option_l();	break;
 			case 5:	                	shot->from_option_l();	break;
 			}
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			shot->set_option_sprite_and_damage(4);
 		} else {
 			shot_velocity_set(&shot->pos.velocity, sai.angle);
@@ -162,7 +162,7 @@ void pascal near shot_mima_l8(void)
 	sai.angle = 186;
 	while(( shot = shots_add() ) != NULL) {
 		if(sai.i <= 6) {
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			switch(sai.i - 1) {
 			// Beware, non-sequential case order!
 			case 0:	VELOCITY_XY( 1.75f, -18.0f); shot->from_option_r(8.0f);	break;
@@ -172,7 +172,7 @@ void pascal near shot_mima_l8(void)
 			case 3:	VELOCITY_X ( 0.5f);          shot->from_option_l();	break;
 			case 5:                              shot->from_option_l();	break;
 			}
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			shot->set_option_sprite_and_damage(4);
 		} else {
 			shot_velocity_set(&shot->pos.velocity, sai.angle);
@@ -190,7 +190,7 @@ void pascal near shot_mima_l9(void)
 	SHOT_FUNC_INIT(6, SC_6X, SC_3X, i += 4);
 	while(( shot = shots_add() ) != NULL) {
 		if(sai.i <= 6) {
-			shot->pos.velocity.y = -20.0f;
+			shot->pos.velocity.y.set(-20.0f);
 			switch(sai.i - 1) {
 			case 0:	VELOCITY_XY( 2.625f, -18.0f); shot->from_option_r(8.0f); break;
 			case 1:	VELOCITY_XY(-2.625f, -18.0f); shot->from_option_l(8.0f); break;

th05/p_yuuka.cpp

@@ -102,7 +102,7 @@ void pascal near shot_yuuka_l4(void)
 					sai.i = 1;
 				}
 			}
-			shot->pos.velocity.y = 2.0f;
+			shot->pos.velocity.y.set(2.0f);
 			shot->set_option_sprite_and_damage(6);
 		}
 		shot_velocity_set(&shot->pos.velocity, sai.angle);