Confusing behaviour when modifying struct fields with #soa

[sumit-sahrawat]

Minimal Example

package example

import "core:fmt"

Entity :: struct {
	moving:   bool,
	position: [2]int,
	velocity: [2]int,
}

update_entity :: proc(entity: ^Entity) {
	if entity.moving {
		entity.position += entity.velocity
	}
}

draw_entity :: proc(entity: Entity) {
	fmt.printfln(
		"[Entity] moving = %v, position = (%v, %v)",
		entity.moving,
		entity.position.x,
		entity.position.y,
	)
}

main :: proc() {
	entities: #soa[2]Entity

	entities[0].moving = true
	entities[0].velocity = {1, 1}

	entities[1].moving = false

	for _ in 1 ..= 3 {
		for &entity in entities {
			update_entity(&entity)
		}

		for entity in entities {
			draw_entity(entity)
		}
	}
}

Output with entities: #soa[2]Entity

[Entity] id = true, position = (0, 0)
[Entity] id = false, position = (0, 0)
[Entity] id = true, position = (0, 0)
[Entity] id = false, position = (0, 0)
[Entity] id = true, position = (0, 0)
[Entity] id = false, position = (0, 0)

Output with entities: [2]Entity

[Entity] moving = true, position = (1, 1)
[Entity] moving = false, position = (0, 0)
[Entity] moving = true, position = (2, 2)
[Entity] moving = false, position = (0, 0)
[Entity] moving = true, position = (3, 3)
[Entity] moving = false, position = (0, 0)

[sumit-sahrawat]

Looks like this is the root cause:

• Can accidently take the address of an soa through for loop reference iteration #5802