Bug Burst executes both branches of if/else, if they are too similar

Hi,

I have this if/else branch in a bursted ISystem, where both, the if and the else part are executed simultaneously for the same entity (There is only one matching entity, as also shown by the debug.logs).
Both branches add an empty (but different) tag component to the entity, and add an element to a DynamicBuffer.

Code (CSharp):

1.  
2. [BurstCompile]
3. public void OnUpdate(ref SystemState state)
4. {
5.     // ...
6.  
7.     var ecb = SystemAPI.GetSingleton<EndSimulationEntityCommandBufferSystem.Singleton>()
8.                    .CreateCommandBuffer(state.WorldUnmanaged);
9.     foreach (var (buffers, battle, match, e) in
10.                 SystemAPI.Query<BattleBuffers, RefRW<OngoingBattle>, RefRO<Match>>()
11.                         .WithAll<BattleState.PerformAction>()
12.                         .WithEntityAccess())
13.     {
14.         // ...
15.         Debug.Log($"--- before IF/ELSE for E{e.Index}:{e.Version}; buffer isEmpty: {buffers.actionQueue.IsEmpty}");
16.         if (buffers.actionQueue.IsEmpty)
17.         {
18.             ecb.AddComponent<BattleState.WaitForTurn>(e); // empty tag component
19.             Debug.Log($"--- execute IF branch for E{e.Index}:{e.Version}");
20.             var stateUpdate = new MatchStateUpdateMessage
21.             {
22.                 ActorId = Actor.Invalid.Id,
23.                 CurrentState = BattleState.State.WaitForTurn,
24.             };
25.             foreach(var player in buffers.players)
26.                 ecb.AppendToBuffer(matchStateBuffer, stateUpdate.For(player.Connection));
27.         }
28.         else
29.         {
30.             ecb.AddComponent<BattleState.PrepareTurn>(e); // empty tag component
31.             Debug.Log($"--- execute ELSE branch for E{e.Index}:{e.Version}");
32.             var stateUpdate = new MatchStateUpdateMessage
33.             {
34.                 ActorId = action.CasterId,
35.                 CurrentState = BattleState.State.PrepareTurn,
36.             };
37.             foreach(var player in buffers.players)
38.                 ecb.AppendToBuffer(matchStateBuffer, stateUpdate.For(player.Connection));
39.         }
40.         Debug.Log($"--- after IF/ELSE E{e.Index}:{e.Version}");
41.         continue;
42.  
43.         // ...
44.     }
45. }
46.  

When this system runs with burst compilation enabled, both if and else branch are executed (however, only one of the

BattleState

tags will be present on the entity afterwards).


This can be fixed by switching the order of the statements, i.e. moving the

ecb.AddComponent

and

Debug.Log

to the bottom of the if branch.
This should not change anything logically, but provides the correct output.


EDIT: Burst version 1.7.4, Entities 1.0.0-pre.15, Unity 2022.2.0f1

 

Same log if you disable Burst compilation?

 

Yes, with Burst compilation disabled, it works as expected, and the else branch never executes. (the condition is always false).

It reproduced right away in an empty project, which I actually didn't expect. Report number is IN-30517. I added a system that logs the number of items in the buffer that is added each frame with

ecb.AppendToBuffer

in both branches, and it always logs 2 when burst is enabled, and 1 when burst is disabled.