Should MonoPInvokeCallback be required to all [BurstCompile] functions in an IL2CPP build?

Hi all,

Quick question about using the new DirectCall burst stuff. Here's a code snippet:

Code (CSharp):

1. public class BurstTest : MonoBehaviour
2. {
3.     // Start is called before the first frame update
4.     void Awake()
5.     {
6.         bool same = BurstUtils.SameSide(float3.zero, new float3(1.0f, 0.0f, 0.0f), new float3(1.0f, 1.0f, 0.0f), new float3(1.0f, -1.0f, 0.0f));
7.         Debug.Log(same);
8.     }
9. }
10.  
11. [BurstCompile]
12. public static class BurstUtils
13. {
14.     public delegate bool bool_float3x4(float3 a, float3 b, float3 c, float3 d);
15.     public delegate bool float_float3x2(float3 a, float3 b);
16.    
17.     [BurstCompile]
18.     [MonoPInvokeCallback(typeof(bool_float3x4))] // <- SHOULD I NEED THIS?!
19.     public static bool SameSide(in float3 p1, in float3 p2, in float3 a, in float3 b)
20.     {
21.         float3 cp1 = math.cross(b - a, p1 - a);
22.         float3 cp2 = math.cross(b - a, p2 - a);
23.  
24.         return math.dot(cp1, cp2) >= 0f;
25.     }
26. }

So, we're calling into some BurstCompiled utility code from a mono-behaviour. Nothing is jobbified. In a Mono build, this all works fine *without* the MonoPInvokeCallback boilerplate. In an IL2CPP build though, without that attribute I get this error:

The method `Boolean SameSide(Unity.Mathematics.float3 ByRef, Unity.Mathematics.float3 ByRef, Unity.Mathematics.float3 ByRef, Unity.Mathematics.float3 ByRef)` must have `MonoPInvokeCallback` attribute to be compatible with IL2CPP!
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:Log(Object)
Unity.Burst.BurstCompiler:Compile(Object, MethodInfo, Boolean, Object)
Unity.Burst.BurstCompiler:CompileUnsafeStaticMethod(RuntimeMethodHandle)
SameSide_00000005$BurstDirectCall:GetFunctionPointer()
SameSide_00000005$BurstDirectCall:Constructor()
BurstTest:Awake()

Just wanted to check that's expected behaviour?

Also, I'm wondering why it's trying to compile the code at run-time? My understanding was that all compilation should happen at build time.

This is using Unity 2021.1.0f1, and Burst 1.5.0-pre.5.

Apologies if I'm missing something obvious.

Thanks,
Rob.

 

Interesting, the stack is telling me that it is compiling function when trying to do a direct call. It seems it needs to get a function pointer. Referring to https://docs.unity3d.com/Packages/c...al/docs/AdvancedUsages.html#function-pointers, MonoPInvokeCallback is required when using function pointers.