Does [NoAlias] help on NativeArray fields?

Burst already infers no-alias for NativeArray fields in IJob/IJobParallelFor. The attribute is no-op there. It matters when you take raw pointers via GetUnsafePtr and pass them around.

Where does [NoAlias] actually help?

Function parameters that are pointers, spans, or shared NativeReferences. Burst can't always prove non-overlap. [NoAlias] tells the optimizer it's safe to vectorize.

How do I see Burst's decisions?

Jobs → Burst → Open Inspector. Look at the LLVM IR. Vectorized loops indicate the optimizer found alias-free spans; scalar loops mean it couldn't.

Fix: Unity Jobs [NoAlias] Doesn't Improve Codegen

Quick answer: Annotation is no-op for NativeArray fields (Burst infers it). Use [NoAlias] on raw pointers / function params where alias analysis fails.

You add [NoAlias] expecting Burst to vectorize a hot loop. Inspector still shows scalar code. The real bottleneck wasn’t alias inference.

The Fix

[BurstCompile]
struct SimJob : IJobParallelFor {
    public NativeArray<float> A;   // Burst already knows non-aliasing
    [ReadOnly] public NativeArray<float> B;
    public void Execute(int i) => A[i] += B[i] * 2f;
}

// Where [NoAlias] does help
[BurstCompile]
static unsafe void Compute(
    [NoAlias] float* a, [NoAlias] float* b, int n) {
    for (int i = 0; i < n; i++) a[i] = b[i] + 1f;
}

Burst Inspector LLVM IR shows whether vectorization happened. If not, look at unrolling, math fast-mode, or struct layout instead.

Verifying

Open Burst Inspector, find the job. Vectorized SIMD instructions indicate optimization. Profiler ms drops accordingly.

“NoAlias is for pointers. NativeArray gets it free.”

Related Issues

For NativeArray dispose, see dispose. For ReadOnly write, see readonly.

Annotation for pointers. Inspector confirms.