Fix: Unity Burst Math Diverges Across Platforms

Lockstep multiplayer sim. Same inputs, two clients diverge after 30 ticks. Burst’s default float mode reorders math; one CPU does an FMA, another doesn’t. Different bits.

The Symptom

Cross-play desync. Replays from one platform fail to reproduce on another. Hash-checks of physics state diverge after a handful of ticks.

The Fix

using Unity.Burst;

[BurstCompile(// strict cross-platform determinism
    FloatMode = FloatMode.Strict,
    FloatPrecision = FloatPrecision.Standard
)]
public struct SimulationStep : IJob {
    public void Execute() {
        // math here is portable
    }
}

Strict mode disables FMA contraction, associativity reorder, and reciprocal-multiply substitutions. Slower — but every ISA produces identical bits.

Better: Fixed-Point

struct Fix64 {  // 32.32 fixed-point
    public long raw;
    public static Fix64 operator +(Fix64 a, Fix64 b) =>
        new Fix64 { raw = a.raw + b.raw };
    // ... mul with shift, div with shift
}

Integer arithmetic is bit-identical across all CPUs Burst targets. Slightly more work to maintain but immune to float-mode regressions.

Verifying

Run the sim on Windows x64, ARM macOS, and IL2CPP-Android. Hash the state every tick. Hashes must match. With FloatMode.Fast: divergence within 100 ticks; with Strict or Fix64: matches to 100,000+.

“Strict mode for floats. Fixed-point for paranoia. Determinism holds.”

Related Issues

For Burst parallel batch size, see batch size. For IL2CPP reflection, see link.xml.

Strict floats. Or fixed-point. Sim agrees.