Pathtracer 16: Burst SIMD Optimization

Introduction and index of this series is here.

When I originally played with the Unity Burst compiler in “Part 3: C#, Unity, Burst”, I just did the simplest possible “get C# working, get it working on Burst” thing and left it there. Later on in “Part 10: Update C#” I updated it to use Structure-of-Arrays data layout for scene objects, and that was about it. Let’s do something about this.

Meanwhile, I have switched from late-2013 MacBookPro to mid-2018 one, so the performance numbers on a “Mac” will be different from the ones in previous posts.

Update to latest Unity + Burst + Mathematics versions

First of all, let’s update the Unity version we use from some random 2018.1 beta to the latest stable 2018.2.13, and update Burst (to 0.2.4-preview.34) & Mathematics (to 0.0.12-preview.19) packages along the way. Mathematics renamed lengthSquared to lengthsq, and introduced a PI constant that clashed with our own one :) These trivial updates in this commit.

Just that got performance on PC from 81.4 to 84.3 Mray/s, and on Mac from 31.5 to 36.5 Mray/s. I guess either Burst or Mathematics (or both) got some optimizations during this half a year, nice!

Add some “manual SIMD” to sphere intersection

Very similar to how in Part 8: SSE HitSpheres I made the C++ HitSpheres function do intersection testing of one ray against 4 spheres at once, we’ll do the same in our Unity C# Burst code.

The thought process and work done is extremely similar to the C++ side done in Part 8 and Part 9; basically:

  • Since data for our spheres is laid out nicely in SoA style arrays, we can easily load data for 4 of them at once.
  • Do all ray intersection math on these 4 spheres,
  • If any are hit, pick the closest one and calculate final hit position & normal.

HitSpheres function code gets to be extremely similar between C++ version and C# version. In fact the C# one is cleaner since float4, int4 and bool4 types in Mathematics package are way more complete SIMD wrappers than my toy manual implementations in the C++ version.

The full change commit is here.

Performance: PC from 84.3 to 133 Mray/s, and Mac from 35.5 to 60.0 Mray/s. Not bad!

Updated numbers for new Mac hardware

Implementation PC Mac
GPU 1854 246
C++, SSE+SoA HitSpheres 187 74
C#, Unity Burst, 4-wide HitSpheres 133 60
C++, SoA HitSpheres 100 36
C#, Unity Burst 82 36
C#, .NET Core 53.0 23.6
C#, mono -O=float32 --llvm w/ MONO_INLINELIMIT=100 22.0
C#, mono -O=float32 --llvm 18.9
C#, mono -O=float32 11.0
C#, mono 6.1
  • PC is AMD ThreadRipper 1950X (3.4GHz, 16c/16t - SMT disabled) with GeForce GTX 1080 Ti.
  • Mac is mid-2018 MacBookPro (Core i9-8950HK 2.9GHz, 6c/12t) with AMD Radeon Pro 560X.
  • Unity version 2018.2.13 with Burst 0.2.4-preview.34 and Mathematics 0.0.12-preview.19.
  • Mono version 5.12.
  • .NET Core version 2.1.302.

All code is on github at 16-burst-simd tag.