SIMD in game development Often in game development we talk about SIMD. It is the holy grail of CPU performance, often out of reach. The conventional wisdom is that it is difficult to achieve real gains from SIMD. It is tempting to build a math library around SIMD hoping to get some performance gains. However, it often has no proven benefit. It just feels good to be using something we know can improve performance.

For Box2D version 3.0 I decided to finally try using SIMD as it is meant to be used for solving contacts. Making contacts solve faster could yield large performance gains so I decided it would be worth the effort.

But how can I gather 4 or 8 contact pairs to be solved simultaneously? The key is graph coloring. The idea is to have a handful of colors to be assigned to all the contact constraints. For example, suppose I have 6 colors and I want to assign all the contacts to one of those 6 colors. Contact constraints act upon two bodies at a time. With graph coloring the restriction is that within a single color a body can only appear once or not at all.

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I did all this work to enable SIMD processing. Did it help? Box2D has a benchmarking console application to help answer this question. I implemented SSE2, Neon, and AVX SIMD instruction sets in the Box2D contact solver. I also implemented a scalar reference implementation. I have 5 benchmarks scenarios that push Box2D in various ways. See the benchmark results here.

The large pyramid benchmark with 4 workers has the following numbers:

  • AVX2 : 1117 fps = 0.90 ms
  • Neon : 1058 fps = 0.95 ms
  • SSE2 : 982 fps = 1.02 ms
  • scalar (AMD): 524 fps = 1.91 ms
  • scalar (M2): 679 fps = 1.47 ms

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The bottom line is that making good use of SIMD can be a lot of work but it is worth the effort because it can make games run significantly faster and handle more rigid bodies.