Rust SIMD on the GPU

The tentative plan is to open source all the compiler and `std` bits with our products built on top (compilers are not good businesses). More about our products coming in the next couple of months!

I'm confused too. How does this fit between these approaches for paraellization:

  - CUDA kernels and Tiles (e.g. Cudarc, cuda-oxide, rust-gpu etc) - SIMD on the GPU. (E.g. as in the title...)
  - CPU SIMD using avx or SSE instructions (And probably thin wrappers for vectors so you can have sane syntax). Or the maybe-upcoming core simd which should abstract over architecture-specific instructions. Magic floats etc which do 4-16 computations at once, but are a bit clumsy to work with
  - Rayon thread pools - arbitrary parallel computations, including SIMD, one per CPU core.

It looks like from the code samples like maybe a cleaner syntax for writing code on the GPU than CUDA kernels? E.g. without mucking with serialization, host and device by abstracting over it? And inspired by core::simd. (Good choice if so, in the interest of standardizing on syntax; I did this for my x86 SIMD vector/quaternion lib as well)

Didn't want to go into crazy detail in the post.

Each family of operations is a trait parameterized by the operation itself:

  pub trait EvaluateReduction<Operation, T>: LaneEvaluator {
      /// Reduce one distributed definition to an ordinary uniform scalar.
      fn evaluate_reduction(&self, value: LaneValue<Self, role::Distributed, T>) -> T;
  }

Call sites name the operation:

  let one   = evaluator.splat::<Splat, _>(1_u32);
  let two   = evaluator.splat::<Splat, _>(2_u32);
  let three = evaluator.binary::<Add, _>(one, two);

  let total   = evaluator.reduce::<Sum, u32>(three);   // a uniform u32
  let running = <Executor as EvaluateScan<Scan<Sum, Exclusive>, u32>>::scan(&evaluator, three);

Operations like Sum, Max, ReduceXor, Inclusive, and Exclusive are all distinct types.

As mentioned in the post, execution shape is typed too. A static shuffle takes its control as a type-level constant, and the shuffle mode constrains which controls are expressible:

  // Shift down one lane, keeping our own value where the source is inactive.
  let down  = <Executor as EvaluateShuffle<Shuffle<Down>, DownOrSelf<1>, u32>>::shuffle(&ev, v);
  // Broadcast from lane zero.
  let bcast = <Executor as EvaluateShuffle<Shuffle<Broadcast>, WarpLane<0>, u32>>::shuffle(&ev, down);
  // Butterfly exchange with the neighbor one bit away.
  let bfly  = <Executor as EvaluateShuffle<Shuffle<Xor>, Butterfly<1>, u32>>::shuffle(&ev, bcast);

For an example of errors caught, a warp-scoped executor for a device-scoped barrier is a compile error:

  <ScopedWarpExecutor<'_, WarpUniform> as EvaluateBarrier<Barrier<Device>>>::barrier(evaluator)
  // error[E0277]: the trait bound `Device: NvptxBarrierScope` is not satisfied
  //               help: the trait `NvptxBarrierScope` is implemented for `Warp`

Strip mining is typed on the amount of work and the lane capacity, and it hands back one chunk at a time along with the predicate saying which lanes live in that chunk:

  // Six work items across four active lanes: two chunks, based at 0 and 4.
  <Executor as EvaluateStripMine<StripMine, (WorkItems, ActiveLanes<StripMined<4>>), i32>>::
      for_each_strip_mined(
          &evaluator,
          (WorkItems::new(6)?, ActiveLanes::new(4)?),
          |index, active| {
           // ...
          },
      );

Hopefully that gives the flavor of it.

It is not currently available but we intend to make it available after we launch our products.

Part of our thesis is that decent GPUs are in every shipping device and most software doesn't use them and should.

Could also just be trolls attracted by the Rust topic.