Use a more optimal formulation of swizzle_dyn for AVX2 - #548
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@dzaima got nerd-sniped by [my blog post](https://shnatsel.github.io/improving-std-simd-swizzle-dyn/) about `swizzle_dyn_precise` and workshopped even faster formulations. I couldn't let that stand and tried optimizing these this myself. All the formulations we tried, their godbolt links and their llvm-mca timings can be found at https://gist.github.com/Shnatsel/38abb51f0dc337837b1c84191461ed56 The one proposed in this PR is the last line of the table, "Blend using control-derived mask". On my Zen2 laptop this improves performance dramatically: 33% less time taken which means 50% higher throughput. Corresponding `std::simd` PR: rust-lang/portable-simd#548 ---- This PR also includes an optimization to swizzle_dyn, the non-precise variant. Details on its performance are in the commit message.
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@dzaima got nerd-sniped by my blog post about
swizzle_dynand workshopped even faster formulations. I couldn't let that stand and tried optimizing these this myself.All the formulations we tried, their godbolt links and their llvm-mca timings can be found at https://gist.github.com/Shnatsel/38abb51f0dc337837b1c84191461ed56
The one proposed in this PR is the last line of the table, "Blend using control-derived mask".
On my Zen2 laptop this improves performance dramatically: 33% less time taken which means 50% higher throughput.