Avoid large table as part of TensorPrimitives remainder mask #92765
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Tagging subscribers to this area: @dotnet/area-system-numerics Issue DetailsAs discussed in #92672 (review)
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...ibraries/System.Numerics.Tensors/src/System/Numerics/Tensors/TensorPrimitives.netstandard.cs
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| Vector512.LoadUnsafe( | ||
| ref Unsafe.As<uint, float>(ref MemoryMarshal.GetReference(RemainderUInt32Mask_16x16)), | ||
| (uint)(validItems * 16)); // all sixteen floats in the row | ||
| Vector512.ConditionalSelect( |
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So for AVX512 it'd looked like
var mmask16 mask = (1 << n) - 1;
var maskedAnd = Avx512.And(vec1, vec2, mask);but we decided not to expose mask registers so if we expose these as a public API, we can intrinsify at leas the AVX512 version to be cheap 🙂 if it matters, presumably, performance of handling of trailing elements is not that much important esp for large data.
| Vector128.ConditionalSelect( | ||
| Vector128.LessThan(Vector128.Create(3, 2, 1, 0), Vector128.Create(validItems)).AsSingle(), | ||
| Vector128<float>.AllBitsSet, | ||
| Vector128<float>.Zero); |
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This is going to be slower and result in a 128-bit constant (or more for other sizes) emitted at runtime per method this gets inlined into.
I think the table is ultimately better and we can get rid of it longer term using a proper JIT intrinsic.
If we really don't want the table, then achieving this with just a broadcast + comparison should be sufficient, since LessThan already produces a per-element mask of AllBitsSet (true) and Zero (false). You can use GreaterThanOrEqual if you need the mask inverted
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Ok, I'll stick with the table. I was on the fence anyway. Just don't like its bulkiness.
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That is, since TrailingMask wants to handle only trailing elements, it will skip processing the first n items that have already been processed.
So if 2 items remain, you have 3 < 2, 2 < 2, 1 < 2, 0 < 2 which produces Zero, Zero, AllBitsSet, AllBitsSet already
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As discussed in #92672 (review)