[API Proposal]: Vector128.ShuffleUnsafe

[MihaZupan]

Background and motivation

When implementing vectorized algorithms using the new Vector128 and friends APIs, Vector128.Shuffle is a case where one sometimes has to drop back down to using architecture-specific instructions.

Vector128.Shuffle guarantees that results for out-of-range indices (values above 15) are normalized to 0.
If the JIT can't prove that indices are already in the [0, 15] range, it is forced to emit extra logic to do the normalization.
For some algorithms, you may also want exactly the behavior of the underlying intrinsic (e.g. X86's PSHUFB will treat indices MOD 16, and set the result to 0 if the high bit is set).

The solution in both cases is to use the architecture-specific instruction directly, often writing a helper such as

private static Vector128<byte> Shuffle(Vector128<byte> vector, Vector128<byte> indices)
{
    return Ssse3.IsSupported
        ? Ssse3.Shuffle(vector, indices)
        : AdvSimd.Arm64.VectorTableLookup(vector, indices);
}

(We already have 5 such helpers in runtime)

I propose adding a Vector128.ShuffleUnsafe helper that does not guarantee identical behavior for out-of-range indices across platforms. Instead, it would be defined as using the underlying intrinsic directly when available.

API Proposal

namespace System.Runtime.Intrinsics;

public static class Vector128
{
    // Existing
    public static Vector128<byte> Shuffle(Vector128<byte> vector, Vector128<byte> indices);

    // Proposed
    public static Vector128<byte> ShuffleUnsafe(Vector128<byte> vector, Vector128<byte> indices);
}

API Usage

-Vector128<byte> bitMask = Shuffle(bitmapLookup, lowNibbles);
-Vector128<byte> bitPositions = Shuffle(Vector128.Create(0x8040201008040201).AsByte(), 
+Vector128<byte> bitMask = Vector128.ShuffleUnsafe(bitmapLookup, lowNibbles);
+Vector128<byte> bitPositions = Vector128.ShuffleUnsafe(Vector128.Create(0x8040201008040201).AsByte(), highNibbles);

-private static Vector128<byte> Shuffle(Vector128<byte> vector, Vector128<byte> indices)
-{
-    return Ssse3.IsSupported
-        ? Ssse3.Shuffle(vector, indices)
-        : AdvSimd.Arm64.VectorTableLookup(vector, indices);
-}

Alternative Designs

No response

Risks

No response

[ghost]

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Issue Details

---

Background and motivation

When implementing vectorized algorithms using the new Vector128 and friends APIs, Vector128.Shuffle is a case where one sometimes has to drop back down to using architecture-specific instructions.

Vector128.Shuffle guarantees that results for out-of-range indices (values above 15) are normalized to 0.
If the JIT can't prove that indices are already in the [0, 15] range, it is forced to emit extra logic to do the normalization.
For some algorithms, you may also want exactly the behavior of the underlying intrinsic (e.g. X86's PSHUFB will treat indices MOD 16, and set the result to 0 if the high bit is set).

The solution in both cases is to use the architecture-specific instruction directly, often writing a helper such as

private static Vector128<byte> Shuffle(Vector128<byte> vector, Vector128<byte> indices)
{
    return Ssse3.IsSupported
        ? Ssse3.Shuffle(vector, indices)
        : AdvSimd.Arm64.VectorTableLookup(vector, indices);
}

(as of #80963, we already have 4 such helpers in runtime)

I propose adding a Vector128.ShuffleUnsafe helper that does not guarantee identical behavior for out-of-range indices across platforms. Instead, it would be defined as using the underlying intrinsic directly when available.

API Proposal

namespace System.Runtime.Intrinsics;

public static class Vector128
{
    // Existing
    public static Vector128<byte> Shuffle(Vector128<byte> vector, Vector128<byte> indices);

    // Proposed
    public static Vector128<byte> ShuffleUnsafe(Vector128<byte> vector, Vector128<byte> indices);
}

API Usage

-Vector128<byte> bitMask = Shuffle(bitmapLookup, lowNibbles);
-Vector128<byte> bitPositions = Shuffle(Vector128.Create(0x8040201008040201).AsByte(), 
+Vector128<byte> bitMask = Vector128.ShuffleUnsafe(bitmapLookup, lowNibbles);
+Vector128<byte> bitPositions = Vector128.ShuffleUnsafe(Vector128.Create(0x8040201008040201).AsByte(), highNibbles);

-private static Vector128<byte> Shuffle(Vector128<byte> vector, Vector128<byte> indices)
-{
-    return Ssse3.IsSupported
-        ? Ssse3.Shuffle(vector, indices)
-        : AdvSimd.Arm64.VectorTableLookup(vector, indices);
-}

Alternative Designs

No response

Risks

No response

Author:	MihaZupan
Assignees:	-
Labels:	api-suggestion, area-System.Runtime.Intrinsics
Milestone:	-

[terrajobst]

Video

• Looks good as proposed
• Overloads for other T's are approved by this, but we don't need them yet.

namespace System.Runtime.Intrinsics;

public partial class Vector64
{
    // Existing
    // public static Vector64<byte> Shuffle(Vector64<byte> vector, Vector64<byte> indices);

    public static Vector64<byte> ShuffleUnsafe(Vector64<byte> vector, Vector64<byte> indices);
}

public partial class Vector128
{
    // Existing
    // public static Vector128<byte> Shuffle(Vector128<byte> vector, Vector128<byte> indices);

    public static Vector128<byte> ShuffleUnsafe(Vector128<byte> vector, Vector128<byte> indices);
}

public partial class Vector256
{
    // Existing
    // public static Vector256<byte> Shuffle(Vector256<byte> vector, Vector256<byte> indices);

    public static Vector256<byte> ShuffleUnsafe(Vector256<byte> vector, Vector256<byte> indices);
}

public partial class Vector512
{
    // Existing
    // public static Vector512<byte> Shuffle(Vector512<byte> vector, Vector512<byte> indices);

    public static Vector512<byte> ShuffleUnsafe(Vector512<byte> vector, Vector512<byte> indices);
}

[tannergooding]

This won't make .NET 8 and isn't critical enough to warrant a bar check given there are several trivial workarounds.

We can finish up the PR once CI is passing anytime after main opens for .NET 9 changes next month