Arm64 SVE: Rename arguments for SVE ConditionalExtract APIs

[a74nh]

Background and motivation

During implementation, the naming of the default value arguments changed from what was reviewed. Original API approval was in #93964

This issue should either approve the differences, or they should be rejected and a PR to fix up the code should be raised.
The implemented APIs need changing to match what was approved.

Approved APIs:

  /// T: float, double, sbyte, short, int, long, byte, ushort, uint, ulong
  public static unsafe Vector<T> ConditionalExtractAfterLastActiveElement(Vector<T> mask, Vector<T> defaultScalar, Vector<T> data); // CLASTA // MOVPRFX
  public static unsafe Vector<T> ConditionalExtractAfterLastActiveElementAndReplicate(Vector<T> mask, Vector<T> defaultValues, Vector<T> data); // CLASTA // MOVPRFX
  public static unsafe T ConditionalExtractAfterLastActiveElement(Vector<T> mask, T defaultValue, Vector<T> data); // CLASTA // MOVPRFX

  /// T: float, double, sbyte, short, int, long, byte, ushort, uint, ulong
  public static unsafe Vector<T> ConditionalExtractLastActiveElement(Vector<T> mask, Vector<T> defaultScalar, Vector<T> data); // CLASTB // MOVPRFX
  public static unsafe Vector<T> ConditionalExtractLastActiveElementAndReplicate(Vector<T> mask, Vector<T> defaultValues, Vector<T> data); // CLASTB // MOVPRFX
  public static unsafe T ConditionalExtractLastActiveElement(Vector<T> mask, T defaultValue, Vector<T> data); // CLASTB // MOVPRFX
}

Currently implemented:

namespace System.Runtime.Intrinsics.Arm;

/// VectorT Summary
public abstract partial class Sve : AdvSimd /// Feature: FEAT_SVE  Category: mask
{
  /// T: float, double, sbyte, short, int, long, byte, ushort, uint, ulong
  public static unsafe Vector<T> ConditionalExtractAfterLastActiveElement(Vector<T> mask, Vector<T> defaultValue, Vector<T> data); // CLASTA // MOVPRFX
  public static unsafe Vector<T> ConditionalExtractAfterLastActiveElementAndReplicate(Vector<T> mask, Vector<T> defaultScalar, Vector<T> data); // CLASTA // MOVPRFX
  public static unsafe T ConditionalExtractAfterLastActiveElement(Vector<T> mask, T defaultValues, Vector<T> data); // CLASTA // MOVPRFX

  /// T: float, double, sbyte, short, int, long, byte, ushort, uint, ulong
  public static unsafe Vector<T> ConditionalExtractLastActiveElement(Vector<T> mask, Vector<T> defaultValue, Vector<T> data); // CLASTB // MOVPRFX
  public static unsafe Vector<T> ConditionalExtractLastActiveElementAndReplicate(Vector<T> mask, Vector<T> fallback, Vector<T> data); // CLASTB // MOVPRFX
  public static unsafe T ConditionalExtractLastActiveElement(Vector<T> mask, T defaultValues, Vector<T> data); // CLASTB // MOVPRFX
}

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[tannergooding]

@a74nh it looks like the originally approved names are the correct ones and we should likely just fix?

We have CLASTA (scalar), CLASTA (SIMD&FP scalar), and CLASTA (vectors)

The first (CLASTA (scalar)) takes and returns a T, so public static unsafe T ConditionalExtractAfterLastActiveElement(Vector<T> mask, T defaultValue, Vector<T> data); is correct

The second (CLASTA (SIMD&FP scalar)) is similar, but takes a Vector<T> and returns a Vector<T> because that's the register the floating-point types live in and makes it convenient to compose with other SIMD operations, so Vector<T> ConditionalExtractAfterLastActiveElement(Vector<T> mask, Vector<T> defaultScalar, Vector<T> data); is correct and the name defaultScalar was used to make it clear only element 0 is used from the input. Notably it zero extends the element up to the result register size.

The third (CLASTA (vectors)) takes the whole Vector<T> as the default and otherwise replicates the element from data, so it is again correct as Vector<T> ConditionalExtractAfterLastActiveElementAndReplicate(Vector<T> mask, Vector<T> defaultValues, Vector<T> data);, with a minor nuance I think that defaultValue is potentially sufficient

[a74nh]

Switched the title and description so that it's no longer an API suggestion change