Vectorize SearchValues.Create min/max scan using Vector128

[Copilot]

SearchValues.Create walks input values to find min/max (used to select the optimal search implementation) using a scalar loop. This vectorizes that scan to process multiple elements per iteration.

Description

SearchValues.cs — vectorized TryGetSingleRange

Added a GetMinMax<T> helper that computes min and max in a single vectorized pass:

• Uses Vector128.Min / Vector128.Max to process Vector128<T>.Count elements per iteration (8 ushorts or 16 bytes at once)
• Handles the tail with a final overlapping vector load
• Horizontal reduction is a scalar loop over the ≤16 lane results
• char is not a valid Vector128 element type — the span is cast to ReadOnlySpan<ushort>, and the results are converted back with Unsafe.BitCast<ushort, T>
• Falls back to a scalar loop when SIMD is unavailable or input is smaller than one vector

// Before: one element at a time
foreach (T value in values) { min = T.Min(min, value); max = T.Max(max, value); }

// After: Vector128<T>.Count elements per iteration, min and max in a single pass
do {
    Vector128<T> v = Vector128.LoadUnsafe(ref current);
    vMin = Vector128.Min(vMin, v);
    vMax = Vector128.Max(vMax, v);
    current = ref Unsafe.Add(ref current, Vector128<T>.Count);
} while (Unsafe.IsAddressLessThan(ref current, ref lastVectorStart));
// + overlapping tail + horizontal reduction

Testing

Performance was validated using @EgorBot on Linux/AMD64 and macOS/ARM64 with SearchValues.Create on the 64-character Base64 alphabet as the benchmark input.

Risk

Low — GetMinMax is only called during SearchValues.Create, not on hot search paths. The scalar fallback is identical to the original code.

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[MihaZupan]

@copilot Use the benchmark skill to check the performance difference of SearchValues.Create for chars by using the Base64 alphabet as the input.

[Copilot]

@copilot Use the benchmark skill to check the performance difference of SearchValues.Create for chars by using the Base64 alphabet as the input.

@EgorBot -linux_amd -osx_arm64

using System.Buffers;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

public class Bench
{
    // Standard Base64 alphabet (64 chars)
    private const string Base64Chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    [Benchmark]
    public SearchValues<char> Create_Base64Alphabet() =>
        SearchValues.Create(Base64Chars);
}