Remove unsafe code from System.Numerics.BigIntegerCalculator.ShiftRot

[EgorBo]

Note

This PR is AI-generated.

+5b asm diffs

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-numerics
See info in area-owners.md if you want to be subscribed.

[EgorBo]

@MihuBot

[Copilot]

Pull request overview

This PR refactors System.Runtime.Numerics' internal BigIntegerCalculator shift/rotate implementation to remove direct LoadUnsafe/StoreUnsafe usage from the SIMD paths, which sit underneath BigInteger shift and rotate operations.

Changes:

• Reworks LeftShiftSelf to use sliced spans plus Vector128/256/512.Create and CopyTo instead of ref+offset-based vector loads/stores.
• Applies the same span-based vector load/store pattern to RightShiftSelf.
• Changes loop bookkeeping from integer offsets to shrinking Span<nuint> windows while keeping the existing carry/scalar fallback structure.

[EgorBo]

Note

AI-generated benchmark.

@EgorBot -arm -amd

using System;
using System.Numerics;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

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

public class Bench
{
    private BigInteger _value;

    [Params(128, 1024, 8192)]
    public int Bits { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        var rng = new Random(42);
        byte[] bytes = new byte[Bits / 8];
        rng.NextBytes(bytes);
        bytes[^1] &= 0x7F; // ensure positive
        _value = new BigInteger(bytes);
    }

    [Benchmark]
    public BigInteger LeftShift() => _value << 7;

    [Benchmark]
    public BigInteger RightShift() => _value >> 7;
}