Improve Utf8Parser UInt64 decimal performance

[thomhurst]

Improves decimal Utf8Parser.TryParse throughput for ulong inputs long enough to require overflow checks.

The new long-input path validates and parses the first 16 digits four bytes at a time, then uses the existing overflow logic for the remaining bytes. Inputs with a non-digit in the first 16 bytes fall back to the existing parser. Dispatch happens above TryParseUInt64D, preserving its leaf codegen for common shorter inputs.

Local BenchmarkDotNet results on Windows x64, Intel Core i7-12700K, .NET 10 source-built baseline/candidate, R2R disabled, 1 launch / 5 warmups / 10 iterations:

Input	Before	After	Ratio
4 digits	1.082 ns	1.062 ns	0.98
9 digits	3.047 ns	2.981 ns	0.98
10 digits	3.554 ns	3.356 ns	0.94
19 digits	7.764 ns	7.901 ns	1.02
ulong.MaxValue	10.561 ns	6.587 ns	0.62
Overflow	10.896 ns	6.535 ns	0.60
19 digits + invalid suffix	10.126 ns	6.357 ns	0.63

Shorter cases were equivalent under the 5% Mann-Whitney threshold. No benchmark allocates.

Validation:

• Current main CoreLib Release build: 0 warnings, 0 errors
• 100,000 randomized ulong format/parse roundtrips
• 100,000 randomized malformed-byte comparisons against the previous parser
• Exhaustive repeated-byte digit validation
• Added long leading-zero and early-invalid-suffix test cases

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[thomhurst]

@EgorBot -windows_x64 -windows_arm64 -linux_x64 -linux_arm64 -osx_arm64

using System.Buffers.Text;
using System.Text;
using BenchmarkDotNet.Attributes;

public class UInt64Utf8ParserBenchmarks
{
    private byte[] _source = null!;

    [ParamsAllValues]
    public InputCase Input { get; set; }

    [Params('\0', 'D', 'G')]
    public char Format { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        string text = Input switch
        {
            InputCase.Empty => "",
            InputCase.Zero => "0",
            InputCase.OneDigit => "7",
            InputCase.FourDigits => "1234",
            InputCase.NineDigits => "123456789",
            InputCase.TenDigits => "1234567890",
            InputCase.NineteenDigits => "1234567890123456789",
            InputCase.Maximum => "18446744073709551615",
            InputCase.Overflow => "18446744073709551616",
            InputCase.TwentyOneDigits => "100000000000000000000",
            InputCase.InvalidSuffix => "1234567890123456789x",
            InputCase.InvalidAt7 => "1234567x901234567890",
            InputCase.InvalidAt15 => "123456789012345x7890",
            InputCase.LeadingZeros => "00000000000000000000",
            _ => throw new InvalidOperationException(),
        };
        _source = Encoding.ASCII.GetBytes(text);
    }

    [Benchmark]
    public ulong TryParse()
    {
        bool success = Utf8Parser.TryParse(_source, out ulong value, out int consumed, Format);
        return value ^ ((ulong)(uint)consumed << 1) ^ (success ? 1UL : 0UL);
    }
}

public enum InputCase
{
    Empty,
    Zero,
    OneDigit,
    FourDigits,
    NineDigits,
    TenDigits,
    NineteenDigits,
    Maximum,
    Overflow,
    TwentyOneDigits,
    InvalidSuffix,
    InvalidAt7,
    InvalidAt15,
    LeadingZeros,
}

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[thomhurst]

@EgorBot -linux_amd -windows_intel -osx_arm64

using System.Buffers.Text;
using System.Text;
using BenchmarkDotNet.Attributes;

public class UInt64Utf8ParserBenchmarks
{
    private byte[] _source = null!;

    [ParamsAllValues]
    public InputCase Input { get; set; }

    [Params('\0', 'D', 'G')]
    public char Format { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        string text = Input switch
        {
            InputCase.Empty => "",
            InputCase.Zero => "0",
            InputCase.OneDigit => "7",
            InputCase.FourDigits => "1234",
            InputCase.NineDigits => "123456789",
            InputCase.TenDigits => "1234567890",
            InputCase.NineteenDigits => "1234567890123456789",
            InputCase.Maximum => "18446744073709551615",
            InputCase.Overflow => "18446744073709551616",
            InputCase.TwentyOneDigits => "100000000000000000000",
            InputCase.InvalidSuffix => "1234567890123456789x",
            InputCase.InvalidAt3 => "123x5678901234567890",
            InputCase.InvalidAt7 => "1234567x901234567890",
            InputCase.InvalidAt11 => "12345678901x34567890",
            InputCase.InvalidAt15 => "123456789012345x7890",
            InputCase.LeadingZeros => "00000000000000000000",
            _ => throw new InvalidOperationException(),
        };
        _source = Encoding.ASCII.GetBytes(text);
    }

    [Benchmark]
    public ulong TryParse()
    {
        bool success = Utf8Parser.TryParse(_source, out ulong value, out int consumed, Format);
        return value ^ ((ulong)(uint)consumed << 1) ^ (success ? 1UL : 0UL);
    }
}

public enum InputCase
{
    Empty,
    Zero,
    OneDigit,
    FourDigits,
    NineDigits,
    TenDigits,
    NineteenDigits,
    Maximum,
    Overflow,
    TwentyOneDigits,
    InvalidSuffix,
    InvalidAt3,
    InvalidAt7,
    InvalidAt11,
    InvalidAt15,
    LeadingZeros,
}

Note

Benchmark request is AI-generated with Codex.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[thomhurst]

@EgorBot -linux_amd -windows_intel -osx_arm64

using System.Buffers.Text;
using System.Text;
using BenchmarkDotNet.Attributes;

public class UInt64Utf8ParserBenchmarks
{
    private byte[] _source = null!;

    [ParamsAllValues]
    public InputCase Input { get; set; }

    [Params('\0', 'D', 'G')]
    public char Format { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        string text = Input switch
        {
            InputCase.Empty => "",
            InputCase.Zero => "0",
            InputCase.OneDigit => "7",
            InputCase.FourDigits => "1234",
            InputCase.NineDigits => "123456789",
            InputCase.TenDigits => "1234567890",
            InputCase.NineteenDigits => "1234567890123456789",
            InputCase.Maximum => "18446744073709551615",
            InputCase.Overflow => "18446744073709551616",
            InputCase.TwentyOneDigits => "100000000000000000000",
            InputCase.InvalidSuffix => "1234567890123456789x",
            InputCase.InvalidAt3 => "123x5678901234567890",
            InputCase.InvalidAt7 => "1234567x901234567890",
            InputCase.InvalidAt11 => "12345678901x34567890",
            InputCase.InvalidAt15 => "123456789012345x7890",
            InputCase.LeadingZeros => "00000000000000000000",
            _ => throw new InvalidOperationException(),
        };
        _source = Encoding.ASCII.GetBytes(text);
    }

    [Benchmark]
    public ulong TryParse()
    {
        bool success = Utf8Parser.TryParse(_source, out ulong value, out int consumed, Format);
        return value ^ ((ulong)(uint)consumed << 1) ^ (success ? 1UL : 0UL);
    }
}

public enum InputCase
{
    Empty,
    Zero,
    OneDigit,
    FourDigits,
    NineDigits,
    TenDigits,
    NineteenDigits,
    Maximum,
    Overflow,
    TwentyOneDigits,
    InvalidSuffix,
    InvalidAt3,
    InvalidAt7,
    InvalidAt11,
    InvalidAt15,
    LeadingZeros,
}

Note

Benchmark request is AI-generated with Codex.