Refactor ValueStringBuilder

[kzrnm]

Extracted the logic into EnsureTerminated()

This allowed removing the overloads of AsSpan(terminate) and GetPinnableReference(terminate).

Unified the implementation of AppendFormat with that of StringBuilder .

Copied from StringBuilder, can't be done via generic extension as ValueStringBuilder is a ref struct and cannot be used in a generic.

In .NET 9, generics can use ref structs.

This is reverted due to observed performance regression.

Remove ValueStringBuilder.TryCopyTo.

Even though TryCopyTo has the unintuitive behavior of also calling Dispose, it’s only used in a single place—Number.BigInteger.cs.

[jkotas]

How does this affect StringBuilder.AppendFormat performance?

[kzrnm]

@jkotas

The benchmark results for string.Format (which internally calls ValueStringBuilder.AppendFormat) and StringBuilder.AppendFormat are as follows. With StringBuilder there is almost no impact, while with ValueStringBuilder performance has improved.

BenchmarkDotNet v0.15.2, Windows 11 (10.0.26100.4770/24H2/2024Update/HudsonValley)
13th Gen Intel Core i5-13500 2.50GHz, 1 CPU, 20 logical and 14 physical cores
.NET SDK 10.0.100-rc.1.25420.111
  [Host]     : .NET 10.0.0 (10.0.25.42111), X64 RyuJIT AVX2
  Job-ZICBXN : .NET 10.0.0 (42.42.42.42424), X64 RyuJIT AVX2
  Job-QENHHP : .NET 10.0.0 (42.42.42.42424), X64 RyuJIT AVX2

Method	Toolchain	Mean	Ratio	Code Size	Gen0	Allocated	Alloc Ratio
ValueStringBuilderSmall	\main\corerun.exe	170.2 ns	1.00	2,673 B	0.0203	256 B	1.00
ValueStringBuilderSmall	\pr\corerun.exe	164.7 ns	0.97	409 B	0.0203	256 B	1.00
StringBuilderSmall	\main\corerun.exe	145.7 ns	1.00	6,266 B	-	-	NA
StringBuilderSmall	\pr\corerun.exe	146.6 ns	1.01	5,452 B	-	-	NA
ValueStringBuilderLarge	\main\corerun.exe	6,534.0 ns	1.00	2,673 B	0.4654	5856 B	1.00
ValueStringBuilderLarge	\pr\corerun.exe	5,228.6 ns	0.80	409 B	0.4654	5856 B	1.00
StringBuilderLarge	\main\corerun.exe	4,900.4 ns	1.00	6,864 B	-	-	NA
StringBuilderLarge	\pr\corerun.exe	4,832.9 ns	0.99	5,860 B	-	-	NA

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using BenchmarkDotNet.Configs;
using System;
using System.Globalization;
using System.Runtime.InteropServices;
using System.Linq;
using System.Text;

[DisassemblyDiagnoser]
[MemoryDiagnoser(false)]
[HideColumns("Job", "Error", "StdDev", "Median", "RatioSD")]
[GroupBenchmarksBy(BenchmarkLogicalGroupRule.ByMethod)]
public class StringBuilderTest
{
    object[] formatArgs;
    string formatSmall, formatLarge;
    StringBuilder sb;

    [GlobalSetup]
    public void Setup()
    {
        sb = new StringBuilder();
        formatSmall = "Text={0}, DateTime={1:G}, Number={2}, NumberX={2:X}, EnumD={3:D}, EnumF={3:F}";
        formatLarge = formatSmall + ", " + string.Join(", ", Enumerable.Repeat("DateTime={1:yyyyMMdd-HH:mm:ss}", 100));
        formatArgs = ["Text", new DateTime(2025, 4, 5, 6, 7, 5), long.MinValue, DayOfWeek.Friday];

        StringBuilderLarge(); // allocation
    }

    [Benchmark] public string ValueStringBuilderSmall() => string.Format(CultureInfo.InvariantCulture, formatSmall, formatArgs);
    [Benchmark]
    public StringBuilder StringBuilderSmall()
    {
        sb.Clear();
        return sb.AppendFormat(CultureInfo.InvariantCulture, formatSmall, formatArgs);
    }

    [Benchmark] public string ValueStringBuilderLarge() => string.Format(CultureInfo.InvariantCulture, formatLarge, formatArgs);
    [Benchmark]
    public StringBuilder StringBuilderLarge()
    {
        sb.Clear();
        return sb.AppendFormat(CultureInfo.InvariantCulture, formatLarge, formatArgs);
    }
}

[jkotas]

@EgorBot -intel

using BenchmarkDotNet.Attributes;
using System.Text;

public class Bench
{
    StringBuilder sb = new StringBuilder(100);

    [IterationSetup]
    public void IterationSetup() => sb.Clear();

    [Benchmark]
    public void AppendFormatSimple() => sb.AppendFormat("{0}{1}{2}", 2, 3, 4);
}

[jkotas]

30+% regression in the simple micro-benchmark for StringBuilder.AppendFormat: EgorBot/runtime-utils#484 (comment)

[kzrnm]

@jkotas I reverted the changes related to AppendFormat. I’ll look into whether this can be addressed separately.

[jkotas]

Thanks