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Use Span.Fill in ValueStringBuilder.Append for repeating chars - #131652

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Use Span.Fill in ValueStringBuilder.Append for repeating chars#131652
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prozolic:valuestringbuilder-append

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@prozolic prozolic commented Jul 31, 2026

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This PR uses Span<char>.Fill in ValueStringBuilder.Append(char c, int count) and ValueStringBuilder<TChar>.Append(TChar c, int count) instead of a for loop to add each char.
This change is expected to improve performance for larger sizes.

Benchmark

The benchmark uses BigInteger.ToString/BigInteger.TryFormat, which internally use ValueStringBuilder<TChar>.Append(TChar c, int count)

Method Job Toolchain Padding Mean Error StdDev Median Min Max Ratio RatioSD Gen0 Allocated Alloc Ratio
ToString_Binary Job-QIDXWP \main\corerun.exe 1 53.49 ns 4.516 ns 5.201 ns 52.80 ns 45.88 ns 62.27 ns 1.00 0.00 0.0049 32 B 1.00
ToString_Binary Job-TLPBEL \pr\corerun.exe 1 47.55 ns 2.400 ns 2.668 ns 47.47 ns 42.59 ns 53.49 ns 0.90 0.10 0.0049 32 B 1.00
TryFormat_Binary Job-QIDXWP \main\corerun.exe 1 38.48 ns 1.970 ns 2.108 ns 37.83 ns 35.55 ns 43.40 ns 1.00 0.00 - - NA
TryFormat_Binary Job-TLPBEL \pr\corerun.exe 1 38.83 ns 1.274 ns 1.308 ns 38.71 ns 35.83 ns 40.48 ns 1.01 0.06 - - NA
ToString_Binary Job-QIDXWP \main\corerun.exe 4 51.56 ns 3.178 ns 3.532 ns 50.35 ns 47.13 ns 61.45 ns 1.00 0.00 0.0063 40 B 1.00
ToString_Binary Job-TLPBEL \pr\corerun.exe 4 48.15 ns 1.408 ns 1.506 ns 48.16 ns 45.64 ns 50.94 ns 0.94 0.07 0.0063 40 B 1.00
TryFormat_Binary Job-QIDXWP \main\corerun.exe 4 39.45 ns 2.126 ns 2.363 ns 39.62 ns 34.10 ns 43.53 ns 1.00 0.00 - - NA
TryFormat_Binary Job-TLPBEL \pr\corerun.exe 4 38.37 ns 1.308 ns 1.399 ns 38.49 ns 35.64 ns 41.50 ns 0.98 0.07 - - NA
ToString_Binary Job-QIDXWP \main\corerun.exe 16 58.34 ns 3.190 ns 3.413 ns 58.62 ns 53.47 ns 65.93 ns 1.00 0.00 0.0101 64 B 1.00
ToString_Binary Job-TLPBEL \pr\corerun.exe 16 52.59 ns 2.213 ns 2.368 ns 52.64 ns 47.66 ns 56.74 ns 0.90 0.06 0.0100 64 B 1.00
TryFormat_Binary Job-QIDXWP \main\corerun.exe 16 41.96 ns 1.108 ns 1.138 ns 42.10 ns 39.02 ns 43.52 ns 1.00 0.00 - - NA
TryFormat_Binary Job-TLPBEL \pr\corerun.exe 16 37.81 ns 1.457 ns 1.496 ns 37.60 ns 35.27 ns 40.15 ns 0.90 0.04 - - NA
ToString_Binary Job-QIDXWP \main\corerun.exe 64 76.80 ns 3.734 ns 3.995 ns 75.83 ns 71.39 ns 84.62 ns 1.00 0.00 0.0252 160 B 1.00
ToString_Binary Job-TLPBEL \pr\corerun.exe 64 59.92 ns 3.232 ns 3.459 ns 59.79 ns 55.52 ns 68.02 ns 0.78 0.06 0.0253 160 B 1.00
TryFormat_Binary Job-QIDXWP \main\corerun.exe 64 59.51 ns 6.173 ns 6.605 ns 59.50 ns 51.53 ns 75.67 ns 1.00 0.00 - - NA
TryFormat_Binary Job-TLPBEL \pr\corerun.exe 64 39.51 ns 1.829 ns 1.878 ns 40.05 ns 35.84 ns 42.44 ns 0.67 0.07 - - NA
ToString_Binary Job-QIDXWP \main\corerun.exe 256 179.44 ns 5.598 ns 5.749 ns 178.89 ns 165.36 ns 191.94 ns 1.00 0.00 0.0861 544 B 1.00
ToString_Binary Job-TLPBEL \pr\corerun.exe 256 86.34 ns 4.322 ns 4.977 ns 85.90 ns 77.26 ns 95.18 ns 0.48 0.03 0.0867 544 B 1.00
TryFormat_Binary Job-QIDXWP \main\corerun.exe 256 130.84 ns 5.883 ns 6.539 ns 128.21 ns 123.39 ns 149.16 ns 1.00 0.00 - - NA
TryFormat_Binary Job-TLPBEL \pr\corerun.exe 256 42.79 ns 2.006 ns 2.230 ns 43.23 ns 38.15 ns 46.16 ns 0.33 0.02 - - NA
Benchmark source 
    [BenchmarkCategory(Categories.Libraries)]
    public class Perf_FormatPaddingForBigInteger
    {
        private const int CharsForBits = 2;

        [Params(1, 4, 16, 64, 256)]
        public int Padding;

        private BigInteger bigInteger;
        private string paddedFormat;
        private char[] destination;

        [GlobalSetup]
        public void Setup()
        {
            bigInteger = BigInteger.One;
            paddedFormat = $"B{(CharsForBits + Padding)}";
            destination = new char[CharsForBits + Padding];
        }

        [Benchmark]
        public string ToString_Binary()
        {
            return bigInteger.ToString(paddedFormat);
        }

        [Benchmark]
        public bool TryFormat_Binary()
        {
            return bigInteger.TryFormat(destination, out _, paddedFormat);
        }
    }

@prozolic
Use Span.Fill in ValueStringBuilder.Append for repeating chars
7313026 
This PR uses Span<char>.Fill in ValueStringBuilder.Append(char c, int
count) and ValueStringBuilder<TChar>.Append(TChar c, int count)
instead of a for loop to add each char.
This change expected to improve performance for larger sizes.
Copilot AI review requested due to automatic review settings July 31, 2026 16:16
@dotnet-policy-service dotnet-policy-service Bot added the community-contribution Indicates that the PR has been added by a community member label Jul 31, 2026
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Pull request overview

This PR replaces manual per-element loops with Span<T>.Fill when appending a repeated character into ValueStringBuilder buffers, leveraging the framework’s optimized fill implementations to improve performance on larger repeat counts.

Changes:

  • Update ValueStringBuilder.Append(char c, int count) to use _chars.Slice(...).Fill(c) instead of a for loop.
  • Update ValueStringBuilder<TChar>.Append(TChar c, int count) to use _chars.Slice(...).Fill(c) instead of a for loop.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File Description
src/libraries/Common/src/System/Text/ValueStringBuilder.cs Replaces the repeated-char append loop with Span<char>.Fill.
src/libraries/Common/src/System/Text/ValueStringBuilder_1.cs Replaces the repeated-char append loop with Span<TChar>.Fill in the generic builder.

@prozolic
prozolic marked this pull request as ready for review July 31, 2026 16:23
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Azure Pipelines:
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tannergooding
tannergooding reviewed Jul 31, 2026
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Comment thread src/libraries/Common/src/System/Text/ValueStringBuilder.cs
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tannergooding approved these changes Jul 31, 2026
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LGTM. CC. @jeffhandley for whether the trivial 2 line perf improvement is ok to take into .NET 11

@github-actions github-actions Bot mentioned this pull request Jul 31, 2026
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@tannergooding tannergooding tannergooding approved these changes
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