[prozolic]

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);
        }
    }

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[Copilot]

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.

[azure-pipelines]

Azure Pipelines:
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13 pipeline(s) were filtered out due to trigger conditions.
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[tannergooding]

LGTM. CC. @jeffhandley for whether the trivial 2 line perf improvement is ok to take into .NET 11

[Copilot]

Pull request overview

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

[Copilot]

🟢 Approval recommended

No unresolved issues were identified, and behavior is preserved.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Lite

[prozolic]

@jeffhandley Gentle ping. Is there anything else I should do before this can be merged?