Add fast path in string.Concat

[prozolic]

This PR adds a fast path for string.Concat(IEnumerable<string?> values) when the input value is List<string?> or string?[]. This fast path is based on the pattern already used in Join(string? separator, IEnumerable<string?> values).
I have also added tests for the fast path and fallback path.

Benchmark

This Change is expected to improve performance when the argument of string.Concat(IEnumerable<string?> values) is List<string?> or string?[].

Method	Job	Toolchain	Size	Mean	Error	StdDev	Median	Min	Max	Ratio	RatioSD	Gen0	Gen1	Gen2	Allocated	Alloc Ratio
ConcatString_Enumerable	Job-QIDXWP	\main\corerun.exe	10	150.22 ns	7.503 ns	8.640 ns	152.11 ns	137.70 ns	165.26 ns	1.00	0.00	0.1197	-	-	752 B	1.00
ConcatString_Enumerable	Job-TLPBEL	\pr\corerun.exe	10	133.46 ns	3.512 ns	3.903 ns	131.45 ns	129.67 ns	142.99 ns	0.89	0.06	0.1195	-	-	752 B	1.00
ConcatString_Array	Job-QIDXWP	\main\corerun.exe	10	54.16 ns	1.042 ns	0.813 ns	54.30 ns	52.53 ns	55.19 ns	1.00	0.00	0.0407	-	-	256 B	1.00
ConcatString_Array	Job-TLPBEL	\pr\corerun.exe	10	37.43 ns	1.786 ns	1.985 ns	36.62 ns	34.19 ns	41.63 ns	0.69	0.04	0.0356	-	-	224 B	0.88
ConcatString_List	Job-QIDXWP	\main\corerun.exe	10	62.35 ns	5.475 ns	6.085 ns	58.71 ns	57.42 ns	76.39 ns	1.00	0.00	0.0419	-	-	264 B	1.00
ConcatString_List	Job-TLPBEL	\pr\corerun.exe	10	38.78 ns	1.355 ns	1.506 ns	38.22 ns	37.06 ns	41.77 ns	0.63	0.06	0.0357	-	-	224 B	0.85
ConcatString_Enumerable	Job-QIDXWP	\main\corerun.exe	100	1,384.76 ns	109.124 ns	125.668 ns	1,338.98 ns	1,240.39 ns	1,560.94 ns	1.00	0.00	1.0524	-	-	6608 B	1.00
ConcatString_Enumerable	Job-TLPBEL	\pr\corerun.exe	100	1,304.29 ns	23.435 ns	19.569 ns	1,302.74 ns	1,283.53 ns	1,343.32 ns	0.95	0.08	1.0494	-	-	6608 B	1.00
ConcatString_Array	Job-QIDXWP	\main\corerun.exe	100	409.49 ns	7.974 ns	7.832 ns	407.95 ns	398.56 ns	424.99 ns	1.00	0.00	0.3218	-	-	2024 B	1.00
ConcatString_Array	Job-TLPBEL	\pr\corerun.exe	100	341.10 ns	21.791 ns	25.095 ns	337.12 ns	310.58 ns	396.64 ns	0.83	0.06	0.3174	-	-	1992 B	0.98
ConcatString_List	Job-QIDXWP	\main\corerun.exe	100	517.63 ns	8.515 ns	7.548 ns	515.12 ns	508.14 ns	533.71 ns	1.00	0.00	0.3227	-	-	2032 B	1.00
ConcatString_List	Job-TLPBEL	\pr\corerun.exe	100	309.35 ns	4.453 ns	3.718 ns	309.90 ns	302.72 ns	314.76 ns	0.60	0.01	0.3173	-	-	1992 B	0.98
ConcatString_Enumerable	Job-QIDXWP	\main\corerun.exe	1000	14,219.06 ns	221.613 ns	185.057 ns	14,227.31 ns	13,951.98 ns	14,659.81 ns	1.00	0.00	10.4576	-	-	65856 B	1.00
ConcatString_Enumerable	Job-TLPBEL	\pr\corerun.exe	1000	13,544.69 ns	124.313 ns	103.807 ns	13,573.65 ns	13,323.62 ns	13,683.07 ns	0.95	0.01	10.4680	-	-	65856 B	1.00
ConcatString_Array	Job-QIDXWP	\main\corerun.exe	1000	4,299.72 ns	82.261 ns	68.692 ns	4,300.00 ns	4,195.10 ns	4,404.01 ns	1.00	0.00	3.1547	-	-	19928 B	1.00
ConcatString_Array	Job-TLPBEL	\pr\corerun.exe	1000	2,999.08 ns	47.892 ns	39.992 ns	3,002.73 ns	2,940.33 ns	3,077.51 ns	0.70	0.01	3.1615	-	-	19896 B	1.00
ConcatString_List	Job-QIDXWP	\main\corerun.exe	1000	5,252.69 ns	75.591 ns	63.122 ns	5,252.78 ns	5,124.19 ns	5,374.56 ns	1.00	0.00	3.1667	-	-	19936 B	1.00
ConcatString_List	Job-TLPBEL	\pr\corerun.exe	1000	3,070.12 ns	60.962 ns	54.042 ns	3,077.02 ns	2,953.34 ns	3,134.09 ns	0.58	0.01	3.1615	-	-	19896 B	1.00
ConcatString_Enumerable	Job-QIDXWP	\main\corerun.exe	10000	305,850.16 ns	9,171.318 ns	9,813.208 ns	307,086.58 ns	280,512.72 ns	325,177.59 ns	1.00	0.00	123.9224	61.4224	61.4224	658830 B	1.00
ConcatString_Enumerable	Job-TLPBEL	\pr\corerun.exe	10000	297,325.54 ns	9,746.004 ns	10,428.116 ns	302,198.41 ns	272,057.94 ns	308,274.79 ns	0.97	0.05	123.9407	61.4407	61.4407	658830 B	1.00
ConcatString_Array	Job-QIDXWP	\main\corerun.exe	10000	108,964.71 ns	4,410.330 ns	4,902.070 ns	110,349.45 ns	97,951.58 ns	116,218.28 ns	1.00	0.00	62.1044	62.1044	62.1044	199543 B	1.00
ConcatString_Array	Job-TLPBEL	\pr\corerun.exe	10000	82,938.57 ns	2,698.977 ns	2,887.876 ns	83,646.74 ns	75,325.70 ns	86,369.22 ns	0.76	0.04	62.2066	62.2066	62.2066	199511 B	1.00
ConcatString_List	Job-QIDXWP	\main\corerun.exe	10000	123,694.56 ns	2,888.695 ns	3,090.871 ns	122,749.77 ns	118,993.66 ns	131,228.33 ns	1.00	0.00	62.0438	62.0438	62.0438	199551 B	1.00
ConcatString_List	Job-TLPBEL	\pr\corerun.exe	10000	92,988.47 ns	3,075.460 ns	3,290.708 ns	92,794.81 ns	87,439.32 ns	100,264.49 ns	0.75	0.03	62.0567	62.0567	62.0567	199511 B	1.00

    [BenchmarkCategory(Categories.Libraries, Categories.Runtime)]
    public class Perf_StringConcatForString
    {
        private int[] array;
#pragma warning disable CS8632 // '#nullable' 
        private IEnumerable<string?> _stringArray;
        private IEnumerable<string?> _stringList;
        private IEnumerable<string?> _stringEnumerable;
#pragma warning restore CS8632

        [Params(10, 100, 1000, 10000)]
        public int Size;

        [GlobalSetup]
        public void Setup()
        {
            var r = new Random(42);
            array = Enumerable.Range(0, Size).Select(_ => r.Next(int.MinValue, int.MaxValue)).ToArray();
            _stringArray = array.Select(i => i.ToString()).ToArray();
            _stringList = array.Select(i => i.ToString()).ToList();
            _stringEnumerable = array.Select(i => i.ToString());
        }

        [Benchmark]
        public string ConcatString_Enumerable()
        {
            return string.Concat(_stringEnumerable);
        }

        [Benchmark]
        public string ConcatString_Array()
        {
            return string.Concat(_stringArray);
        }

        [Benchmark]
        public string ConcatString_List()
        {
            return string.Concat(_stringList);
        }

    }

[dotnet-policy-service]

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

[Copilot]

Pull request overview

This PR optimizes the System.String.Concat(IEnumerable<string?> values) overload in System.Private.CoreLib by adding type-specialized fast paths for common inputs (arrays and List<T>), and extends existing String.Concat tests to cover these additional enumerable shapes.

Changes:

• Add a fast path for List<string?> inputs by extracting a span via CollectionsMarshal.AsSpan and forwarding to the span-based Concat implementation.
• Add a fast path for string?[] inputs by forwarding to the span-based Concat implementation.
• Extend StringTests.Concat_String to validate results for a non-array/non-list enumerable (LINQ Select) and for a List<string?>.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs	Adds List/array fast paths for Concat(IEnumerable<string?>) by forwarding to span-based concat.
src/libraries/Common/tests/Tests/System/StringTests.cs	Adds test coverage for the new fast-path inputs and a fallback enumerable shape.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[MihaZupan]

Thanks

[Copilot]

Pull request overview

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

[prozolic]

@MihaZupan CI seems to be failing due to a timeout. Could you check the current status?

[prozolic]

Thank you!