Avoid ToString allocations for each ISpanFormattable value in string.Concat/Join(..., IEnumerable<T>)

[stephentoub]

We can use a T's ISpanFormattable implementation to avoid the individual ToStrings. We also don't need to maintain a separate implementation for Concat; at the cost of one branch per value, we can just reuse the Join implementation and pick up all of its optimizations for Concat.

private IEnumerable<int> _intValues;
private IEnumerable<string> _stringValues;
private IEnumerable<object> _objectValues;

[GlobalSetup]
public void Setup()
{
    var r = new Random(42);
    _intValues = Enumerable.Range(0, 5).Select(_ => r.Next(int.MinValue, int.MaxValue)).ToList();
    _stringValues = _intValues.Select(i => i.ToString()).ToList();
    _objectValues = Enumerable.Range(0, 5).Select(i => new MyObj()).ToList();
}

[Benchmark] public string ConcatInt() => string.Concat(_intValues);
[Benchmark] public string ConcatString() => string.Concat<string>(_stringValues);
[Benchmark] public string JoinInt() => string.Join(", ", _intValues);
[Benchmark] public string JoinObject() => string.Join(", ", _objectValues);

private sealed class MyObj
{
    public override string ToString() => "Something";
}

Method	Toolchain	Mean	Ratio	Allocated	Alloc Ratio
ConcatInt	\main\corerun.exe	175.78 ns	1.00	408 B	1.00
ConcatInt	\pr\corerun.exe	101.45 ns	0.58	168 B	0.41
ConcatString	\main\corerun.exe	83.84 ns	1.00	168 B	1.00
ConcatString	\pr\corerun.exe	51.36 ns	0.61	128 B	0.76
JoinInt	\main\corerun.exe	141.82 ns	1.00	424 B	1.00
JoinInt	\pr\corerun.exe	116.11 ns	0.82	184 B	0.43
JoinObject	\main\corerun.exe	98.92 ns	1.00	168 B	1.00
JoinObject	\pr\corerun.exe	99.97 ns	1.01	168 B	1.00

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Issue Details

---

We can use a T's ISpanFormattable implementation to avoid the individual ToStrings. We also don't need to maintain a separate implementation for Concat; at the cost of one branch per value, we can just reuse the Join implementation and pick up all of its optimizations for Concat.

private IEnumerable<int> _intValues;
private IEnumerable<string> _stringValues;
private IEnumerable<object> _objectValues;

[GlobalSetup]
public void Setup()
{
    var r = new Random(42);
    _intValues = Enumerable.Range(0, 5).Select(_ => r.Next(int.MinValue, int.MaxValue)).ToList();
    _stringValues = _intValues.Select(i => i.ToString()).ToList();
    _objectValues = Enumerable.Range(0, 5).Select(i => new MyObj()).ToList();
}

[Benchmark] public string ConcatInt() => string.Concat(_intValues);
[Benchmark] public string ConcatString() => string.Concat<string>(_stringValues);
[Benchmark] public string JoinInt() => string.Join(", ", _intValues);
[Benchmark] public string JoinObject() => string.Join(", ", _objectValues);

private sealed class MyObj
{
    public override string ToString() => "Something";
}

Method	Toolchain	Mean	Ratio	Allocated	Alloc Ratio
ConcatInt	\main\corerun.exe	175.78 ns	1.00	408 B	1.00
ConcatInt	\pr\corerun.exe	101.45 ns	0.58	168 B	0.41
ConcatString	\main\corerun.exe	83.84 ns	1.00	168 B	1.00
ConcatString	\pr\corerun.exe	51.36 ns	0.61	128 B	0.76
JoinInt	\main\corerun.exe	141.82 ns	1.00	424 B	1.00
JoinInt	\pr\corerun.exe	116.11 ns	0.82	184 B	0.43
JoinObject	\main\corerun.exe	98.92 ns	1.00	168 B	1.00
JoinObject	\pr\corerun.exe	99.97 ns	1.01	168 B	1.00

Author:	stephentoub
Assignees:	stephentoub
Labels:	area-System.Runtime
Milestone:	-