Improve Directory.CreateDirectory performance

[stephentoub]

This does a few things:

• On all OSes, makes DirectoryInfo.Name and FileInfo.Name lazy. For a case like DirectoryInfo.CreateDirectory(fullPath) where the returned DirectoryInfo is ignored, previously we would still promptly allocate a string for the name, and now we won't, waiting instead until the property is accessed.
• On Windows, after checking whether the target directory exists and finding it doesn't, rather than proceeding with the full logic to parse the path, find the first segment that exists, and then create them all, it adds a fast-path check to just try to create the target, under the assumption that creating a directory in a directory that already exists is very common. In such a case, it measurably speeds up throughput. If the parent doesn't already exist, this does add one syscall of overhead, and it can be measurable, but it's relatively small compared to the other costs involved on this slower path.
• On Windows, on the slow path rather than allocating a List<string> to store the segments, it uses a ValueListBuilder<string> seeded with some stack space.
• Fixes the nullable annotation on the internal DirectoryExists helper
• Normalizes some parameter names

Contributes to #61954

[ghost]

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

Issue Details

---

This does a few things:

• On all OSes, makes DirectoryInfo.Name and FileInfo.Name lazy. For a case like DirectoryInfo.CreateDirectory(fullPath) where the returned DirectoryInfo is ignored, previously we would still promptly allocate a string for the name, and now we won't, waiting instead until the property is accessed.
• On Windows, after checking whether the target directory exists and finding it doesn't, rather than proceeding with the full logic to parse the path, find the first segment that exists, and then create them all, it adds a fast-path check to just try to create the target, under the assumption that creating a directory in a directory that already exists is very common. In such a case, it measurably speeds up throughput. If the parent doesn't already exist, this does add one syscall of overhead, and it can be measurable, but it's relatively small compared to the other costs involved on this slower path.
• On Windows, on the slow path rather than allocating a List<string> to store the segments, it uses a ValueListBuilder<string> seeded with some stack space.
• Fixes the nullable annotation on the internal DirectoryExists helper
• Normalizes some parameter names

Contributes to #61954

Author:	stephentoub
Assignees:	-
Labels:	area-System.IO, tenet-performance
Milestone:	7.0.0

[stephentoub]

@tmds, in #58799, how much of a regression did you see on Linux for CreateDirectory when the path already exists? I didn't see numbers for that in the PR nor the concerns about that regression addressed. I tried something similar on Windows, and opted against it as it ended up regressing that case by almost 2x, and I expect that's a reasonably common case.

[stephentoub]

Method	Toolchain	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
Exists	\main\corerun.exe	17.82 us	0.192 us	0.180 us	1.00	144 B	1.00
Exists	\pr\corerun.exe	17.70 us	0.228 us	0.213 us	0.99	96 B	0.67
DoesntExist	\main\corerun.exe	473.60 us	8.972 us	9.973 us	1.00	1056 B	1.00
DoesntExist	\pr\corerun.exe	352.16 us	5.199 us	4.609 us	0.74	664 B	0.63
ParentDoesntExist	\main\corerun.exe	766.22 us	7.961 us	6.647 us	1.00	1968 B	1.00
ParentDoesntExist	\pr\corerun.exe	786.89 us	15.585 us	18.553 us	1.03	1560 B	0.79

private string _exists;
private string _doesntExist;
private string _doesntExistChild;

[GlobalSetup]
public void Setup()
{
    _exists = Path.GetTempFileName();
    File.Delete(_exists);
    Directory.CreateDirectory(_exists);

    _doesntExist = Path.GetRandomFileName();
    _doesntExistChild = Path.Combine(_doesntExist, Path.GetRandomFileName());
}

[GlobalCleanup]
public void Cleanup()
{
    Directory.Delete(_exists);
}

[Benchmark]
public void Exists()
{
    Directory.CreateDirectory(_exists);
}

[Benchmark]
public void DoesntExist()
{
    Directory.CreateDirectory(_doesntExist).Delete();
}

[Benchmark]
public void ParentDoesntExist()
{
    Directory.CreateDirectory(_doesntExistChild).Delete();
    Directory.Delete(_doesntExist);
}

[adamsitnik]

LGTM, thank you @stephentoub!

[adamsitnik]

nit: I know it was named like this before your changes

Suggested change

	bool somepathexists = false;
	bool somePathExists = false;

[jozkee]

nit: remove whitespace

Suggested change

[stephentoub]

This is hitting failures on pre-Windows 10 OSes due to some kind of device path validation issue. I'm going to close it for now. I separated out the FileInfo/DirectoryInfo.Name laziness part into #73983.

[tmds]

@tmds, in #58799, how much of a regression did you see on Linux for CreateDirectory when the path already exists? I didn't see numbers for that in the PR nor the concerns about that regression addressed. I tried something similar on Windows, and opted against it as it ended up regressing that case by almost 2x, and I expect that's a reasonably common case.

I had mentioned this regression in the initial comment. I expect it to be similar: 2x slower because it is making 2 syscalls instead of 1.

I opted to optimize the non-exists case at the cost of the exists case because that improves new directory creation from a loop. A loop should be written so it doesn't create the same directory over and over.

An Exists check can still be added by the user (and may already be part of the code).