Avoid allocations for Unicode data tries

[MrJul]

What does the pull request do?

This PR completely avoids allocations for Unicode data tries on .NET 7+, removing 195 kB of always allocated managed memory.

How was the solution implemented (if it's not obvious)?

The trie's data is now a property returning ReadOnlySpan<uint> instead of ReadOnlySpan<byte>, that's used directly instead of copying the bytes to a runtime uint[].

Starting with .NET 7 and Roslyn 17.5, this syntax makes the compiler use RuntimeHelpers.CreateSpan, which references the data embedded inside the assembly directly on little-endian systems, without having to allocate or copy anything!
(For .NET Standard, the compiler falls back to an array.)

Performance

I expected better memory usage, but it turns out we also get some performance improvement. With these changes, the JIT is able to inline some constants from the Unicode data directly, generating better code.

Before:

Method	CodepointValue	Mean	Error	StdDev	Allocated
GetGeneralCategory	65	0.4334 ns	0.0118 ns	0.0110 ns	-
GetGeneralCategory	4660	0.4076 ns	0.0215 ns	0.0201 ns	-

After:

Method	CodepointValue	Mean	Error	StdDev	Allocated
GetGeneralCategory	65	0.2274 ns	0.0101 ns	0.0089 ns	-
GetGeneralCategory	4660	0.2292 ns	0.0056 ns	0.0050 ns	-

[avaloniaui-bot]

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[maxkatz6]

Starting with .NET 7 and Roslyn 17.5, this syntax makes the compiler use RuntimeHelpers.CreateSpan, which references the data embedded inside the assembly directly on little-endian systems, without having to allocate or copy anything!

I am pretty sure we expected old code to work exactly in this way, i.e. returning reference to the data inside of the assembly. I.e. for .NET 6 as well.
But I guess it was wrong.

[MrJul]

I am pretty sure we expected old code to work exactly in this way, i.e. returning reference to the data inside of the assembly. I.e. for .NET 6 as well.
But I guess it was wrong.

You were right, it's been working for a while with ReadOnlySpan<byte>, but the old code then copied that to a runtime uint[], so we still had allocations.

Supporting other primitive types than byte with RuntimeHelpers.CreateSpan is much more recent: dotnet/roslyn#61414

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0046814-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]