Replace usages of Marshal.AllocHGlobal with malloc in libraries

[reflectronic]

Contributes to #54297

Probably best to review commit-by-commit, I tried not to backtrack too much.

Usages in tests will be addressed in a separate PR.

[jkotas]

This should be called NativeMemory and only included for pre-.NET 6 targets. We do not want to have the wrapper on .NET 6+.

[reflectronic]

That can work for Alloc and Free, but it also has some replacements for Marshal.StringToHGlobalUni that would have to stay on a type like this. Is that okay?

[jkotas]

I would avoid touching StringToHGlobalUni in this PR. If it is useful to have NativeMemory variant of StringToHGlobalUni and similar APIs, we should introduce a proper public API for it first. Duplicating NativeMemory variant of StringToHGlobalUni via private helper is not an improvement over using StringToHGlobalUni directly.

[tannergooding]

This might be worth profiling more and its probably worth opening a proposal.

On my box, a benchmark which does simply the following, over a 260 character string:

var ptr = Marshal.StringToHGlobalUni(s);
Marshal.FreeHGlobal(ptr);

Compared to an equivalent API using NativeMarshal.Alloc/Free under the covers:

BenchmarkDotNet=v0.13.0, OS=Windows 10.0.19043.1052 (21H1/May2021Update)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK=6.0.100-preview.5.21302.13
  [Host]     : .NET 6.0.0 (6.0.21.30105), X64 RyuJIT
  DefaultJob : .NET 6.0.0 (6.0.21.30105), X64 RyuJIT

|  Method |     Mean |    Error |   StdDev |
|-------- |---------:|---------:|---------:|
|   Alloc | 41.92 ns | 0.051 ns | 0.045 ns |
| HGlobal | 47.37 ns | 0.071 ns | 0.066 ns |

Of course, in comparison to the GC transition and any work the call itself does, this is probably not that meaningful but ~10% overhead can add up.

[tannergooding]

• To clarify, I don't think anything needs to be done in this PR, just calling out there is a measurable difference here and so it might be worth opening a proposal because of that. In more extreme cases, many Alloc calls can be up to 20x faster than many LocalAlloc calls.

[jeffhandley]

@tannergooding This PR is assigned to you for follow-up/decision before the RC1 snap.

[danmoseley]

@reflectronic you might prefer to push commits in groups, as each push restarts the build and test.

[reflectronic]

Yes; I am sorry, just got a little lazy, been fixing these last errors on GitHub.

[ghost]

Draft Pull Request was automatically closed for inactivity. Please let us know if you'd like to reopen it.