Consider emitting methods with SkipLocalsInitAttribute

[AArnott]

The LibraryImportAttribute source generator emits code that applies SkipLocalsInitAttribute to each method. Perhaps we should too for perf reasons.

[elachlan]

There are a couple of scenarios to consider when outputting it.
https://www.meziantou.net/csharp-9-improve-performance-using-skiplocalsinit.htm

[elachlan]

Do we only want to apply SkipLocalsInitAttribute to methods and not classes/structs?

[AArnott]

We can completely control the methods we emit to ensure they are safe. Structs we emit are expected to be initialized by the users, which are accustomed to .NET behavior of clearing memory for a new struct, so I do not think we should apply this attribute on structs.
And I don't think we emit any classes (with fields, anyway).

[jnm2]

Is there any difference between placing [SkipLocalsInit] on a type and placing it only on each of the type's methods and accessors (and nested types)?

[elachlan]

From the docs. I actually had no idea. It does apply to all methods. It doesn't say it applied to the fields on the module though.
https://docs.microsoft.com/en-us/dotnet/api/system.runtime.compilerservices.skiplocalsinitattribute?view=net-6.0

This attribute is unsafe, because it may reveal uninitialized memory to the application in certain instances (for example, reading from uninitialized stack-allocated memory). If applied to a method directly, the attribute applies to that method and all its nested functions, including lambdas and local functions. If applied to a type or module, it applies to all methods nested inside. This attribute is intentionally not permitted on assemblies. To apply the attribute to multiple type declarations, use it at the module level instead.

[jnm2]

How could it apply to fields? It only affects the locals block of method bodies so that they are not locals-init blocks.

[AArnott]

I think you're right. I am not too familiar with the attribute yet so I probably added more confusion than clarity with my last comment.

[JeremyKuhne]

One other option here is to use Unsafe.SkipInit() explicitly on structs that don't need initialization. That doesn't help for stackalloc of course.