No sharing of generic instantiations using reference types

[drcjt]

For a generic class that has > 1 instantiation using reference types then separate copies of the methods will be compiled rather than sharing the same code.

Example where Person and Order are reference types:

var personList = new List<Person>();
Console.WriteLine(personList.Count);

var orderList = new List<Order>();
Console.WriteLine(orderList.Count);

In this case two copies of the code for List.Count will be included in the generated assembly code when the same code could be shared for both instantiations.

This will involve looking into the canonical forms used in the dotnet type system.

Quote from Michal Strehovsky:

"https://github.com/dotnet/coreclr/blob/775003a4c72f0acc37eab84628fcef541533ba4e/Documentation/botr/type-loader.md has a short paragraph about it. It has to do with sharing code (and potentially other data structures) of generic instantiations. Generic instantiations that map to the same canonical form can do the sharing between each other."

Also see unit tests in native aot ilc here - https://github.com/dotnet/corert/blob/c6af4cfc8b625851b91823d9be746c4f7abdc667/src/ILCompiler.TypeSystem/tests/CanonicalizationTests.cs#L12

Very good explanation of shared generics can be seen here - dotnet/corert#7248
Also see original shared generics in corert using cpp code gen here - dotnet/corert#6467

Design document on shared generics in dotnet:
https://github.com/dotnet/runtime/blob/main/docs/design/coreclr/botr/shared-generics.md

Post explaining "CLR Generics and Code Sharing" -> https://learn.microsoft.com/en-gb/archive/blogs/joelpob/clr-generics-and-code-sharing

• Canonicalisation of code
• Pass Generic Context to shared generic code
• Use Generic Context in shared generic code

The generic context is an extra parameter that specifies the instantiation information - think of this as an exact instantiation MethodDesc.