R2R contains unnecessary compilations for certain generic types

[tannergooding]

As part of #120802, R2RDump over S.P.Corelib shows that there are various generic compilations for methods that "don't make sense".

For example, Vector64/128/256/512<T> are special types to the runtime and only support a finite set of primitive T (byte, sbyte, short, ushort, int, uint, long, ulong, nint, nuint, float, and double). However, we get a number of R2R methods generated for the __Canon type which will always throw:

void System.Runtime.Intrinsics.Vector64.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector64`1<__Canon>, __Canon[])
void System.Runtime.Intrinsics.Vector64.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector64`1<__Canon>, __Canon[])
void System.Runtime.Intrinsics.Vector64.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector64`1<__Canon>, __Canon[], int)
void System.Runtime.Intrinsics.Vector64.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector64`1<__Canon>, __Canon[], int)
void System.Runtime.Intrinsics.Vector64.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector64`1<__Canon>, System.Span`1<__Canon>)
void System.Runtime.Intrinsics.Vector64.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector64`1<__Canon>, System.Span`1<__Canon>)

void System.Runtime.Intrinsics.Vector128.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector128`1<__Canon>, __Canon[])
void System.Runtime.Intrinsics.Vector128.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector128`1<__Canon>, __Canon[])
void System.Runtime.Intrinsics.Vector128.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector128`1<__Canon>, __Canon[], int)
void System.Runtime.Intrinsics.Vector128.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector128`1<__Canon>, __Canon[], int)
void System.Runtime.Intrinsics.Vector128.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector128`1<__Canon>, System.Span`1<__Canon>)
void System.Runtime.Intrinsics.Vector128.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector128`1<__Canon>, System.Span`1<__Canon>)

void System.Runtime.Intrinsics.Vector256.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector256`1<__Canon>, __Canon[])
void System.Runtime.Intrinsics.Vector256.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector256`1<__Canon>, __Canon[])
void System.Runtime.Intrinsics.Vector256.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector256`1<__Canon>, __Canon[], int)
void System.Runtime.Intrinsics.Vector256.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector256`1<__Canon>, __Canon[], int)
void System.Runtime.Intrinsics.Vector256.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector256`1<__Canon>, System.Span`1<__Canon>)
void System.Runtime.Intrinsics.Vector256.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector256`1<__Canon>, System.Span`1<__Canon>)

void System.Runtime.Intrinsics.Vector512.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector512`1<__Canon>, __Canon[])
void System.Runtime.Intrinsics.Vector512.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector512`1<__Canon>, __Canon[])
void System.Runtime.Intrinsics.Vector512.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector512`1<__Canon>, __Canon[], int)
void System.Runtime.Intrinsics.Vector512.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector512`1<__Canon>, __Canon[], int)
void System.Runtime.Intrinsics.Vector512.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector512`1<__Canon>, System.Span`1<__Canon>)
void System.Runtime.Intrinsics.Vector512.CopyTo<__Canon>(System.Runtime.Intrinsics.Vector512`1<__Canon>, System.Span`1<__Canon>)

Ideally, we would special-case R2R to avoid generating for these types and potentially have a reusable way to annotate that other types should equally not have R2R entries generated if they will "always throw". This should save a bit of space in the output assembly.

There's potentially other fine tuning that could also be done, such as marking some of these APIs as [NonVersionable] (where there is no actual intrinsic handling in the JIT) so that they can be inlined or ensuring that specific T are pre-compiled.

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[EgorBo]

I've just checked - not compiling these saves 132 bytes, so not sure it's worth the effort?

  // There is no good reason for Vector APIs to be compiled for __Canon types
  if (methodNeedingCode.ImplementationType is EcmaType ecmaType && methodNeedingCode.HasInstantiation &&
      ecmaType.Namespace.SequenceEqual("System.Runtime.Intrinsics"u8) &&
      ecmaType.Name.StartsWith("Vector"u8))
  {
      foreach (TypeDesc typeDesc in methodNeedingCode.Instantiation)
      {
          if (typeDesc.IsCanonicalDefinitionType(CanonicalFormKind.Any))
          {
              return true;
          }
      }
  }

in ShouldSkipCompilation

[EgorBo]

I guess this is what we get when we relax constraints, obviously, these methods won't be compiled if the APIs had where T : unmanaged