Add Unsafe.CopyBlockUnaligned

[benaadams]

That uses the .unaligned prefix

Has come up in dotnet/corefxlab#848 and aspnet/HttpAbstractions#717

/cc @jkotas

[benaadams]

It will currently work with core clr but might not with other runtimes and may not work in future dotnet/coreclr#7198 (comment)

ECMA-355 says:
"cpblk assumes that both destaddr and srcaddr are aligned to the natural size of the machine (but see the unaligned. prefix instruction). The operation of the cpblk instruction can be altered by an immediately preceding volatile. or unaligned. prefix instruction.

[jkotas]

We should also add InitBlockUnaligned to make the surface symmetric. Looks good to me otherwise.

[nietras]

I am looking at adding this. Would this be fine:

  .method public hidebysig static void CopyBlockUnaligned(void* destination, void* source, uint32 byteCount) cil managed aggressiveinlining
  {
        .custom instance void System.Runtime.Versioning.NonVersionableAttribute::.ctor() = ( 01 00 00 00 )
        .maxstack 3
        ldarg.0
        ldarg.1
        ldarg.2
        unaligned. 0x1
        cpblk
        ret
  } // end of method Unsafe::CopyBlockUnaligned

and:

  .method public hidebysig static void InitBlockUnaligned(void* startAddress, uint8 'value', uint32 byteCount) cil managed aggressiveinlining
  {
        .custom instance void System.Runtime.Versioning.NonVersionableAttribute::.ctor() = ( 01 00 00 00 )
        .maxstack 3
        ldarg.0
        ldarg.1
        ldarg.2
        unaligned. 0x1
        initblk
        ret
  } // end of method Unsafe::InitBlockUnaligned

any suggestions for tests?

[nietras]

And what about the uint32 byteCount as far as I understand Span also need UIntPtr version, correct?