swiftself This indicates that the parameter is the self/context parameter. This is not a valid attribute for return values and can only be applied to one parameter.

“swiftcc” - This calling convention is used for Swift language. On X86-64 RCX and R8 are available for additional integer returns, and XMM2 and XMM3 are available for additional FP/vector returns.

sret This indicates that the pointer parameter specifies the address of a structure that is the return value of the function in the source program. This pointer must be guaranteed by the caller to be valid: loads and stores to the structure may be assumed by the callee not to trap and to be properly aligned. This is not a valid attribute for return values.