XPLink and C and C++ programming

Extra Performance Linkage, normally abbreviated to XPLink, is a z/OS feature which provides high performance subroutine call and return mechanisms. This results in short and highly optimized execution path lengths.

Object Oriented programming is built upon the concept of sending 'messages' to objects which result in that object performing some actions. The message sending activity is implemented as a subroutine invocation. Subroutines, known as member functions in C++ terminology, are normally small pieces of code. The characteristic execution flow of a typical C++ program is of many subroutine invocations to small pieces of code. Programs of this nature benefit from the XPLink optimization technology.

MVS has a standard subroutine calling convention which can be traced back to the early days of System/360. This convention was optimized for an environment in which subroutines were more complex, there were relatively few of them, and they were invoked relatively infrequently. Object oriented programming conventions have changed this. Subroutines have become simpler but they are numerous, and the frequency of subroutine invocations has increased by orders of magnitude. This change in the size, numbers, and usage pattern, of subroutines made it desirable that the system overhead involved be optimized. XPLink is the result of this optimization.

To use XPLink, your C or C++ application code must be reentrant and threadsafe. The same code instance can be executing on more than one MVS TCB and, without threadsafe mechanisms to protect shared resources, the execution behavior of application code is unpredictable. This cannot be too strongly emphasized.