The <builtins.h> header file contains a section at the top
  conditionalized to just __ia64 with the support for built-in
  functions specific to I64 systems.  This includes macro definitions
  for all of the registers that can be specified to the __getReg,
  __setReg, __getIndReg, and __setIndReg built-in functions.
  Parameters that are const-qualified require an argument that is a
  compile-time constant.

     Notes:

      o  The <builtins.h> header file contains comments noting which
         built-in functions are not available or are not the
         preferred form for I64 systems.  The compiler issues
         diagnostics where using a different built-in function for
         I64 systems would be preferable.

      o  The comments in <builtins.h> reflect only what is explicitly
         present in that header file itself, and in the compiler
         implementation.  You should also consult the content and
         comments in <pal_builtins.h> to determine more accurately
         what functionality is effectively provided by including
         <builtins.h>.  For example, if a program explicitly declares
         one of the Alpha built-in functions and invokes it without
         having included <builtins.h>, the compiler might issue a
         BIFNOTAVAIL error message, regardless of whether or not the
         function is available through a system service.  If the
         compilation does include <builtins.h>, and BIFNOTAVAIL is
         issued, then either there is no support at all for the
         built-in function or a new version of <pal_builtins.h> is
         needed.