/IEEE_MODE=option
     /IEEE_MODE=DENORM_RESULTS (D)

  Selects the IEEE floating-point mode to be used.

  Options:

   FAST              During program execution, only finite values (no
                     infinities, NaNs, or denorms) are created.
                     Exceptional conditions, such as floating point
                     overflow and divide by zero, are fatal.

   UNDERFLOW_TO_ZERO Generate infinities and NaNs.  Flush
                     denormalized results and underflow to zero
                     without exceptions.

   DENORM_RESULTS    Same as UNDERFLOW_TO_ZERO, except that denorms
                     are generated.  This is the default for I64
                     systems.

   INEXACT           Same as DENORM_RESULTS, except that inexact
                     values are trapped.  This is the slowest mode.

  On Alpha sytems, the /IEEE_MODE qualifier generally has its
  greatest effect on the generated code of a compilation.  When calls
  are made between functions compiled with different /IEEE_MODE
  qualifiers, each function produces the /IEEE_MODE behavior with
  which it was compiled.

  On I64 systems, the /IEEE_MODE qualifier primarily affects only the
  setting of a hardware register at program startup.  In general, the
  /IEEE_MODE behavior for a given function is controlled by the
  /IEEE_MODE option specified on the compilation that produced the
  main program:  the startup code for the main program sets the
  hardware register according the command-line qualifiers used to
  compile the main program.

  When applied to a compilation that does not contain a main program,
  the /IEEE_MODE qualifier does have some effect:  it might affect
  the evaluation of floating-point constant expressions, and it is
  used to set the EXCEPTION_MODE used by the math library for calls
  from that compilation.  But the qualifier has no effect on the
  exceptional behavior of floating-point calculations generated as
  inline code for that compilation.  Therefore, if floating-point
  exceptional behavior is important to an application, all of its
  compilations, including the one containing the main program, should
  be compiled with the same /IEEE_MODE setting.

  Even on Alpha systems, the particular setting of
  /IEEE_MODE=UNDERFLOW_TO_ZERO has this characteristic:  its primary
  effect requires the setting of a runtime status register, and so it
  needs to be specified on the compilation containing the main
  program in order to be effective in other compilations.