XL Fortran Enterprise Edition for AIX : User's Guide - IBM

v
The fltint suboption speeds up float-to-integer conversions by reducing error
checking for overflows. You should make sure that any floats that are converted
to integers are not outside the range of the corresponding integer types.
In cases where speed is so important that you can make an informed decision to
sacrifice correctness at boundary conditions, you can replace hssngl and fltint with
the hsflt suboption; it does the same thing as fltint and suppresses rounding
operations.
In suppressing rounding operations, hsflt works like hssngl, but it also suppresses
rounding operations when double-precision values are assigned to single-precision
memory locations. Single-precision overflow is not detected in such assignments,
and the assigned value is not correctly rounded according to the current rounding
mode.
Attention: When you use the hsflt suboption, observe these restrictions, or your
program may produce incorrect results without warning:
v Your program must never convert overly large floats to integer.
v Your program must never compute NaNs, or values outside the range of single
precision.
v Your program must not depend on results to be correctly rounded to single
precision: for example, by comparing two single-precision values for equality.
Therefore, we recommend that you use this suboption only with extreme caution.
It is for use by knowledgeable programmers in specific applications, such as
graphics programs, where the computational characteristics are known. If you are
at all unsure whether a program is suitable or if the program produces unexpected
results when you use this suboption, use hssngl instead.
Related Information: “Technical Details of the -qfloat=hsflt Option” on page 412
provides additional technical information about this
suboption.
Detecting and Trapping Floating-Point Exceptions
As stated earlier, the IEEE standard for floating-point arithmetic defines a number
of exception (or error) conditions that might require special care to avoid or
recover from. The following sections are intended to help you make your programs
work safely in the presence of such exception conditions while sacrificing the
minimum amount of performance.
The floating-point hardware always detects a number of floating-point exception
conditions (which the IEEE standard rigorously defines): overflow, underflow,
zerodivide, invalid, and inexact.
By default, the only action that occurs is that a status flag is set. The program
continues without a problem (although the results from that point on may not be
what you expect). If you want to know when an exception occurs, you can arrange
for one or more of these exception conditions to generate a signal.
The signal causes a branch to a handler routine. The handler receives information
about the type of signal and the state of the program when the signal occurred. It
can produce a core dump, display a listing showing where the exception occurred,
modify the results of the calculation, or carry out some other processing that you
specify.
296 XL Fortran Enterprise Edition for AIX : User’s Guide