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

XL Fortran Floating-Point Processing
IEEE Floating-Point Overview
This section answers some common questions about floating-point processing, such
as:
v How can I get predictable, consistent results?
v How can I get the fastest or the most accurate results?
v How can I detect, and possibly recover from, exception conditions?
v Which compiler options can I use for floating-point calculations?
Related Information: This section makes frequent reference to the compiler
options that are grouped together in “Options for
Floating-Point Processing” on page 86, especially the “-qfloat
Option” on page 163. The XL Fortran compiler also provides
three intrinsic modules for exception handling and IEEE
arithmetic support to help you write IEEE module-compliant
code that can be more portable. See IEEE Modules and
Support in the XL Fortran Enterprise Edition for AIX Language
Reference for details.
The use of the compiler options for floating-point calculations affects the accuracy,
performance, and possibly the correctness of floating-point calculations. Although
the default values for the options were chosen to provide efficient and correct
execution of most programs, you may need to specify nondefault options for your
applications to work the way you want. We strongly advise you to read this
section before using these options.
Note: The discussions of single-, double-, and extended-precision calculations in
this section all refer to the default situation, with -qrealsize=4 and no
-qautodbl specified. If you change these settings, keep in mind that the size
of a Fortran REAL, DOUBLE PRECISION, and so on may change, but
single precision, double precision, and extended precision (in lowercase) still
refer to 4-, 8-, and 16-byte entities respectively.
The information in this section relates to floating-point processing on the
PowerPC family of processors. The section “Floating-Point Processing on the
POWER and POWER2 Architectures” on page 303 describes the differences
between floating-point processing on the PowerPC processors and
floating-point processing on the POWER and POWER2 processors.
Here is a brief summary of the IEEE Standard for Floating-Point Arithmetic and the
details of how it applies to XL Fortran on specific hardware platforms. For
information on the draft Fortran 2003 IEEE Module and arithmetic support, see the
XL Fortran Enterprise Edition for AIX Language Reference.
Compiling for Strict IEEE Conformance
By default, XL Fortran follows most, but not all of the rules in the IEEE standard.
To compile for strict compliance with the standard:
v Use the compiler option -qfloat=nomaf.
v If the program changes the rounding mode at run time, include rrm among the
-qfloat suboptions.
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