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

The following example shows the difference between using the norndsngl and the
rndsngl suboptions:
REAL(4) a,b,c,d
...
a = b + c + d
With norndsngl, the compiler does the following:
1. Performs the intermediate computation of b + c in double precision without
rounding.
2. Adds the double-precision result to d.
3. Rounds the final double-precision result and then stores it into variable a.
With rndsngl, the compiler follows the same steps except that it performs
rounding in the first step. Note that norndsngl maintains increased precision for
intermediate results and improves performance, but you may need to specify
rndsngl to produce results that are bit-for-bit identical to those computed on other
systems.
norndsngl is the default when you use -qarch to target a POWER, a POWER2, or a
common architecture. rndsngl is the default when you target a PowerPC
architecture. You can also explicitly set the rndsngl suboption for any target
architecture.
Invalid Operation Exceptions for SQRT Operations on POWER
Processors
The POWER architecture does not include a hardware status flag to indicate IEEE
invalid operation exceptions that are caused by attempting to compute the square
root of a negative number. Instead, the operating system must handle those
exceptions by using a software mechanism. Therefore, using SQRT for a negative
number may not reliably generate invalid operation exceptions on POWER
platforms, depending on the level of the operating system installed on the
computer.
The POWER2 architecture and many PowerPC architectures each has a hardware
status flag for invalid SQRT operations, and each reliably generates exceptions.
304 XL Fortran Enterprise Edition for AIX : User’s Guide