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

For example, you might compile a program like the one in “Selecting the
Rounding Mode” on page 292 with this command if the program consistently uses
round-to-plus-infinity mode:
xlf95 -qieee=plus -qfloat=rrm changes_rounding_mode.f
Duplicating the Floating-Point Results of Other Systems
To duplicate the double-precision results of programs on systems with different
floating-point architectures (without multiply-add instructions), specify the nomaf
suboption of the -qfloat option. This suboption prevents the compiler from
generating any multiply-add operations. This results in decreased accuracy and
performance but provides strict conformance to the IEEE standard for
double-precision arithmetic.
To duplicate the results of programs where the default size of REAL items is
different from that on systems running XL Fortran, use the -qrealsize option (page
222) to change the default REAL size when compiling with XL Fortran.
If the system whose results you want to duplicate preserves full double precision
for default real constants that are assigned to DOUBLE PRECISION variables, use
the -qdpc or -qrealsize option.
If results consistent with other systems are important to you, include norsqrt and
nofold in the settings for the -qfloat option. If you specify the option -O3, include
-qstrict too.
Maximizing Floating-Point Performance
If performance is your primary concern and you want your program to be
relatively safe but do not mind if results are slightly different (generally more
precise) from what they would be otherwise, optimize the program with the -O
option, and specify -qfloat=rsqrt:hssngl:fltint. The following section describes the
functions of these suboptions:
v
v
The rsqrt suboption replaces division by a square root with multiplication by the
reciprocal of the root, a faster operation that may not produce precisely the same
result.
The hssngl suboption is the opposite of rndsngl; it improves the performance of
single-precision (REAL(4)) floating-point calculations by suppressing rounding
operations that are required by the Fortran language but are not necessary for
correct program execution. The results of floating-point expressions are kept in
double precision where the original program would round them to
single-precision. These results are then used in later expressions instead of the
rounded results.
To detect single-precision floating-point overflows and underflows, rounding
operations are still inserted when double-precision results are stored into
single-precision memory locations. However, if optimization removes such a
store operation, hssngl also removes the corresponding rounding operation,
possibly preventing the exception. (Depending on the characteristics of your
program, you may or may not care whether the exception happens.)
The hssngl suboption is safe for all types of programs because it always only
increases the precision of floating-point calculations. Program results may differ
because of the increased precision and because of avoidance of some exceptions.
XL Fortran Floating-Point Processing 295