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

2. For thread-safety and reentrancy, the include file /usr/include/fpdc.h contains a
THREADLOCAL directive that is protected by the trigger constant IBMT. The
invocation commands xlf_r, xlf_r7, xlf90_r, xlf90_r7, xlf95_r, and xlf95_r7 turn
on the -qthreaded compiler option by default, which in turn implies the trigger
constant IBMT. If you are including the file /usr/include/fpdc.h in code that is
not intended to be thread-safe, do not specify IBMT as a trigger constant.
Related Information: For more information about the bits in the FPSCR register
that correspond to the fpstat array elements, see the
POWERstation and POWERserver ® Hardware Technical
Reference - General Information.
Minimizing Rounding Errors
There are several strategies for handling rounding errors and other unexpected,
slight differences in calculated results. You may want to consider one or more of
the following strategies:
v Minimizing the amount of overall rounding
v Delaying as much rounding as possible to run time
v Ensuring that if some rounding is performed in a mode other than
round-to-nearest, all rounding is performed in the same mode
Minimizing Overall Rounding
Rounding operations, especially in loops, reduce code performance and may have
a negative effect on the precision of computations. Consider using double-precision
variables instead of single-precision variables when you store the temporary results
of double-precision calculations, and delay rounding operations until the final
result is computed. You can also specify the hssngl suboption of -qfloat instead of
converting a stored single-precision result back to double-precision. This suboption
preserves computed double-precision results so that they can be used again later.
Delaying Rounding until Run Time
The compiler evaluates floating-point expressions during compilation when it can,
so that the resulting program does not run more slowly due to unnecessary
run-time calculations. However, the results of the compiler’s evaluation might not
match exactly the results of the run-time calculation. To delay these calculations
until run time, specify the nofold suboption of the -qfloat option.
The results may still not be identical; for example, calculations in DATA and
PARAMETER statements are still performed at compile time.
The differences in results due to fold or nofold are greatest for programs that
perform extended-precision calculations or are compiled with the -O option or
both.
Ensuring that the Rounding Mode is Consistent
You can change the rounding mode from its default setting of round-to-nearest.
(See for examples.) If you do so, you must be careful that all rounding operations
for the program use the same mode:
v Specify the equivalent setting on the -qieee option, so that any compile-time
calculations use the same rounding mode.
v Specify the rrm suboption of the -qfloat option, so that the compiler does not
perform any optimizations that require round-to-nearest rounding mode to work
correctly.
294 XL Fortran Enterprise Edition for AIX : User’s Guide