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

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Consider using only a subset of the overflow, underflow, zerodivide, invalid,
and inexact suboptions with the -qflttrap option if you can identify some
conditions that will never happen or you do not care about. In particular,
because an inexact exception occurs for each rounding error, you probably
should not check for it if performance is important.
Include the imprecise suboption with the -qflttrap option, so that your compiler
command looks similar to this:
xlf90 -qflttrap=underflow:enable:imprecise does_underflows.f
imprecise makes the program check for the specified exceptions only on entry
and exit to subprograms that perform floating-point calculations. This means
that XL Fortran will eventually detect any exception, but you will know only the
general area where it occurred, not the exact location.
When you specify -qflttrap without imprecise, a check for exceptions follows
each floating-point operation. If all your exceptions occur during calls to
routines that are not compiled with -qflttrap (such as library routines), using
imprecise is generally a good idea, because identifying the exact location will be
difficult anyway.
Note that enable has no effect if using the nanq suboption. nanq generates
trapping code after each floating point arithmetic, load instruction and
procedure returning floating point values even if imprecise is specified.
Floating-Point Processing on the POWER and POWER2 Architectures
The following section provides information on floating-point processing on the
POWER and POWER2 processors.
Precision of Computations
POWER and POWER2 floating-point hardware performs all calculations in IEEE
double-precision mode. The hardware does not directly perform single-precision
calculations, but it is capable of generating single-precision results by using the
following sequence of operations:
1. Convert all single-precision operands of a single-precision operation to
double-precision.
2. Perform the equivalent double-precision operation.
3. Round the result to single-precision.
This sequence always produces exactly the same bit-for-bit result, as if the
single-precision IEEE operation had been performed.
As on the PowerPC machines, conversions from single-precision to
double-precision have no negative performance impacts, but rounding operations
from double-precision to single-precision do. Since the performance penalty of
rounding operations would normally impact all single-precision computations on a
POWER or POWER2 machine, the compiler attempts to reduce the number of
rounding operations. It does this under the control of the norndsngl suboption of
the -qfloat option.
When you specify the norndsgnl suboption, the compiler leaves all intermediate
results of single-precision operations in double-precision. That is, it suppresses the
rounding operation in the above sequence. The compiler only performs a rounding
operation on the final result of an expression, when it stores that result into a
single-precision memory location.
XL Fortran Floating-Point Processing 303