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

How XL Fortran Rounds Floating-Point Calculations
Understanding rounding operations in XL Fortran can help you get predictable,
consistent results. It can also help you make informed decisions when you have to
make tradeoffs between speed and accuracy.
In general, floating-point results from XL Fortran programs are more accurate than
those from other implementations because of MAF operations and the higher
precision used for intermediate results. If identical results are more important to
you than the extra precision and performance of the XL Fortran defaults, read
“Duplicating the Floating-Point Results of Other Systems” on page 295.
Selecting the Rounding Mode
To change the rounding mode in a program, you can call the fpsets and fpgets
routines, which use an array of logicals named fpstat, defined in the include files
/usr/include/fpdt.h and /usr/include/fpdc.h. The fpstat array elements correspond
to the bits in the floating-point status and control register.
For floating-point rounding control, the array elements fpstat(fprn1) and
fpstat(fprn2) are set as specified in the following table:
Table 18. Rounding-Mode Bits to Use with fpsets and fpgets
fpstat(fprn1) fpstat(fprn2) Rounding Mode Enabled
.true. .true. Round towards -infinity.
.true. .false. Round towards +infinity.
.false. .true. Round towards zero.
.false. .false. Round to nearest.
For example:
program fptest
include ’fpdc.h’
call fpgets( fpstat ) ! Get current register values.
if ( (fpstat(fprn1) .eqv. .false.) .and. +
(fpstat(fprn2) .eqv. .false.)) then
print *, ’Before test: Rounding mode is towards nearest’
print *, ’ 2.0 / 3.0 = ’, 2.0 / 3.0
print *, ’ -2.0 / 3.0 = ’, -2.0 / 3.0
end if
call fpgets( fpstat ) ! Get current register values.
fpstat(fprn1) = .TRUE. ! These 2 lines mean round towards
fpstat(fprn2) = .FALSE. ! +INFINITY.
call fpsets( fpstat )
r = 2.0 / 3.0
print *, ’Round towards +INFINITY: 2.0 / 3.0= ’, r
call fpgets( fpstat ) ! Get current register values.
fpstat(fprn1) = .TRUE. ! These 2 lines mean round towards
fpstat(fprn2) = .TRUE. ! -INFINITY.
call fpsets( fpstat )
r = -2.0 / 3.0
print *, ’Round towards -INFINITY: -2.0 / 3.0= ’, r
end
! This block data program unit initializes the fpstat array, and so on.
292 XL Fortran Enterprise Edition for AIX : User’s Guide