HP Fortran Programmer Guide (766160-001, March 2014)

The following program illustrates the different effects of the +autodbl and +autodbl4 options. The
program assigns the same quad-precision constant to three variables:
• x, a default (that is, single-precision) real
• y, a real that is declared as double-precision with the kind parameter
• z, a double-precision real that is declared with the DOUBLE PRECISION statement
Example 3-2 precision.f90
PROGRAM main
REAL x
REAL(KIND=16) y
DOUBLE PRECISION z
! Assign a quad-precision constant to a default real:
x = 3.14159265358979323846_16
PRINT 10, ‘Stored in x: ‘, x
! Assign a quad-precision constant to a variable that
! has been explicitly sized for quad-precision:
y = 3.14159265358979323846_16
PRINT 10, ‘Stored in y: ‘, y
! Assign a quad-precision constant to a variable
! declared with the DOUBLE PRECISION statement:
z = 3.14159265358979323846_16
PRINT 10, ‘Stored in z: ‘, z
10 FORMAT (A, F22.20)
END PROGRAM main
Following are three different sets of command lines to compile and execute this program, including
sample output from each compilation. Note that variable y remains the same for each compilation:
the compiler does not promote variables that are sized with the kind parameter.
First, the program is compiled without any option:
$ f90 precision2.f90
$ a.out
Stored in x: 3.14159274101257320000
Stored in y: 3.14159265358979323846
Stored in z: 3.14159265358979310000
Next, the program is compiled with the +autodbl option. As shown in the output, x is promoted to
double-precision and z to quad-precision:
$ f90 +autodbl precision2.f90
$ a.out
Stored in x: 3.14159265358979310000
Stored in y: 3.14159265358979323846
Stored in z: 3.14159265358979323846
Finally, the program is compiled with the +autodbl4 option. As shown in the output, x is promoted,
but z is not:
$ f90 +autodbl4 precision2.f90
$ a.out
Stored in x: 3.14159265358979310000
Stored in y: 3.14159265358979323846
Stored in z: 3.14159265358979310000
Though useful for increasing the range and precision of numerical data, the +autodbl and +autodbl4
options are especially useful when porting; see “Large word size” (page 145). For detailed
information about these options, see the HP Fortran Programmer’s Reference. For detailed information
about how floating-point arithmetic is implemented on HP 9000 computers and how floating-point
behavior affects the programmer, see the HP-UX Floating-Point Guide.
Increasing default data sizes 71