HP Fortran Programmer's Guide (B3908-90031; September 2011)

Porting to HP Fortran
Using porting options
Chapter 11 245
Instead, it causes the compiler to search for all local, nonarray, nonstatic variables that have not been defined
before being used. Any that it finds are initialized to 0 on the stack each time the procedure in which they are
declared is invoked.
For detailed information about the +save and +Oinitcheck options, see HP Fortran Programmer’s
Reference.
Large word size
The word size of default integers, reals, and logicals in HP Fortran is 4 bytes. However, some
implementations of Fortran 90—notably, Cray—use an 8-byte word size. Programs written for these
implementations may rely on the increased precision and range in their computations.
You can double the sizes of default integer, real, and logicals by compiling with the +autodbl option,
making them compatible with implementations that use the larger word size. This option also doubles the
sizes of items declared with the COMPLEX and DOUBLE PRECISION statements, but not the BYTE and
DOUBLE COMPLEX) statements.
Increasing the size of double-precision items can degrade the performance of your program. If you do not
need the extra precision for items declared with the DOUBLE PRECISION statement, use the +autodbl4
option, which increases single-precision items only. Compiling with this option results in items declared as
default real and double precision real having the same precision—a violation of the Standard.
For usage information about the +autodbl and +autodbl4 options, see “Increasing default data sizes” on
page 106). For detailed descriptions of these options, refer to the HP Fortran Programmer’s Reference.
One-trip DO loops
If a DO loop is coded so that its initial loop count is greater than its final loop count, standard Fortran 90
requires that the loop never execute. However, under some implementations of FORTRAN 66, if a DO loop
is reached, it executes for at least one iteration, even if the DO variable is initialized to a value greater than
the final value. This is called a one-trip DO loop.
To duplicate the behavior of a one-trip DO loop in an HP Fortran program, compile with the +onetrip
option. To see the effects of this option, consider the following program:
PROGRAM main
DO 10 i = 2, 1
PRINT *, 'Should never happen in standard Fortran 90.'
10 CONTINUE
END PROGRAM main
When compiled with the command line:
$ f90 test_loop.f90