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

-qautodbl Option
Syntax
-qautodbl=setting
AUTODBL(setting)
Provides an automatic means of converting single-precision floating-point
calculations to double-precision and of converting double-precision calculations to
extended-precision.
You might find this option helpful in porting code where storage relationships are
significant and different from the XL Fortran defaults. For example, programs that
are written for the IBM VS FORTRAN compiler may rely on that compiler’s
equivalent option.
Rules
Although the POWER and POWER2 floating-point units perform REAL(4)
calculations internally using fast REAL(8) arithmetic, it is often better to have these
calculations done entirely using data entities that are REAL(8) or
DOUBLE PRECISION. If the calculations are coded using REAL or REAL(4) data
entities, the REAL(4)-REAL(8)-REAL(4) conversions take away the extra precision
and range and also lessen performance, even though the intermediate calculations
are done in IEEE double-precision.
Arguments
The -qautodbl suboptions offer different strategies to preserve storage relationships
between objects that are promoted or padded and those that are not.
The settings you can use are as follows:
none
dbl4
dbl8
dbl
dblpad4
Does not promote or pad any objects that share storage. This
setting is the default.
Promotes floating-point objects that are single-precision (4 bytes in
size) or that are composed of such objects (for example, COMPLEX
or array objects):
v REAL(4) is promoted to REAL(8).
v COMPLEX(4) is promoted to COMPLEX(8).
This suboption requires the libxlfpmt4.a library during linking.
Promotes floating-point objects that are double-precision (8 bytes in
size) or that are composed of such objects:
v REAL(8) is promoted to REAL(16).
v COMPLEX(8) is promoted to COMPLEX(16).
This suboption requires the libxlfpmt8.a library during linking.
Combines the promotions that dbl4 and dbl8 perform.
This suboption requires the libxlfpmt4.a and libxlfpmt8.a libraries
during linking.
Performs the same promotions as dbl4 and pads objects of other
types (except CHARACTER) if they could possibly share storage
with promoted objects.
This suboption requires the libxlfpmt4.a and libxlfpad.a libraries
during linking.
134 XL Fortran Enterprise Edition for AIX : User’s Guide