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

-qipa=list
cross-file type checking of external variables and functions. This
option requires the linker -btypchk option to be active.
Instructs the compiler to emit an object listing that provides
information for IPA optimization.
Different Results in Optimized Programs
Here are some reasons why an optimized program might produce different results
from those of an unoptimized one:
v
v
v
Optimized code can fail if a program contains code that is not valid. For
example, failure can occur if the program passes an actual argument that also
appears in a common block in the called procedure, or if two or more dummy
arguments are associated with the same actual argument.
If a program that worked without optimization fails when compiled with it,
check the cross-reference listing and the execution flow of the program for
variables that are used before they are initialized. Compile with the
-qinitauto=hex_value option to try to produce the incorrect results consistently.
For example, using -qinitauto=FF gives REAL and COMPLEX variables an
initial value of ″negative not a number″ (-NAN). Any operations on these
variables will also result in NAN values. Other bit patterns (hex_value) may
yield different results and provide further clues as to what is going on.
(Programs with uninitialized variables may appear to work properly when
compiled without optimization, because of the default assumptions the compiler
makes, but may fail when compiled with optimization. Similarly, a program may
appear to execute correctly when optimized, but fails at lower optimization
levels or when run in a different environment.)
Use with caution debugging techniques that rely on examining values in storage.
The compiler might have deleted or moved a common expression evaluation. It
might have assigned some variables to registers, so that they do not appear in
storage at all.
Related Information: See “-g Option” on page 108, “-qinitauto Option” on page
177, and “Problem Determination and Debugging” on page
369.
Compiler-friendly Programming
Compiler-friendly programming idioms can be as useful to performance as any of
the options or directives. Here are some suggestions.
General
v Where possible, use command invocations like xlf90 or xlf95 to enhance
standards conformance and code portability. If this is not possible,
consider using the -qnosave option to have all local variables be
automatic; doing this provides more opportunities for optimization.
v Use modules to group related subroutines and functions.
v Consider using the highly tuned MASS and ESSL libraries rather than
custom implementations or generic libraries.
Hand-tuning
v Do not excessively hand-optimize your code. Unusual constructs can
confuse the compiler (and other programmers), and make your
application difficult to optimize for new machines.
v Do limited hand tuning of small functions by inlining.
322 XL Fortran Enterprise Edition for AIX : User’s Guide