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

Summary of Compiler Options for Optimization
The following table describes the compiler options that have the most effect on
performance. You can experiment with different combinations of compiler options
to see which options are most suitable for your application.
Table 20. Compiler options for optimization
Option
Overview
-O0 or -qnoopt The compiler performs very limited optimization. This is the default
optimization level. Before you start optimizing your application, ensure
that it compiles and executes successfully at optimization level 0.
-qarch
-qtune
The compiler generates instructions for the specified processor
architecture. This option allows the compiler to take advantage of
processor instructions that exist on the specified architecture but do not
exist on other architectures.
The compiler optimizes the application for the specified processor
architecture. The application will run on all of the processors specified
by the -qarch option, but its performance will be tuned for the
processors specified by the -qtune option.
-O2 The compiler performs basic optimization. This option provides a
balance between compilation speed and run-time performance.
-O3 The compiler performs aggressive optimization, including optimizations
that are memory-intensive, compile-time-intensive, or both.
-qhot
-qcache
-qipa
The compiler performs additional loop optimization, automatic
vectorization, and optionally performs array padding. This option is
most useful for scientific applications that contain numerical processing.
If you want better performance than -O3 with less compile time than
-O4, try -O3 -qhot.
The compiler assumes that the processor will have the specified cache
configuration. This option can improve performance if all of the
processors that execute the application have the same non-default cache
configuration.
The compiler performs interprocedural analysis to optimize the entire
application as a unit. This option is most useful for applications that
contain a large number of frequently used routines. In many cases, this
option significantly increases compilation time.
-O4 This is equivalent to: -O3 -qipa -qhot -qarch=auto -qtune=auto
-qcache=auto
-O5 This is equivalent to: -O4 -qipa=level=2
-qpdf1
-qpdf2
The compiler uses profile-directed feedback to optimize the application
based on an analysis of how often your application executes different
sections of code.
Choosing an Optimization Level
Optimization requires additional compilation time, but usually results in a faster
run time. XL Fortran allows you to select whether you want optimization to be
performed at compile time. By default, the compiler performs very limited
optimization (-O0 or -qnoopt).
To enable compiler optimization, specify the -O compiler option with an optional
digit that signifies the level. The following table summarizes compiler behavior at
each optimization level.
Optimizing XL Fortran Programs 307