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

v -qarch=auto generates code that may take advantage of instructions available
only on the compiling machine (or similar machines).
v To get sqrt optimization, you need to specify -qarch=ppc64grsq or another
-qarch option that supports the square root instruction set.
v Specifying a -qarch option that is not compatible with your hardware, even
though your program appears to work, may cause undefined behaviour; the
compiler may emit instructions not available on that hardware.
Try to specify with -qtune the machine where performance should be best. If you
are not sure, let the compiler determine how to best tune for optimization for a
given -qarch setting.
Before using the -qcache option, look at the options sections of the listing using
-qlist to see if the current settings are satisfactory. The settings appear in the listing
itself when the -qlistopt option is specified. Modification of cache geometry may
be useful in cases where the systems have configurable L2 or L3 cache options or
where the execution mode reduces the effective size of a shared level of cache (for
example, two-core-per-chip SMP execution on POWER4).
If you decide to use -qcache, use -qhot or -qsmp along with it.
Optimizing Floating-Point Calculations
Special compiler options exist for handling floating-point calculations efficiently. By
default, the compiler makes a trade-off to violate certain IEEE floating-point rules
in order to improve performance. For example, multiply-add instructions are
generated by default because they are faster and produce a more precise result
than separate multiply and add instructions. Floating-point exceptions, such as
overflow or division by zero, are masked by default. If you need to catch these
exceptions, you have the choice of enabling hardware trapping of these exceptions
or using software-based checking. The option -qflttrap enables software-based
checking. On the POWER4, POWER5, or PowerPC 970 processor, hardware
trapping is recommended.
Options for handling floating-point calcluations
Option
-qfloat
-qflttrap
Description
Provides precise control over the handling of floating-point
calculations.
Enables software checking of IEEE floating-point exceptions. This
technique is sometimes more efficient than hardware checking
because checks can be executed less frequently.
To understand the performance considerations for floating-point calculations with
different combinations of compiler options, see “Maximizing Floating-Point
Performance” on page 295 and “Minimizing the Performance Impact of
Floating-Point Exception Trapping” on page 302.
High-order Transformations (-qhot)
High-order transformations are optimizations that specifically improve the
performance of loops and array language. Optimization techniques can include
interchange, fusion, and unrolling of loops, and reducing the generation of
temporary arrays. The goals of these optimizations include:
Optimizing XL Fortran Programs 311