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

Compiling and linking
Compiling with the f90 command
Chapter 264
Fine-tuning optimization options
The following options allow you to fine-tune the optimization process by providing control over the specific
techniques that the optimizer applies to your program. The syntax for using these options is
+O[no]optimization
where optimization is a parameter that specifies an optimization technique to apply
to your program. The different parameters are described below. The prefix no negates
the effect of optimization.
The options do not override a specified level of optimization, nor do they imply a particular level. To use
any of these options you must also include the +On option on the same command line, where n specifies the
level at which the type of optimization can be performed.
For example, if you find that the optimizer is causing your program to produce different floating-point
results from those produced by the unoptimized program, you could use the following command line to
suppress optimizations that affect floating-point calculations:
f90 +O3 +Onomoveflops +Ofltacc my_prog.f90
If an option is mistakenly used at a level for which the corresponding optimization is not performed, the
compiler will issue a warning message.
The defaults given in the following descriptions are in effect only at the specified optimization levels, unless
stated otherwise.
+O[no]cache_pad_common
+Ocache_pad_common can improve program performance by padding common
blocks to avoid cache collisions. Cache-line collisions occur when the difference
between the addresses of two data points is a multiple of the cache size. By inserting
empty space between large variables (for example, arrays), the optimizer ensures that
they do not start at nearby addresses, where the possibility of a cache collision is greater.
This option is only effective at optimization level 3 or higher.
Note the following precautions when using this option:
• All program modules that reference the common block must be compiled with the
+Ocache_pad_common option.
• Each common block in the program should have the same layout in all program
units within which it is declared. If the layouts are different, they must be fully
independent—that is, they must not pass values between them.
The default, +Onocache_pad_common, disables padding.
+O[no]cxlimitedrange
Enables [disables] the use of floating point math in the compilation unit.