HP UX Fortran Software manual Fine-tuning optimization options

	The +Onoconservative option relaxes the optimizer’s
	assumptions about the target program.
	The +Onoconservative option relaxes the optimizer’s
	assumptions about the target program.
	The default is +Onoconservative. This option has been
	deprecated starting with HP-UX version 11i and later.
+O[no]limit	+Olimit suppresses optimizations that significantly
	increase compilation time or that can consume large
	amounts of memory at compile time. This option is only
	effective at optimization level 2 or higher.
	The +Onolimit option allows optimizations to be
	performed regardless of their effect on compilation time or
	memory usage. The default is +Olimit.
+O[no]loop_transform	Enables [disables] the following transformations: loop unroll
	and jam, loop distribution, loop interchange, loop blocking,
	loop fusion, and loop unroll. The default is
	+Oloop_transform.
+O[no]size	+Osize suppresses optimizations that significantly increase
	code size. This option is only effective at optimization level
	2 or higher. The +Onosize option permits optimizations
	that can increase code size. The default is +Onosize.

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.

40 Compiling and linking