HP UX Fortran Software manual Conservative vs. aggressive optimization, +Onowholeprogrammode

Table 29 Fine-tuning optimization options (continued)

Level	Level	Function
		NOTE: This option is valid
		only on the PA-RISC systems.
+O[no]whole_program_mode	+O4or higher	Enable the assertion that
		only the files that are
		compiled with this option
		directly reference any global
		variables and procedures
		that are defined in these
		files. In other words, this
		option asserts that there are
		no unseen accesses to the
		globals. When this assertion
		is in effect, the optimizer can
		hold global variables in
		registers longer and delete
		inlined or cloned global
		procedures. This option is in
		effect only at +O4level of
		optimization.All files
		compiled with
		+Owhole_program_mode
		must also be compiled with
		+O4. If any of the files are
		compiled with +O4but are
		not compiled with
		+Owhole_program_mode,
		the linker disables the
		assertion for all files in the
		program.Use this option to
		increase performance speed,
		but only when you are
		certain that only the files
		compiled with
		+Owhole_program_modedirectly
		access any globals that are
		defined in these files.The
		default is
		+Onowhole_program_modewhich
		disables the assertion.

Conservative vs. aggressive optimization

At optimization level 2 or higher, the optimizer makes a number of assumptions about the program it is optimizing—for example, that re-ordering an expression for improved instruction scheduling will not change its results. In general, these assumptions relate to how closely the target program conforms to the Fortran90 Standard. For programs that conform to the Standard, it is safe for the optimizer to apply certain optimizations that can significantly improve performance. For nonstandard-conforming programs, these same optimizations could change the results or behavior of the program in ways that may not be acceptable to the programmer.

The +Oconservative and +Oaggressive options enable you to set the optimizer’s assumptions about which optimizations it can and cannot apply to a program. Each option invokes a subset of the fine-tuning options that balances safety and performance according to the coding style of the target program. You can use either option at optimization level 2 or higher.

NOTE: +Oaggressiveand +Oconservativeare incompatible and must not appear on the same command line.

Conservative vs. aggressive optimization 99