Options That Control Optimization

Most Fortran users will want to use no optimization when developing and testing programs, and use `-O' or `-O2' when compiling programs for late-cycle testing and for production use. However, note that certain diagnostics--such as for uninitialized variables--depend on the flow analysis done by `-O', i.e. you must use `-O' or `-O2' to get such diagnostics.

The following flags have particular applicability when compiling Fortran programs:

-malign-double

(Intel x86 architecture only.) Noticeably improves performance of g77 programs making heavy use of REAL(KIND=2) (DOUBLE PRECISION) data on some systems. In particular, systems using Pentium, Pentium Pro, 586, and 686 implementations of the i386 architecture execute programs faster when REAL(KIND=2) (DOUBLE PRECISION) data are aligned on 64-bit boundaries in memory. This option can, at least, make benchmark results more consistent across various system configurations, versions of the program, and data sets. Note: The warning in the gcc documentation about this option does not apply, generally speaking, to Fortran code compiled by g77. Also note: `-malign-double' applies only to statically-allocated data. Double-precision data on the stack can still cause problems due to misalignment. See section Aligned Data. Also also note: The negative form of `-malign-double' is `-mno-align-double', not `-benign-double'.

-ffloat-store

Might help a Fortran program that depends on exact IEEE conformance on some machines, but might slow down a program that doesn't.

-fforce-mem

-fforce-addr

Might improve optimization of loops.

-fno-inline

Don't compile statement functions inline. Might reduce the size of a program unit--which might be at expense of some speed (though it should compile faster). Note that if you are not optimizing, no functions can be expanded inline.

-ffast-math

Might allow some programs designed to not be too dependent on IEEE behavior for floating-point to run faster, or die trying.

-fstrength-reduce

Might make some loops run faster.

-frerun-cse-after-loop

-fexpensive-optimizations

-fdelayed-branch

-fschedule-insns

-fschedule-insns2

-fcaller-saves

Might improve performance on some code.

-funroll-loops

Definitely improves performance on some code.

-funroll-all-loops

Improves performance on some code.

See section `Options That Control Optimization' in Using and Porting GNU CC, for more information on options to optimize the generated machine code.

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