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'
*Note Aligned Data::, for more information on alignment issues.
_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.
This option is effective when the floating-point unit is set to
work in IEEE 854 `extended precision'--as it typically is on x86
and m68k GNU systems--rather than IEEE 754 double precision.
`-ffloat-store' tries to remove the extra precision by spilling
data from floating-point registers into memory and this typically
involves a big performance hit. However, it doesn't affect
intermediate results, so that it is only partially effective.
`Excess precision' is avoided in code like:
a = b + c
d = a * e
but not in code like:
d = (b + c) * e
For another, potentially better, way of controlling the precision,
see *Note Floating-point precision::.
`-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. Sets
`-funsafe-math-optimizations', and `-fno-trapping-math'.
`-funsafe-math-optimizations'
Allow optimizations that may be give incorrect results for certain
IEEE inputs.
`-fno-trapping-math'
Allow the compiler to assume that floating-point arithmetic will
not generate traps on any inputs. This is useful, for example,
when running a program using IEEE "non-stop" floating-point
arithmetic.
`-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'
Typically improves performance on code using iterative `DO' loops
by unrolling them and is probably generally appropriate for
Fortran, though it is not turned on at any optimization level.
Note that outer loop unrolling isn't done specifically; decisions
about whether to unroll a loop are made on the basis of its
instruction count.
Also, no `loop discovery'(1) is done, so only loops written with
`DO' benefit from loop optimizations, including--but not limited
to--unrolling. Loops written with `IF' and `GOTO' are not
currently recognized as such. This option unrolls only iterative
`DO' loops, not `DO WHILE' loops.
`-funroll-all-loops'
Probably improves performance on code using `DO WHILE' loops by
unrolling them in addition to iterative `DO' loops. In the absence
of `DO WHILE', this option is equivalent to `-funroll-loops' but
possibly slower.
`-fno-move-all-movables'
`-fno-reduce-all-givs'
`-fno-rerun-loop-opt'
In general, the optimizations enabled with these options will lead
to faster code being generated by GNU Fortran; hence they are
enabled by default when issuing the `g77' command.
`-fmove-all-movables' and `-freduce-all-givs' will enable loop
optimization to move all loop-invariant index computations in
nested loops over multi-rank array dummy arguments out of these
loops.
`-frerun-loop-opt' will move offset calculations resulting from
the fact that Fortran arrays by default have a lower bound of 1
out of the loops.
These three options are intended to be removed someday, once loop
optimization is sufficiently advanced to perform all those
transformations without help from these options.
*Note Options That Control Optimization: (gcc)Optimize Options, for more information on options to optimize the generated machine code.
---------- Footnotes ----------
(1) "loop discovery" refers to the process by which a compiler, or indeed any reader of a program, determines which portions of the program are more likely to be executed repeatedly as it is being run. Such discovery typically is done early when compiling using optimization techniques, so the "discovered" loops get more attention--and more run-time resources, such as registers--from the compiler. It is easy to "discover" loops that are constructed out of looping constructs in the language (such as Fortran's `DO'). For some programs, "discovering" loops constructed out of lower-level constructs (such as `IF' and `GOTO') can lead to generation of more optimal code than otherwise.Created Mon Nov 8 17:42:14 2004 on tillpc with info_to_html version 0.9.6.