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GNU Info (g77-295.info)Optimize OptionsOptions 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'. 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. `-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' *Version info:* These options are not supported by versions of `g77' based on `gcc' version 2.8. Each of these might improve performance on some code. Analysis of Fortran code optimization and the resulting optimizations triggered by the above options were contributed by Toon Moene (<toon@moene.indiv.nluug.nl>). These three options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving the performance of Fortran code. Please let us know how use of these options affects the performance of your production code. We're particularly interested in code that runs faster when these options are *disabled*, and in non-Fortran code that benefits when they are *enabled* via the above `gcc' command-line options. Note: Options That Control Optimization, 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. automatically generated by info2www version 1.2.2.9 |