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Using and Porting GNU Fortran

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14.5 Overly Convenient Command-line Options

These options should be used only as a quick-and-dirty way to determine how well your program will run under different compilation models without having to change the source. Some are more problematic than others, depending on how portable and maintainable you want the program to be (and, of course, whether you are allowed to change it at all is crucial).

You should not continue to use these command-line options to compile a given program, but rather should make changes to the source code:

-finit-local-zero
(This option specifies that any uninitialized local variables and arrays have default initialization to binary zeros.)

Many other compilers do this automatically, which means lots of Fortran code developed with those compilers depends on it.

It is safer (and probably would produce a faster program) to find the variables and arrays that need such initialization and provide it explicitly via DATA, so that `-finit-local-zero' is not needed.

Consider using `-Wuninitialized' (which requires `-O') to find likely candidates, but do not specify `-finit-local-zero' or `-fno-automatic', or this technique won't work.

-fno-automatic
(This option specifies that all local variables and arrays are to be treated as if they were named in SAVE statements.)

Many other compilers do this automatically, which means lots of Fortran code developed with those compilers depends on it.

The effect of this is that all non-automatic variables and arrays are made static, that is, not placed on the stack or in heap storage. This might cause a buggy program to appear to work better. If so, rather than relying on this command-line option (and hoping all compilers provide the equivalent one), add SAVE statements to some or all program unit sources, as appropriate. Consider using `-Wuninitialized' (which requires `-O') to find likely candidates, but do not specify `-finit-local-zero' or `-fno-automatic', or this technique won't work.

The default is `-fautomatic', which tells g77 to try and put variables and arrays on the stack (or in fast registers) where possible and reasonable. This tends to make programs faster.

Note: Automatic variables and arrays are not affected by this option. These are variables and arrays that are necessarily automatic, either due to explicit statements, or due to the way they are declared. Examples include local variables and arrays not given the SAVE attribute in procedures declared RECURSIVE, and local arrays declared with non-constant bounds (automatic arrays). Currently, g77 supports only automatic arrays, not RECURSIVE procedures or other means of explicitly specifying that variables or arrays are automatic.

-fgroup-intrinsics-hide
Change the source code to use EXTERNAL for any external procedure that might be the name of an intrinsic. It is easy to find these using `-fgroup-intrinsics-disable'.


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