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Using and Porting GNU Fortran

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14.6.2 Prefer Automatic Uninitialized Variables

If you're using `-fno-automatic' already, you probably should change your code to allow compilation with `-fautomatic' (the default), to allow the program to run faster.

Similarly, you should be able to use `-fno-init-local-zero' (the default) instead of `-finit-local-zero'. This is because it is rare that every variable affected by these options in a given program actually needs to be so affected.

For example, `-fno-automatic', which effectively SAVEs every local non-automatic variable and array, affects even things like DO iteration variables, which rarely need to be SAVEd, and this often reduces run-time performances. Similarly, `-fno-init-local-zero' forces such variables to be initialized to zero--when SAVEd (such as when `-fno-automatic'), this by itself generally affects only startup time for a program, but when not SAVEd, it can slow down the procedure every time it is called.

See section Overly Convenient Command-Line Options, for information on the `-fno-automatic' and `-finit-local-zero' options and how to convert their use into selective changes in your own code.


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