# Is -Ofast in gfortran or -fast flag in Intel Fortran safe to use?

**URL:** <https://fortran-lang.discourse.group/t/is-ofast-in-gfortran-or-fast-flag-in-intel-fortran-safe-to-use/2755>\
**Category:** Uncategorized\
**Created:** [February 9, 2022, 9:32am UTC](https://fortran-lang.discourse.group/t/is-ofast-in-gfortran-or-fast-flag-in-intel-fortran-safe-to-use/2755 "2022-02-09T09:32:26Z")\
**Posts on this page:** 1\
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**Author:** ![certik](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/certik/32/4_2.png) [@certik](https://fortran-lang.discourse.group/u/certik)\
**Post date:** [February 11, 2022, 8:51pm UTC](https://fortran-lang.discourse.group/t/is-ofast-in-gfortran-or-fast-flag-in-intel-fortran-safe-to-use/2755/8 "2022-02-11T20:51:14Z")

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> [@gnikit](#):
>
> I think it would be a good idea to elaborate a bit about what the phrase _“exact implementation of IEEE or ISO rules”_ means in practice and the implications of using `-ffast-math` can have on one’s code.

You can see this post: [https://fortran-lang.discourse.group/t/can-one-design-coding-rules-to-follow-so-that-ffast-math-is-safe/](https://fortran-lang.discourse.group/t/can-one-design-coding-rules-to-follow-so-that-ffast-math-is-safe/) where I posted a link to a document which has examples of codes that break when `-ffast-math` is used, and also how to fix them.

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