What does gcc’s ffast-math actually do?

I understand gcc’s --ffast-math flag can greatly increase speed for float ops, and goes outside of IEEE standards, but I can’t seem to find information on what is really happening when it’s on. Can anyone please explain some of the details and maybe give a clear example of how something would change if the flag was on or off?

I did try digging through S.O. for similar questions but couldn’t find anything explaining the workings of ffast-math.

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