-ffast-mast does a whole bunch of things, which I wish people's didn't so commonly combine.

For example, I think the things it does which are sensible for most people are:

* Rounding subnormals to zero

* Disabling signed zeroes

* Disables support for 'trapping' (throwing SIGFPE)

Then there are the 'middle' things, which annoy some people:

* Allow associative operations, and things like sqrt(xy)=sqrt(x)sqrt(y), exp(x)*exp(y)=exp(x+y)

However, it also (which I often find break code) assumes no operation will make a NaN or an Infinity -- these last two don't really help, and also break code in confusing ways. This being gcc, they don't just change things like std::isnan or std::isinf into an 'abort' (which would make sense, in -ffast-math they don't make sense), they just return nonsense instead.