[Athas]

Consider the documentation for the similar compiler flag in the OpenCL specification:

> -cl-unsafe-math-optimizations

> Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid, (b) may violate IEEE 754 standard and (c) may violate the OpenCL numerical compliance requirements as defined in section 7.4 for single-precision floating-point, section 9.3.9 for double-precision floating-point, and edge case behavior in section 7.5. This option includes the -cl-no-signed-zeros and -cl-mad-enable options.

While it stops short of saying "this will likely break your code" (maybe because it doesn't have the nonlocal effects of -ffast-math), it makes it much more clear that this flag is generally unsafe and fragile, except under rather specific circumstances. Also, it is reasonably exact about what those circumstances are. I'm not sure -ffast-math is documented with enough precision for a programmer to even know whether it will break their code. Best you can do is try and see if the program still works.

[nsajko]

The relevant GCC man page entries are even more clear than the OpenCL spec excerpt.

-ffast-math:

> This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

It also point to the -funsafe-math-optimizations sub-option, where it is said that:

> Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. [...]