Unfortunately, it doesn't tend to get used automatically in languages like C. The result of rsqrtss is slightly different from 1/sqrtf(x) as two seperate operations, so it cannot be applied as an optimization.

If the rules for floating point optimization are loosened by passing -ffast-math to GCC, the compiler will use it. That being said, -ffast-math is a shotgun that affects a lot of things. If you need signed zeros, Infs, NaNs or denormals that flag may break your program.