How to force GCC to assume that a floating-point expression is non-negative?

Question

There are cases where you know that a certain floating-point expression will always be non-negative. For example, when computing the length of a vector, one does sqrt(

Answer 1

After about a week, I asked on the matter on GCC Bugzilla & they've provided a solution which is the closest to what I had in mind

float test (float x)
{
    float y = x*x;
    if (std::isless(y, 0.f))
        __builtin_unreachable();
    return std::sqrt(y);
}

that compiles to the following assembly:

test(float):
    mulss   xmm0, xmm0
    sqrtss  xmm0, xmm0
    ret

I'm still not quite sure what exactly happens here, though.