-Ofast produces incorrect code while using long double

With -Ofast, -ffast-math is enabled, which can cause some operations to be calculated in a different and faster way. In your case, (long double)val / 500) can be calculated as (long double)val * (1.0L / 500)). This can be seen in the generated assembly when you compare -O2 and -Ofast for the following function:

long double f(long double a)
{
    return a / 500.0L;
}

The assembly generated with -O2 involves fdiv instruction, while the assembly generated with -Ofast involves fmul instruction, see https://gcc.godbolt.org/z/58VHxb.

Next, 1/500, that is, 0.002, is not representable by long double exactly. Therefore, some rounding occurs and, seemingly, in your case, this rounding happens to be down. This can be checked by the following expression:

500.0L * (1.0L / 500.0L) < 1.0L

which is evaluated as true: https://gcc.godbolt.org/z/zMcjxJ. So, the exact stored multiplier is 0.002 - some very small delta.

Finally, the result of the multiplication is 500 * (0.002 - delta) = 1 - some small value. And when this value in converted into int, it's truncated, therefore the result in int is 0.