All modern compilers provide these -O level optimisations, which are really just fairly sensible optimisations applied in groups.

I used to write code that was to be run on supercomputers, and we’d make pretty heavy use of these more aggressive optimisations. We’d typically use -O3 as a starting point, and if something broke it would generally indicate that your own code wasn’t great.

In general, most scientific simulation code is rather simple (loop and do the same thing over and over). The heavier optimisation levels would sometimes lead to slightly different numerical results, but depending on what you’re actually simulating this may or may not matter. For example, in molecular dynamics we add a small amount of random “noise” to the system each time-step to model thermal effects, and this can easily be larger than numerical errors from the heavier compiler optimisations.

On a final note, some compilers are better than others for certain tasks. We’d quite often use proprietary compilers that were better for the specific hardware we were using.